Wood floors follow moisture and temperature
When choosing and designing wooden floors, account should be taken of the dimensional changes that will naturally occur in the floor surface due to seasonal variations in humidity and temperature. Similarly, the increasing dimensional changes (especially shrinkage) that occur when using underfloor heating and ventilation systems should be taken into account.
The guide describes the natural variations that can occur in wooden floors depending on the structure and type of wood.
The moisture content of wooden floors will always seek to adjust to equilibrium with the air's relative humidity (RF) and temperature. The wood absorbs and releases moisture by changes in relative humidity. It occurs in all wooden floors, but the movement of the wood floor in the form of shrinkage and expansion (swelling) depends on the product type, wood type, thickness and surface treatment. The largest movements are usually seen in floors of solid wood, while shrinkage and swelling in lamella and veneer floors are usually considerably smaller.
Floors must be laid in an indoor climate where the relative humidity (RF) is 30-65% and at a room temperature of approx. 20 ° C. Most floors are therefore produced with a moisture content of 8 ± 2%, and thus optimized for the normal Danish indoor climate and will 'be best' when the humidity is around 50% RH.
The relative humidity varies over the year, and is typically highest in late summer. Here, the moisture content of the wood and the corresponding dimension will be greatest, and the floor will be roughly free of moisture. In contrast to the winter period, when the relative humidity is lowest, there will be joints between floorboards and parquet bars.
Wooden floors are moving
How much a floor moves (fades and swells) due to variations in air humidity depends on the type of wood selected, the thickness of the floor and the type of floor.
Wood species behave differently
Generally, heavy tree species will shrink and expand most, while lighter tree species have less shrinkage and expansion when the air humidity changes from summer to winter.
Among the tree species that have the least shrinkage and expansion are maple, teak, iroko, merbau and doussié. Wood species with large shrinkage are e.g. beech, keruing and ipé.
Wood species with moderate shrinkage and expansion are, for example. pine, douglas, oak, ash, bilinga, walnut, press dried beech and partly jatoba.
The thickness of the wooden floor
The thickness of the wooden floor also plays a role in shrinkage and expansion. The thicker a floor, the slower it reacts to the variations in humidity.
A 22 mm wooden floor will be considerably longer to react to moisture variations in the air than a 12 or 14 mm floor.
If the floor is also of a heavy wood species, it will take even longer before it reacts to changes in humidity.
The direction of the wood also plays a role in moisture absorption. Side wood absorbs moisture more slowly than end wood. Therefore, flooring floors, and especially thin flooring floors, will respond more rapidly to changes in air moisture content than normal thin wood floors.
Floor shrinkage and expansion
There is a difference between how floors react to changing humidity during the year. It depends on how the wooden floor is constructed.
Slat and veneer floors , which are laid floating by means of click systems or by gluing the boards together, form a continuous floor surface virtually without joints, because the boards are tightly bonded together. The movements that come into the floor surface due to fluctuations in the air humidity will primarily be transverse movements of the entire floor surface. The total movement (shrinkage and expansion) must therefore be taken along the walls below the skirting boards.
During the heating season there may be minor joints in click floors between the individual floorboards, the joining profiles may seem loose and joints may appear along the walls.
The glued floors will only have joints along the walls, which in large spaces may possibly be seen as joints along the skirtings on the long sides.
In combination with floor heating, both joints and cross movements (the total shrinkage across) will grow further.
Slat floors that are glued to the substrate will usually be free of joints at the time of installation.
During the heating season, the individual slat boards will shrink and usually leave uniform joints between the individual boards. The joint size will depend on the type of product and the wood species. In combination with floor heating, the joints will become clearer.
Slat floors (min. 22 mm), which are stitched to joists or joists, are usually mounted tightly together without joints between the boards. Along the walls, a small distance joint is established to accommodate any extensions primarily across the floor surface.
During the heating season there will be joints between the individual floorboards. For narrow boards, the joints will be small, but the wider boards used, the wider the joints become when the wood during the winter period has the lowest moisture content. Height differences between boards can occur. In combination with floor heating, the joints will generally become larger.
Solid pine floors (min. 20 mm) , which are stitched to joists or joists, are usually mounted tightly together without joints between the boards and without the use of 10-board dimensions. Along the walls, a small distance joint is established to accommodate any extensions primarily across the floor surface.
During the heating season there will be joints between the individual floorboards. For narrow boards, the joints will be small, but the wider boards used, the wider the joints become when the wood during the winter period has the lowest moisture content. With completely narrow boards <75 mm, the joints will be insignificant. In combination with floor heating, the joints will generally become larger.
Solid hardwood floors , which are floating with a clamp system, will usually have smaller joints between the boards at the time of assembly, depending on the chosen hinge dimension.
During the summer, rising humidity will close any joints between the boards. The wetting will possibly cause a transverse expansion of the floor surface, which is usually occupied by the spacer joints long walls.
During the heating season, the individual floorboard will shrink and leave uniform joints between the boards. The size depends on the product type and the wood species. In combination with floor heating, the joints will generally become larger.
Solid hardwood floors (min. 20 mm), which are stitched to joists or joists, are usually mounted after 10-board dimensions, indicating the width 10 boards are expected to have in the humid state of use. The 10-board target leaves smaller joints between the boards at the time of laying.
In late summer, the floor will appear roughly free of moisture, while during the heating season there will be joints whose size depends on the product type and type of wood. In combination with floor heating, the joints will generally become larger.
Not all suppliers recommend laying after 10-board dimensions and use in connection with floor heating
Solid wood parquet floors of smaller rods (often laid in pattern), which are glued or stitched to the substrate, will normally be free of joints at the time of laying. Rods that are laid after 10-board dimensions (eg with spacers) can have smaller joints between the rods when laying, so that there is room for expansion during later wetting.
During the heating season, the individual rods will shrink and leave virtually uniform joints between the individual rods. The size will depend on the type of product and the wood species. In combination with floor heating, the joints will become clearer.
Practical rules for shrinkage
In practice, there is often a need to know how much a floorboard or an entire floor fades or expands.
If there is a need to calculate precisely the shrinkage and expansion of the individual tree species, mean width and thickness wind can be found in tables in the handbook TRÆ 70 Wood Materials.
As a realistic average for a number of commonly used solid wood floors of e.g. pine, beech, oak, ash, bilinga, iroko and walnut, can in practice be counted with a mean number of 2.2 mm per. meters for a 1% change in the moisture of the wood.
During the year, the humidity in normal housing, offices, etc. will increase. change 20-25% RH corresponding to a change in wood moisture of 4-5% - but often less. A floor board that is 120 mm wide will thus vary approx. 1 mm in width.
For lamella floors, in practice it can be assumed that the shrinkage will be between one-fifth and one-tenth of the corresponding shrinkage for solid wood floors. For a floating floor that is 6 m in width, this means that there should be space along both walls to be able to absorb a width change of 10-12 mm.
If there is underfloor heating under the wooden floor, the shrinkage will be increased by 15-25%.