Wood and moisture


The fact that wood fades and expands depending on the humidity and temperature of the air is a material property that should be taken into account when planning and performing wooden floors to ensure a good result.

In practice, it is often necessary to calculate how much a cross-dimension changes when the wood moisture changes as a result of ordinary use. As a realistic average for a number of commonly used tree species, for practical calculations it can be assumed that a change in one wood moisture percentage leads to a shrinkage in the dimension of approx. 0.15% (1.5 mm / m) radially and approx. 0.30% (3.0 mm / m) tangentially, see figure 14. For the calculation of expansion, the same percentages given for shrinkage can be used.

Fig.14. Main directions for shrinkage and expansion in wood

If the timber item is planed or cut, the two shrinkage rates can be used directly. In other cases, ie. In common practice, a mean of 0.22% (2.2 mm / m) between the two directions can be used.

FIG. 15. Wastage and expansion in wood depends on the cutting / ringing location


Wood and moisture - The importance of moisture for wooden floors


When choosing and designing wooden floors, account must be taken of the inevitable dimensional changes due to the seasonal variations in humidity.

Joints between floor boards cannot be avoided, but when choosing the floor, the size of future joints can be limited.

The word grout is used here in the sense of openings between boards or rods in the floor.

The joint size can be reduced by:

"Use narrow boards instead of wide, because the joint width follows the board width.

"Choose products with low dimensional change due to moisture.

"Controlling the climate, eg by moderate wetting in office buildings in winter to avoid drying out.

"Do not use underfloor heating or radiant heat in ceilings.

Example 1

If a lot of parquet sticks have been stored for a long time at about 20 ° C and about 78% relative humidity, they will have a moisture content of about 16%.

After laying, the parquet bars will give water to the surroundings, and the moisture content in the rods over the year will vary between approx. 13% in late summer and 6% in coldest periods in winter. The moisture content of the wood thus falls between 3 and 10%.

If the width of the bars is 65 mm and the shrinkage is 0.22% per. percent change in moisture content, the shrinkage - and thus the joint width - can be calculated as:

65 x 0.22% x 3 = 0.42 mm in late summer. 65 x 0.22% x 10 = 1.43 mm in winter.

The joints are thus relatively large, and in practice the joints will also often be of different sizes, which entails a great risk of an unsatisfactory appearance.

Note that if a 180 mm wide floor board had been used instead, the joints would instead be approx. 1.2 and 4 mm.

Example 2

A traditional pine floor has 18 boards per floor. 2 meters. The floorboards are laid without space. After some months, there are gaps between the boards of varying sizes from 0 to 5 mm.

The 17 spaces with a blade finder are measured to have a total width of 48 mm. 48 mm of 2 meters corresponds to a shrinkage of 2.4%.

With a mean crosswind of 0.22% for every one percent moisture change (2.4%: 0.22%) = 11% change in board moisture content to give a shrinkage of 2.4%. Since the actual wood moisture is 7%, the floorboards have a moisture content of approximately 18% during installation.


10-board rule


When laying solid wooden floors, care must be taken that the wooden floor is able to absorb moisture movements. This is often done by laying out the wooden floor after a 10-board measure which indicates the width 10 boards or rods are expected to have in the humid state of use.

The wettest period is usually late summer, where the average moisture content in the air can reach 65% RH with a consequent moisture content in the wood of 12 13%, however, depending on wood species etc.

If the floor is laid with a moisture content of 8% wood moisture, a small distance must therefore be ensured between the individual rods or boards when laying. The required distance can be ensured by using spacers (small spacers) between rods or boards when laying the floor.



Use of spacers


The use of spacers is a method for securing 10-board targets, provided that the wood moisture is known.



Use of spacers - Theoretical calculation of spacers - example

444 A floor must be laid out of 65.5 mm width rods with a moisture content of approx. 8%.

Assuming that the highest expected humidity in the use period will be 65% RH and the rods thereby obtain a moisture content of approx. 12%, the required distance can be calculated as follows:

(0.22% x 4) x 65.5 mm = 0.57 mm

A distance of approx. 0.5 mm to avoid pressure debris in the wood or bulging of the floor during the wettest period.

If the floor is instead of 130 mm width boards, the necessary distance becomes:

(0.22% x 4) x 130 mm = 1.1 mm.

Securing 10-board targets using spacers Remember that spacers must be removed again.


Actual calculation of spacers


Since wood is a natural product with the variations that characterize wood, it is not so easy to calculate 10-board targets for wood floors, as the calculation above expresses.

The equilibrium moisture in wood varies depending on the wood species and it fades and expands differently depending on the direction of the earrings. In addition, the dimensioning moisture load depends on where the wooden floor is laid, for example on terrain decks or storey decks.

The wood flooring supplier must therefore always be taken on board when determining 10-board dimensions and calculating the thickness of the spacers.



Moisture technical requirements for the construction site


Wood dimensions depend on the moisture content of the wood, which in turn depends on the relative humidity (RF) and temperature of the environment. As the relative humidity varies with the season, the use of the premises, etc., the dimensions of the wood will also vary. This must be taken into account when designing and laying wooden floors.

To avoid unnecessary wetting, the laying of wood floors should be done as late in the construction process as possible. Before installation, the building must be closed and dry and heat must be placed on the building.

All work that can add moisture to the building, eg masonry work and basic painting work, must be completed. The building must be in equilibrium with a normal humidity for the season ie. 35 - 65% relative humidity at about 20 ° C.

For concrete or light concrete elements, at least a couple of months of drying time must be expected. If necessary, drying must be done by moderate use of dehumidifiers.

Before laying wooden floors directly on new concrete or light concrete decks, the moisture content of the tire should be checked by measuring, see section 2 Moisture and flooring.

Insulating materials, etc. must be dry. If clusters are made of concrete or are walled, they must be hardened and dry.

If the building does not dry out properly, the relative humidity will be so high that the wood expands after laying. Hereby, the boards / rods can be pushed live apart, or they can deform each other to get pressure debris. This can result in the floor getting large joints when drying out later. If there is not enough space, the extension can cause the floor to bulge or, at worst, push the walls out.

Examination of moisture conditions in connection with the laying of wooden floors on concrete cannot normally be done by measuring relative humidity, but must be done in the substrate. The reason is that air conditioning or heavy ventilation can lower the relative humidity of the air without a sufficient reduction of the moisture in the concrete.

Should wooden floors be used under conditions where it is only necessary to secure against moderate building moisture from underlying concrete, ie. with pore moisture of 65 - 80% RH, wetting can be prevented by using a moisture barrier, eg at least 0.15 mm PE film.

The moisture barrier is laid out with at least 200 mm overlay (wallpaper or squeezed) on the damp surface before laying the floor.


Moisture technical requirements for flooring materials


Boards and parquet are usually supplied oven-dried and packaged in strong plastic foil, with a moisture content of 8 ± 2%.

Of this, 2/3 of the batch should be between 7 and 9% moisture content.

The moisture content of wooden beams, joists and bricks should, as far as possible, correspond to the moisture content with which the wooden floor is supplied. The average moisture content during laying should not exceed 12% in beams and 13% in beams. This means that the average value of 12% (13%) must be adhered to, while normally no moisture content above 14% for bedding and 15% for beams should be measured.

If it is not possible to have the flooring materials delivered with the correct moisture content, they must be delivered so early that they can reach acclimatization, ie. get in equilibrium with the temperature and humidity conditions in the room before laying. Acclimatization can even take several weeks, even during the onset.

If the wooden floor is laid with too much moisture, larger joints will come between the boards than if they were dry.


Requirements for temperature and humidity conditions during use of the floor


Wood floors are a natural product with the variations in the material that characterize wood. As the tree at the same time follows the variations in the humidity of the air over the year, there will be natural variations in the rod or board width and thus in the joint width.

The humidity in homes and similar buildings will, under normal conditions, move between 35% and 65% RH. A change in relative humidity of 30% will usually result in dimensional changes in the transverse direction of solid wood floors of 1-2%. The change in dimension will depend on floor construction, wood type, surface treatment and the time-related moisture impact.

Owners and users of buildings with wooden floors should follow the temperature and relative humidity throughout the year. Floors of wood are best at a temperature of approx. 20 ° C and a humidity between 35% and 65% RH. Especially in larger buildings with wooden floors, it is recommended that control of temperature and humidity be put into system, eg by incorporating fixed control routines into the building's operating manual, so that registration is carried out regularly.

GSO has prepared a folder in which the importance of the moisture for the wooden floor is explained in more detail. Folder - Prevention of damage to hardwood floors - can be purchased by contacting GSO.




Joints in wooden floors can be made either to absorb deformations and / or continue forces, or simply to make a significant division of the floor surface.

For smaller joints, sealants or joint profiles in rubber or plastic are used. Moisture profiles in metal are mainly used where larger movements are expected in the floor surface.

A distinction is made between expansion joints and insulation joints.

Dilation joints are used to absorb moisture and temperature-related movements between materials in the floor construction.

Insulation joints are used to separate the floor from adjacent building parts, eg columns and walls. Insulation joints can be designed as expansion joints.

In the case of floors with irregular geometry or with columns through the floor surface and with floors with large point loads, eg offices with very heavy filing cabinets, fire-resistant boxes and the like, the floor's movement possibilities must not be prevented.

If there is expansion joint in the concrete substrate, this must be continued through the wooden floor construction.

Continuation of underlying dilatation joints may, for reasons of traffic and other considerations, necessitate a slight displacement of the dilatation joint in the floating wooden floor construction to a more convenient location in the floor. This disposition must, however, take place while observing the size, geometry and expected load of the floor. The width of the joint must be at least the width of the underlying expansion joint.

The number and location of joints is determined by the structural design of the floor, the load on the floor, the expected moisture variations and the wood species.

Joints in floors should always be planned in consultation with the wooden floor supplier.

For dilatation joints, a suitable joint sealant is used, which should have a hardness of 40 - 65 ° Shore A.

If insulation joints are made along walls and columns as a replacement for skirtings, the joint width must be 20 - 25 mm. Depending on the movement of the floor, the elasticity and hardness properties of the joint material can be adapted to the actual conditions. However, it must be ensured that the joint is the weakest place in the floor.

Joints are made with a bottom stop, eg slip tape, to avoid bonding in the joint.

For the sake of compatibility between grout, wood species and surface treatment agent, the wood flooring supplier should always be advised.

For further information on joints, please refer to the Fugebranken's Cooperation and Information Council (FSO).


Floor laying conditions

42 When fully gluing wooden floors to concrete with underfloor heating, the subfloor temperature must be lowered to 18-20 ºC to prevent the adhesive's adhesiveness from being reduced. Early two days after the floor has been laid, the temperature must be raised again, but this must be done gradually over a period of 5-8 days until the maximum surface temperature is reached again.

Moisture control in concrete with underfloor heating


Moisture control in concrete subfloors with underfloor heating can, in view of the risk of damaging the floor heating system, only take place in embedded bushings, ie. bushings that are suitable for the purpose and inserted at the concrete casting and the control must then always be made at the future subfloor temperature.

In concrete substrates without underfloor heating, the residual building moisture must be a maximum of 65% RH when laying wooden floors, however up to 85% RH if a glue with moisture-inhibiting properties * is used, and the glue and wood flooring supplier has been advised.

However, the above-mentioned rules on maximum relative humidity in concrete do not apply if the subfloor is affected by floor heating, where an exact moisture limit value is difficult to determine.

(Note *) Glue with moisture-inhibiting properties (MS glue) is mentioned in the section: Selection of glue


Example of shrinkage in wood floors:


In a pine floor of 100 mm wide boards, the moisture content fluctuates between approx. 6% wood moisture in winter and approx. 13% in summer.

The boards are assumed to be quite close during the wettest period (summer).

When the floor in the winter is most dry, there will be a joint of (13 - 6) x 0.22% of 100 mm = 1.5 mm.

If floor heating is used where the temperature in the wooden floor increases on average from 21 to 30 ° C, the moisture content in the winter will fall further to approx. 4% wood moisture, and the joint width will thereby increase to (13 - 4) x 0.22% of 100 mm = 2 mm.


Missing Table 1. Overview of the use of flooring materials for different floor types. The colored fields indicate that the product can be used for the purpose mentioned. The blank fields indicate that the product cannot normally be used for this purpose.

Missing Table 2. Beam and beam distances counted from the middle to the middle. The distances ensure floors without annoying deflections or vibrations. For thicknesses that lie between the stated, the distances are used for the nearest smaller dimension .

Missing Table 3. Floor clearance gates L. The cargo distances are for homes, offices and light businesses calculated for a point load of 3 kN and a surface load of 2 kN / m2. For assembly rooms and shops, they are calculated for point loads, which, depending on the type of premises, are 3, 4 or 5 kN and a surface load of 4 kN / m2. The deflection must be less than 2.5 mm and, in addition, it must be less than L / 500 for surface loading and L / 200 for point load. The balancing distances in the table apply to spreading distances up to 950 mm. The first column (L Max) indicates the distances in the ordinary subjects (normal subjects). The second column (L End) indicates the distances of the bedding, ie. by wall or where two joists are knocked together. Blocking distances for floor joists are under review by, among other things. TOP and SBI. It is therefore expected that, in the near future, changes will be made to the reported distance between the blocks.

Deficiencies Figure 16. Expansion of joists. Note the extra litter along the walls and the alternately staggered blockage under the joists.

Figure 17. Blocking of struts with plastic wedges.

Figure 18. Blocking of joists with wooden pieces and possibly. roofing felt. If the cleavage is done on soft pieces for sound reasons, do not sew the soft part of the piece.

Figure 19. If cut into joints, eg for pipes, the joists must be supported on both sides of the cut.

Figure 20. Extra support must always be carried out, where joists are impacted or assembled with joints.

Table 4. Guiding dimensions of nails, staples and screws for load-bearing floor slabs on beams and beams. The screw dimensions can also be used when attaching floorboards to subfloor of chipboard, boards and the like. Hot-dip galvanized nails provide the best adhesion in wood. Be careful not to use longer nails, staples or screws than those listed here, as this may cause problems with creaking in shrinkage and swelling in joists and beams. For direct mounting on concrete, special screws, eg Monta-flex, can be used. Always consult the nail, staple or screw supplier