English Timbers

Solid Hardwood flooring

Site Survey and predicting the performance of Hardwood Flooring

Introduction

Because of the the variety of property types, climate ranges and site conditions, fitting hardwood flooring can be potentially risky for even the most proficient floor layer.

He must consider the variation in type and age of building (which affect the internal conditions), the type and level of heating, the ambient conditions at the time of laying, the possibility of damp in the structure - especially with a new build and, of course, the moisture content of the new floor.

Guesswork and experience usually play a large part in getting this right, but to ensure that the site conditions are correct and that the job is fitted perfectly first time, it is best to take a more scientific approach. A little investment in knowledge, some new skills and equipment will save costly returns to site, potential litigation in the case of a complaint and pay long term dividends. In order to predict how a floor will behave it is necessary to know the following information:

  1. The eventual operating temperature and relative humidity (RH) of the building whenin use.
  2. The recommended moisture content of the timber flooring specified by the designer or manufacturer to suit the conditions in service.
  3. The actual moisture content of the flooring when it arrives on site and when it is to be fitted. It is essential that a reputable supplier is used and especially one who stores the timber correctly and knows its moisture content accurately.
  4. The level of moisture within the structure itself - this includes masonry, concrete floors, existing timber etc. This is important to assess if structural moisture is likely to migrate into the atmosphere or adjacent flooring.
  5. The ambient relative humidity and temperature of the air in the building to determine when it is suitable for the floor to be laid.

Finally it is necessary to understand how both site conditions and final occupation conditions affect the moisture content of the wood and what the consequences of any changes may be. Application of this knowledge to the information gathered permits the fitter to predict what will happen to the wood floor at any stage in its life, whether there will be any movement and most importantly when and if site conditions are suitable for laying the floor.

The next sections explain how these methods and measurements can be used to predict and prevent problems arising and how they may be used to give the fitter a framework in which to work as well as recording the evidence of correct fitting procedure should any complaint arise at a later date.

Explanation of Terms

For starters an explanation of some of the terms may be useful:- Moisture Content is a measure of how much water is contained in the wood. It is normally measured with a meter which measures the electrical resistance of the wood between two pins. Simple meters give only an idea of the moisture content while the more sophisticated ones adjust the readings for temperature and species type and are much more accurate. However they only read the moisture content between the pins and it is essential to take the average of several readings.

An other type of meter uses an electromagnetic field to measure the moisture content. These are dependant on the correct estimation of timber density.

Relative Humidity (RH) is a measure of how much moisture ( As water vapour ) the air contains at a particular temperature and the temperature must be measured at the same time as the RH. Relative humidity is best measured with a an accurately calibrated hygrometer such as a Protimeter Hygromaster or a Protimeter MMS. The capacity of air to hold moisture increases as the temperature increases and visa versa. So warm air will hold more moisture than cool air and this may be seen when warm humid air cools in early morning and dew is formed because the cooling air cannot hold the same quantity of water vapour.

Equilibrium Moisture Content (EMC) is the moisture content that a material eventually achieves if exposed to a stable combination of temperature and relative humidity for sufficient time. So if a piece of wood was left in a room where the temperature was 20°C and the RH was 75%, the moisture content of the timber would finally reach about 16%. The EMCs for a whole range of temperatures and humidities can be predicted for wood with reasonable accuracy regardless of specie. However other materials react differently and have very different EMCs. So, for example concrete in the same conditions would achieve about 5% moisture content. It is important not to compare moisture contents of different materials as this can lead to costly mistakes and in this case if dry timber (at say 8%) was placed in contact with concrete at 5%, the moisture content of the timber would increase to about 16% with disastrous results!

Equilibrium Relative Humidity (ERH ) is the relative humidity that would be achieved in the air in contact with a particular material and it is a measure of the amount of moisture in the material. For example if an open box was placed open side down on a concrete slab and sealed around its edges, the relative humidity of the air in the box would eventually reach equilibrium with the moisture in the slab. If the concrete is damp it will release moisture into the air in the box, but if it is very dry it will draw moisture out of the air until equilibrium is reached. By measuring the ERH in the box after say 24 hours we have a measure of the moisture in the slab. This method is independent of the type of floor slab and also gives an indication of the amount of moisture deep within the slab. It is also more accurate than trying to measure the moisture content of the concrete with a meter because the electrical resistance is influenced by different concrete mixes, types of aggregate and salts within the slab. Protimeter have developed a simple and effective system of measuring the ERH in concrete called a Humidity Sleeve and this is discussed in the next section.

Measurement of Site Conditions

This section explains how to measure and assess site conditions and materials when laying a wood floor.

Wood is a hygroscopic material and it adjusts to the surrounding conditions absorbing or losing moisture until it is in equilibrium with them. It is influenced by the relative humidity (RH) and the temperature of the surrounding air and the moisture content of the adjacent structure.

To ensure stability, the moisture content of the wood should be in equilibrium with all of these factors both when the floor is installed and during service. For this to occur it is important for the floor layer to measure the site conditions accurately and interpret the information correctly.

Measurement of Timber Moisture Content.

The best site method of measuring moisture content is to use a meter, which measures the electrical resistance between 2 pins driven into the timber. There is a range of meters available, some simple and others sophisticated.

If only a general idea of moisture content is wanted, use a simple, low cost meter which is better than nothing, but the readings are not accurate enough to predict potential movement. For the purposes of an accurate survey and to be able to monitor the hardwood flooring with confidence the more sophisticated version is a must.

The meter must be able to compensate for the timber species and the temperature of the timber - all of which will affect the accuracy of readings. There are a number of meters on the market and a typical meter would be the Protimeter Timber Master or a Delmhorst J 2000. For the thinner timber - say up to 1" thick it is normally sufficient to use a push-in electrode. Thicker timbers require a hammer electrode which drives the pins in to the required depth. If just the surface of the timber is wet the meter will give a higher than true reading because it will effectively short circuit across the surface. Insulated pins are available where contact is only made at the point and these will give the moisture content within the wood. Some useful pointers are: -

  • The meter will always give a localised moisture content - just between the pins and this must not be taken as representative of the whole batch.
  • The moisture content will vary within one piece of wood along its length and from the surface to the core and between each face. It will also vary from piece to piece, as the drying rate in the kiln varies with the different densities and characteristics of the wood.
  • The moisture content quoted by the hardwood floor manufacturer is the average moisture content of the whole batch of flooring so one or two checks have little value and may be misleading. It is best to take as many readings as possible across the whole batch. If for example the average moisture content is 8% you will probably get most of the readings in the range 6 to 10 but there will always be a few outside that range.
    At English Timbers we take between 500 & 1000 readings per 1000m² of flooring produced and another 300 as orders are prepared for dispatch.
  • It is usual for the surface moisture content to be a little higher than that at the core. If you find the readings higher than expected with a push in probe, use the hammer probe and check the moisture content at the centre of the wood - if it is lower than the surface, it indicates that the flooring may have been badly stored and has absorbed moisture on the outside. Similarly if the moisture contents of the pieces inside a bundle of flooring are significantly lower than those on the outside, it may indicate poor conditions during storage or transit. Flooring should be fully wrapped in storage and transit.
  • In addition to checking the new flooring the meter may be used to check the following: -
    • Any plywood or chipboard which is to be used in contact with the flooring. · Any existing wood floor for signs of damp.
    • Existing woodwork in the building - for example the moisture content in the skirtings will indicate if there is damp in the walls.
    • Underneath or back of furniture in the room - this will indicate the equilibrium moisture content achieved in service in that particular room.
  • The meter calibration should be checked regularly.

Measurement of Ambient Air Conditions.

UK WOOD FLOORS

The measurement of Relative Humidity is best done with a hygrometer which should also measure the ambient temperature. Choose a reputable brand and pay for accuracy and quality. Both the Protimeter Hygromaster and the MMS devices are recommended.

Survey the building well in advance of laying any floor. Readings should ideally be taken in different parts of each room where the flooring is to be laid. For the purposes of the survey, the conditions within the rooms should have been stable for the previous 24 hours. In existing buildings which are lived in this is not too critical because conditions have already stabilised but in new buildings the air conditions give a good indication of the free moisture in the structure. However any artificial ventilation or heating within the previous 24 hours could produce incorrect readings.

It is a good idea to draw up a standard survey document in which all the readings are recorded with the localities and date. This will provide a proof of survey if there is a problem in the future and allow monitoring of the building at the same locations in the future.

Measurement of the Relative Humidity of Concrete Slabs.

The moisture content of concrete is difficult to measure accurately with a meter because different mixes, aggregates and salts within the concrete affect the readings. The only accurate method of assessing the free moisture in the slab is to measure the Equilibrium Relative Humidity of the concrete. Two basic methods are described briefly.

Using a Humidity Box - This is a small sealed box, open on one side, which is placed open side down on the concrete and sealed round the edges so that the air in the box is in contact with the concrete and cannot be influenced by the surrounding air.

In the older types the hygrometer is built into the top of the box and it works by using the response of natural fibres to the change in humidity of the air in the box. It is essential to frequently calibrate these. A modern approach is to use an electronic meter and sensor, which plugs into the box. It is normal to leave the box for about 24 hours before measuring the relative humidity and temperature. As before, record the readings, the location, and the date so that the same areas can be monitored.

They have the disadvantages of :-

  • Inaccuracy if the concrete has any surface coatings or it has been power floated.
  • Being accidentally moved or disturbed by others and giving a false result.

Using a Humidity Sleeve - This is a method developed by Protimeter and has the advantage of providing permanent, safe monitoring sites within the concrete. They are flush with the concrete and do not have the same disadvantages.

Instead of measuring the relative humidity at the surface of the slab it is measured within the slab. This actually gives a better indication of the overall relative humidity of the slab and permits readings to be taken within a shorter time period.

Holes are drilled into the slab at the selected sites (see below) cleaned and the plastic sleeves are pushed into place. An end cap seals the sleeve. Each sleeve is perforated so that the air within the sleeve adjusts to the equilibrium relative humidity of the slab. Drilling the hole heats up the surrounding concrete and changes the RH in the hole. So, to ensure accuracy and that the conditions in the sleeve are stable and have reached equilibrium with those in the concrete, allow 24 hours before testing. However readings may be taken after a few hours have elapsed to give an indication of how wet or dry the concrete is.

UK WOODEN FLOORS

Measurements are taken with the remote sensor on the Protimeter Hygromaster or MMS by removing the end cap and carefully inserting the sensor into the sleeve. For the first reading only, about 30 minutes is required for the sensor to adjust from the conditions in its carrying case to those in the slab. Subsequent measurements require less time for adjustment because conditions within the slab are unlikely to greatly vary and 10 minutes should cover this. However, always refer to the manufacturers instructions and when in doubt ask them or allow further time for conditions to stabilise.

Again the relative humidity and temperature at each of the sites should be recorded and the sleeve caps replaced so that the sites may be used at a later date.

Selecting the Best Site to Measure Relative Humidity

An experienced floor layer may guess the position of the wettest regions but there is no substitute for science. The Protimeter MMS has a search mode and the meter can be used to find the area with the highest moisture level.The instrument is placed on the slab at suitable intervals across the floor and will show relative readings indicating which areas have the highest moisture level. This only takes a few moments and allows the floor layer to find the wettest areas to positionthe sleeves. However it is best to monitor the whole floor incase the wettest areas dry significantly faster than others and become less critical. Full information of these materials and their use may be obtained from Protimeter Plc.

Measurement of Moisture in the Rest of the Structure.

For the hardwood floor layer this actual measurement may not be necessary provided the recent history of the building is known and both ambient air conditions and the relative humidity of the concrete slab (if appropriate) are measured.

A visual survey of the existing building together with some moisture contents of skirtings and other woodin the room will indicate any obvious damp in contact with the walls. High relative humidity may indicate if the structure is damp or poorly heated.

If the building is new or renovation has recently taken place, the type of construction and the timee lapsed since completion of the wet work could indicate the possibility of high moisture contents in the building fabric.

For example, traditionally built walls with rendering and skim coats of plaster may take weeks or months to dry out and this will lead to increased air relative humidity within the buildings.

So to summarise the requirements of the survey:-

  1. If there is a concrete slab to which a wood floor is to be fixed, find the wettest area using the search mode of the Protimeter MMS. Place humidity sleeves at this point and other representative positions across the slab and monitor the relative humidity andtemperature.
  2. Measure the stable ambient relative humidity and temperature of the air in the rooms/building where the floor is to be laid.
  3. Measure the moisture content of existing timber floors, skirting, furniture and woodwork in the room with an accurate moisture meter.
  4. UK WOODEN FLOORINGGather information about the building, how it is to be heated,structure, recent building work etc. and generally look at the building for signs of damp and condensation. Ascertain the future living conditions.

This information can then be used to assess whether a wood floor should be laid. If not it can be used to decide what measures should be taken to provide the correct conditions for laying the floor. How to assess and use this information will be dealt with in the next article. Advice given is genericand regardless of the product or equipment, always refer to individual manufacturers instructions.

Site Conditions & Their Effect on Hardwood Floor Moisture Content

The previous section explained howto measure and assess site conditions. This section explains some of the theory and how to apply this to gauge if site conditions are satisfactory.

To summarise the requirements for a site survey, the following need to be measured and assessed:

  1. The ambient relative humidity(RH) and temperature of the air within the rooms at the time of the survey.
  2. The relative humidity of the concrete slab (if applicable).
  3. The moisture content of any wood sub-floor (if applicable).
  4. The moisture content of any wood fittings and furniture in the area where the wood floor isto be laid.
  5. A general assessment of the condition of the building, signs of damp etc.
  6. The recent history of occupation(if any), if any building work, ambient living conditions (relative humidity and temperature), type of heating.

This will give the factual site information. It is helpful to standardise the survey and record the informationin an easily recognisable form. This record will help should there be any future problems, litigation etc.

If the building is new, or has not been occupied for some time, it is necessary to determine what the actual conditions in the building will be when later occupied.i.e.:-

  1. The ambient air relative humidity and temperature.
  2. Type of heating - under-floor, radiators etc. The question of under-floor heating will be dealt with elsewhere.
  3. The type of floor to be laid and method of fixing - Wide boards, strip etc, nailing, gluing etc.

Assuming that under-floor heatinghas not been installed, the average ambient relative humidity and temperature, if reasonably constant, will determine the final moisture content of the wood floor in service.

Seasonal variations will cause some expansion and contraction. The time of year when the floor is laid together with the moisture content of the new flooring will determine if it will expand or contract after laying.

The table below shows the equilibrium moisture content (EMC) for the average of most common timbers for a range of temperatures and humidities. It shows the approximate moisture content that the timber would achieve if left in a particular set of conditions for a sufficient length of time. Values have been rounded upor down to nearest 0.5%.

Table - APPROXIMATE EQUILIBRIUM MOISTURE CONTENTS ACHIEVED FOR A RANGE OF RELATIVE HUMIDITIES AND TEMPERATURES.

Ambient Temperature °C
Relative Humidity 15°C 20°C 25°C 30°C 35°C
20% 6 5.5 5.5 5.5 5
25% 7 7 6.5 6 6
30% 8 7.5 7 7 6.5
35% 9 8.5 8 7.5 7.5
40% 9.5 9.5 9 8.5 8
45% 10 10 9.5 9.5 9
50% 11 11 10.5 10 10
55% 12 12 11.5 11 10.5
60% 13 13 12.5 12 11.5
65% 14.5 14 13.5 13 12.5
70% 15.5 15 14.5 14 14
75% 16.5 16 16 15.5 15
80% 18 18 17.5 17 16.5
85% 20 20 19 18.5 18

Values are based on an average of common wood types. Some variation may exist between individual species.

So for example if the average temperature is 20°C and the average relative humidity is 55% the timber would eventually reach an EMC of about 12%. At 25°C and 40% relative humidity the EMC would be 9%.

One of the difficulties of laying a wood floor in the U.K. is the wide range of conditions encountered in the different styles of houses with a variety of heating systems and seasonal variations between summer and winter.

Older properties tend to be more draughty, less well insulatedand have higher humidities. If the house is well ventilated in a warm humid summer period then the humidity may be 60/70%internally with a temperature of 20°C. This would givean EMC in the floor of about 13% to 15%. In winter, if the building was continuously heated, the conditions may be 40 to 45% RHand 25°C giving a moisture content of about 9% - a differenceof 4% or 5%.

  • In reality the change in moisture content should be less than calculated because:-
  • The floor should be well sealed and would react slowly to changes in ambient conditions.
  • Current lifestyle trends are towards warmer drier conditions within buildings. Also modern buildings are usually drier and warmer with more stable conditions.
  • It can be seen from the table that the EMC is more sensitive to changes in relative humidity than temperature. However the two are closely linked and an increase in temperature normally lowers the RH and visa versa.

Movement of Timber

The amount the floor moves with changing moisture content will depend on the species and how the tree is cut.So for example Afrormosia is a very stable timber and has low movement while oak and most of the temperate timbers have medium to high movement characteristics.

Movement is the expansion or contraction of the wood across the width of the board or at 90 degrees to the direction of the grain. For practical purposes there is little movement parallel to the grain.

If the moisture content of the timber falls then it will contract across the grain, if the moisture increases it will expand. Additionally wide boards will tend to cup or bow and adequate fixings must be provided to prevent this.

In general terms Oak, Ash, Elm, Cherry, Walnut and Maple havesimilar characteristics and are classed as medium movement timbers. Beech is classed as a large movement timber with about 20% moremovement for the same change in conditions. Plain sawn or crown-cut timber moves more than quarter sawn or radially sawn timber.

As a very approximate rule of thumb it can be said that the maximum movement for medium movement timbers is about 2 to 3mm per metre width of floor per 1% change in moisture content -across the grain only. So for example in the old house, if the change in moisture content is 4% the movement would be about 10mm max. per metre width of floor. This is only a guidelinebut it does serve to show the magnitudes of change which can occur given sufficient time and lack of restraint.

Also, because there are variations in characteristics and moisture content of individual boards within the floor, some boards may move more than others as they adjust to any changes. Localised movement may occur near sources of heat and low humidity suchas radiators, uninsulated pipework, wood burners and in andaround cooker ranges which are constantly heated.

New properties are built to higher standards and tend to have less variation in ambient conditions once the property has fully dried out from new. High levels of insulation, double glazing and less natural ventilation mean less heat input and smaller seasonal variations in RH andtemperature and in consequence more stable floors. Modern timber framed buildings are particularly stable if there is little or no wet work and only the RH of the base slab needs to be considered a potential source of moisture.

Under-floor heating uses the floor itself as one large low temperature radiator and theamount of heat passing through the floor (as well as the relativehumidity in the room) will determine the moisture content in the timber. The heat output from the floor depends on the temperature required in the building and heat lost through the building fabric and via air exchange or ventilation. A well insulated structure loses less heat and requires less energy to maintain stable conditions and therefore less heat input via the floor. An older or less well insulated building requires more heat input and the seasonal range of conditions within such a building may lead to a very large movement in the timber.

As a guideline, moisture contents in the order of 6 to 8% are likely in winter in a well insulated building with an under-floorheating system and perhaps 9% or 10% in summer.

For the manufacturer of flooring it is very difficult to accommodate all conditions, especially with the current trend towards under-floorheating, but it can be seen how important it is to try and achieve a happy medium that will suffice for most types of building and conditions. At English Timbers we consider that the correct moisture content of the floor is the single most important factorin the performance of a hardwood floor that we can actually influence. In cases of poor performance or failure of a wood floor, it is invariably incorrect installation or site conditions or an incorrect moisture content in the new floor that is to blame.

In consequence we have a very stringent quality control system which monitors all aspects of the manufacturing process with particular emphasis on moisture content. We can trace all orders back through the system via records of machining tolerances and the moisture content checks - at least 500 checks per 1000 square metres produced.

We normally dry to around 8 to 10% average. This is suitable for a normally heated building but a little too high for underfloor heating. This would require a moisture content in the region of about 6% to 8%

We do occasionally have batches of flooring with moisture contents of about 8% and these could be used with care and some acclimatisation over underfloor heating.
We enclose a written record of the average moisture contents with each order and this shows the average of the whole production run, the average of your order & how many readings have been taken. All our timber and flooring is fully wrapped at every stage of manufacture, storage and during delivery to maintain the moisture content and it is important to re-wrapon site if the conditions are not suitable for laying.

Predicting Movement

The moisture content of the new floor should be compatible with the correct conditions at the time of laying and ideally be closeto the average EMC produced by the summer and winter conditions.If this is the case then:-

Floors laid in winter will normally expand in summer when the heating is off.

Floors laid in summer will initially expand and then contract in winter when the heating is on.

It is unusual for the conditions in every building to be exactly right and due allowance should be made for any movement either way. Use the results of the survey to predict any movement.

For example if the winter conditions are 25°C and 40% relative humidity, a floor at about 9% moisture content will be stable in these conditions. If the expected summer conditions are 23°Cand 55% relative humidity the moisture content will increase to about 11.5% with a corresponding expansion of about 7mm per metre width of floor. Allowance should be made for some of this movement. Actual movement is rarely as much as theoretical.

If site conditions are checked and found to be considerably different from the occupation conditions then the consequencescan be estimated and the information used to persuade the clientor contractor to remedy this.

For example, if occupation conditions are 23°C and 40% RH and the site conditions are 18°C and 80% RH then the EMCs are 9%and 18% respectively. This difference of 9% means that if the floor were to be laid in these conditions it could potentially expand by about 20 to 25mm per metre width of floor and would fail. Smaller differences in moisture contents, outsidethe acceptable conditions, may not cause really obvious failure at the time. However the floor could expand to such a point that parts of the timber are compressed and damaged permanently and when the building finally dries out, unsightly gaps anda rippled surface may appear.

Summary

Determine the conditions to be achieved in service. If a new build or refurbishment, ask the client, architect or main contractor. If the building has been and is permanently occupied, check the existing conditions and make some estimation of seasonal variation.

Check with the supplier that the moisture content of the new wood floor is going to be compatible with these conditions -with or without some acclimatisation.

Check that the conditions within the building at the time of installation or survey are close to the conditions required in service. If they are not use the table to calculate the changein moisture content and any likely movement.

Check the moisture content of the new floor as soon as it arrives on site - take plenty of readings in order to gain a fair average. This must be done at the time of delivery, it is unrealisticto expect the supplier to accept any complaints at a later date because dry timber will immediately start to adjust to the new conditions as soon as it is unwrapped.

If the site conditions are compatible or reasonably close to the correct, specified conditions then it is acceptableto lay the floor. If the site conditions differ considerably from the correct, specified conditions then wait until they are correct or thereabouts.

Acclimatisationof the flooring

This is an established practice which is intended to adjust the moisture content of the new flooring to the equilibrium moisture content at the site conditions at the time of laying. Where the flooring is likely to expand, recent research by ourselves has shown that there can be problems and drawbacks with acclimatisation:-

To be effective the flooring must be fully unpackedand stacked in such a way to permit good circulation around every piece of flooring. Even so it takes much longer than the usual1 4 days to make any appreciable change in the moisture content and it may be necessary to leave the timber for several months to fully adjust. This is obviously unacceptable.

If there is a considerable difference between theactual site conditions and those specified, the potential changein moisture content caused by fully acclimatising the floor could be enough to produce a significant change in the width of the strips, some of which will expand more than others. This will lead to variations in the widths and possible difficulties with installation.

If the flooring is acclimatised for a sufficiently long period at incorrect site conditions, the floor, after installation, will re-adjust to the eventual occupation conditionsand further movement will occur - usually shrinkage.

There is really no escape from the establishment of correct site conditions at the time of laying and if this is done acclimatisation has little value. If some expansion is unavoidable then install the flooring with a pneumatic nailer which gives a slacker fit or use a Powernailer but build in sufficient small expansion gaps within the floor.

Remember to keep the floor fully wrapped if the conditions are not compatible with those in service.

See also "ACCLIMATISATION" in the "Laying Guidelines" section.

Effect of Sub-floors & Building Structure on a New Hardwood floor

It is important that the existing sub-floorand the structure are in good condition and dry.If there are signs of damp in the building, walls etc this will show up in the moisture content of adjacent skirtings and woodwork and increased levels of relative humidity. This should be remedied.

If the existing sub-floor consists of timber boards or chipboard, it should be in good condition, well ventilated, if suspended, and have a reasonable moisture content - about 12 -15% for aground floor suspended floor and about 10% in a first floor.

Always use a vapour barrier over a ground floor. Concrete floors are more problematical but once the relative humidity has been determined there are a number of options depending on the type of floor to be laid.

Firstly it is important to restate that different materials reach different EMCs in the same conditions, the industry standard for a dry concrete floor is 75% relative humidity at 20°C and in these conditions concrete has a moisture content of about 5%, depending on the type of mix.

However (refer to table) timber at 75% relative humidity and 20°C has a moisture content of about 16% and if timber at 9% moisture content is placed in direct contact with concrete at 75% it will take in moisture until it reaches 16% with a corresponding potential expansion of about 15 to 20mm per metre width of floor.

If timber at 9% is to be bonded directly to concrete then the relative humidity of the concrete shouldbe 40% or below.

As concrete takes a long time to dry out, the safest optionwould be to apply a damp proof membrane (DPM) to the concrete.If the floor is to be bonded as above, an epoxy DPM must beused and this would usually be sandwiched between two cement aciousscreed layers which also have the advantage of levelling the floor and permit the use of standard adhesives. If the floor is bonded directly to the epoxy a special adhesive must be used- Contact Sika for more info on Epoxy DPMs, self levelling compounds etc.

If the floor is to be laid on to battens or plywood then aDPM must be used to prevent excess moisture migrating from the concrete into the wood. Fixing the battens and floor may puncture the DPM.

Virtually all DPMs - epoxies, polythenesheet and building paper will transmit some moisture and provided that it is only a small amount the wood floor will remain stable.

The amount of moisture transmitted depends on the type of DPMand the difference in temperature and relative humidity of theslab and the wood.

So the drier the conditions are above the DPM and the wetter the concrete, the greater the vapour pressure and the corresponding amount of water vapour forced through the DPM.

As a general rule, always use a DPM but reduce the relative humidity of the slab as much as possible before applying theDPM - below 75% if possible. The type of DPM, number of layers of epoxy etc will depend on the relative humidity of the floor slab and manufacturers of the particular products will adviseon this.

As well as checking the slab relative humidity it is important to check the moisture content of any plywood, battens etc if they are in direct contact with the timber. If the moisture contents are significantly high, the moisture content of the new floor will also increase.

New Building Work

Try to ensure that the contractor/client understands the importance of completing the wet work early and not to cover any floors lab which would inhibit drying out. Heating should be installed early to help the drying process.

Monitor the floor slab as well as the ambient conditions in the building.

Dry out the building and the slab as much as possible - Use the heating and dehumidifiers where necessary.

Always use a DPM under the new floor.

Conclusion

Conducting a systematic and accurate survey of the building provides the floor layer with a record which may be sufficient in itself to permit the installation of a wood floor safely. It will provide a baseline for further monitoring until acceptable site conditions for laying are achieved. Using the information gathered, it is possible to estimate the consequences of laying if the conditions are incorrect and provide evidence to the client of these consequences as well as having written records in the case of unfair litigation.

The survey will also indicate problems with the structure and help the floor layer instruct the client or contractor to remedy these. Living conditions, site laying conditions and the moisture content of the new floor must all be compatible for the successful installation and life of a hardwood floor.

For further information see the rest of the English Timbers Technical Guide or e-mailinfo@englishtimbers.co.ukor contact English Timbers Ltd on 01377 229301.

For details on Protimeter check out their web site atwww.protimeter.comor call 01628 472 722.

Regardless of the product, always refer to individual manufacturers instructions.

For info on DPMs and screeds contact:-

  • F Ball at www.f-ball.co.ukor tel. 01538 361 633 for DPMs screedsand adhesives.
  • Tremco on 01753 691 696 for DPMs.
  • Ardex at www.ardex.co.ukor tel. 01440 714 939 for screeds.
  • Call Sika on 01707 394 444

Revised/Checked 25/11/05