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Tongue & Groove

What are the advantages of T+G profiles compared to straight edge OSB boards?

In case of sheathing with straight-edge panels, an expansion gap of 2-3mm must always be left between the adjacent boards (on all 4 sides) when fixing them to the supports.

If working with T&G milled-edge boards (asymmetrical 2 or 4 sides), this issue is no longer a problem. The T&G profile already has a 1mm dilatation gap incorporated, so this saves time as you can skip checking the correct alignment.

Secondly, when using straight-edge boards for floor decking or wall sheathing, the panels must be supported on all 4 edges, in order to reduce deflection and improve the stiffness. This means the long edge of the boards must be fixed on additional supports (noggins).

In case of roof sheathing, the noggins must be replaced by H-clips to reduce deflection between adjacent panels, and additional bracing stripes must be provided for stiffening the structure. This results in additional labor and material costs, which can be eliminated by using T&G boards instead, which require only their short edges to lay on supports.

Moisture Behaviour & Testing

How OSB behaves when exposed to moisture?

As any other wood product, OSB exhibits mild dimensional changes (shrinkage or dilatation in length/width) when exposed to changes in moisture and temperature of the surrounding environment.
This effect is covered in the design of the structure by using sufficient expansion gaps. In order to limit the dimensional change effects, different technical classes (types of boards) are available, so the right board must always be chosen depending on the conditions of each application (external wall, roof).

As a general rule, OSB must always be protected against direct contact with water (rain, snow, infiltrations) during all stages of its intended use – from warehouse to job-site storage, transportation, installation and daily use within the building lifetime.

In the particular case of concrete screeds casted on OSB subfloors, it is mandatory to cover the OSB flooring panels with a waterproof membrane in order to avoid the moisture uptake caused by this wet building process.

What are the negative effects of excessive moisture upon OSB?

Excessive moisture generally favours the creation and spread of fungus and mould inside the building envelope or on the surface of building components and their connections when ventilation is poor (room corners, wall-to-ceiling edge, balconies, etc), but most important is that a moisture content > 18% reduces the load-bearing capacity of the wood structural elements.

The following effects are associated with high moisture in the building elements:

  • greater deformation caused by swelling of wood and wood-based materials
  • higher pull-out (withdrawal) stress on fasteners (nails / screws / staples)
  • reduced load-bearing capacity, and thereby increased deformation under static loads
  • reduced efficiency of the embedded thermal insulation (mineral wool, hemp, blown cellulose)

What is to be done if an unexpected rain is wetting the OSB boards already installed as structural flooring, partition walls or roof sheathing?

The builder is fully liable for any damage caused by improper protection of the building elements against rain during all stages of construction process.
Anticipation and effective preventive measures for avoiding such unpleasant situations is therefore mandatory.

However, when such situations occur, the exposed panels and wood components (open beams, studs and rafters) must be covered immediately in the best possible way to limit their soaking, and the water puddles must be drained away as soon as possible by suitable measures.

The protection must be removed only when rain has stopped and no other precipitations are forecasted, so the surface can dry out again. Before proceeding with further installation, loosen strands should be removed by sanding.

For how long could OSB boards stay uncovered (exposed), without the risk of altering their stiffness and bending potential?

It depends on the climatic conditions that are specific to each country, on the type of boards used, and of course, on the duration and intensity of any unexpected rain, that can range from episodic shower to unstopped rainy days.

From our experience gained by weathering tests (over a 2 month period), depending on the moisture uptake, the OSB 3 and OSB 4 TOP can lose about 20-25% in strength and stiffness when re-dried to moisture content < 18%, which is the limit value for service class 2 panels (humid conditions).

As long as the boards do not exceed 18% in moisture content, it can be assumed that there is no critical effect on the structural use.
Considering the above, a maximum of 8 weeks open construction (exposure to weathering) shall not be exceeded during installation, presuming that only episodic rain showers take place.

For how long should carpenters wait for the installed OSB panels to dry-out (if wetted by rain) before proceeding further with installation?

From our own experience, panels that have been wet by a normal rain shower (a few hours) must be left to dry-out at least one full day in summer and 2-3 days in autumn, provided that day (days) is/are warm and shiny, so that it is not very humid.

Fire behaviour

How do timber frame houses behave in case of a fire?

Wood and wood-based panels are combustible materials.

Used properly in suitable combination with non-combustible materials, they can provide sufficient protection to the structural elements (columns, beams, floors) of the house, thus allowing these elements to retain their load-bearing capacity for a longer period of time.

Many studies have been conducted on improving the fire regulations regarding the structural fire design of buildings, allowing wood to be safely used as structural component in single-family houses, and also in the erection of office and multi-storey apartment buildings.

EGGER has very good experience and can provide tested and practical solutions for walls and floors to safely sustain fire to 30-90 minutes. From our own experience, fire resistance up to 45 minutes can be achieved by using only combustible materials, while for higher fire requirements it is necessary to combine wood-based panels with other non-combustible materials (stonewool, fire-resistant plasterboards, gypsum fibreboards, surface coating with fireproofing agents, etc).

More information regarding fire tested solutions for walls, roofs and floors can be found on EGGER’s website and on, to which EGGER contributed to the research program. 

What is the fire behaviour of OSB?

The standard EN 13501-1 establishes seven reaction to fire classes of building materials - also known as Euro-classes: A1, A2, B, C, D, E and F, of which A1 & A2 are incombustible.

Standard wood-based panels (including OSB) belong to Euro-classes D and E. The reaction of OSB to fire is:

OSB 2 ( ρ ≥ 580 kg/m3):

  • Class E (for thickness t ≤ 12mm)
  • Class D-s1, do (for t > 12mm)

OSB 3 ( ρ ≥ 600 kg/m3):

  • Class E (for t ≤ 8mm)
  • Class D-s2, do (for t ≥ 9mm, CWFT)

OSB 4 TOP ( ρ ≥ 600 kg/m3)

  • Class D-s2, do (CWFT)

What does CWFT mean and when does it apply?

Classification Without Further Testing (CWFT) is a procedure established for standardised products, based on extensive research supported by the wood-based panels industry.
It can be found in Table 8 from EN 13986.

Essentially, it is a method used for the easy identification of the reaction to the fire class (Euroclass) of wood-based panels according to EN 13501-1, and applies to boards of minimal thickness 9mm and densities ρ ≥ 600 kg/m3, when referring to OSB.

For a manufacturer, this means that as long as the product complies with the requirement values given in Table 8, additional fire testing within the scope of FPC need to be performed.

However, for OSB boards of thicknesses lower than 9mm and densities under 600 Kg/m3 (ex: OSB 2, having declared density ρ ≥ 580 kg/m3), CWFT is not applicable, so the panels must be tested by the manufacturer, who is obliged to declare the values and to mark the Euroclass on the panel or on the package.

What does "s" or "d" mean in the reaction to fire classification?

“s” stands for smoke release, and “d” for burning droplets potential of the product.

There are 3 smoke classes (s1, s2, s3) and 3 burning droplets classes (do, d1, d2) assessed.

The OSB boards classified as CWFT (t ≥ 9mm and ρ ≥ 600 kg/m3) belong to Euroclass D-s2, do, meaning: normal combustible (D) – medium smoke release (s2) – no burning droplets (do).

The reason why the smoke release potential of building products is given so much importance is because smoke and gases produced by combustion are the main killers in a fire.

On the other hand, the potential of a building material to burn with droplets is also important, as they can facilitate the rapid spread of fire to other materials or different sections of the building (typically, from one storey to another, through the façade).

The meaning of burning droplets is:

  • do = droplets formed by burn are self-extinguishing within 10 seconds from ignition
  • d1 = droplets are self-extinguishing within 10 seconds in the first 10 minutes
  • d2 = the material is burning with droplets

What is the meaning of each Euroclass?

The harmonized European standard EN 13501-1 assesses the reaction to fire of building products. These tests differ from national classification and assess building materials in terms of their combustibility, either as incombustible and combustible (with different segmentation, ranging from hardly combustible to heavy combustible & highly flammable).

Can the difference between E-class and D-class be proved by a job-site testing?

No, that is impossible.
Fire testing of building materials is strictly regulated by standard procedures and requires laboratory conditions that cannot be provided empirically on the job-site.

How is it possible that OSB 2 of thickness ≥ 15mm has better fire behaviour than OSB 3 for the same thickness range?

This comes from the different procedure used when the products have been tested and classified:

OSB 3 of thickness ≥ 9mm and density ≥ 600 kg/m3 meets the requirement criteria used for CWFT, so therefore they were classified directly as “D-s2, do” by using Table 8 from EN 13986. The values in Table 8 are the result of an intensive EN research program conducted by the Swedish Fire Testing Institute “SP BORAS” in 2004, which performed on the behalf of the CEN Committee a cross-testing of many boards, selected from different wood-based panel manufacturers.

The minimal achieved results were documented as representative for each product type and accepted by the whole wood industry ever since.

The classification “D-s1, do” of OSB 2 for board thickness > 12mm and density ≥ 580 kg/m3 was gained and declared by EGGER’s own testing.

In 2004 when the test was done, it was sufficient to test the boards tightly fixed to an A2 substrate (mineral wool), so there wasn’t much oxygen accelerating the burning and smoke release.

This is why the smoke release rate was “s1” (low smoke release), which is better than “s2” (normal smoke release).
However, for thicknesses ≤ 12mm, the testing result lead to an “E-class” classification for OSB 2, and that was the same situation with OSB 3 of thickness range 6-8mm.

Fungus / Decay / Mould

Does OSB encourage the formation and spread of fungus or mould?

Being an engineered panel manufactured under high pressure and temperature conditions (around 1500°C), the wood strands within OSB are totally inert and no harmful wood pests can survive the process.

If correctly installed in diffusion open cladding systems and sufficient ventilated roofing systems, provided the building is constantly heated, its external walls are sufficiently thermally insulated and the rooms are naturally aerated on regular basis, mould formation should not occur.

Can OSB be affected by termites or other insects decay?

Yes, it can.
To withstand termite attack, OSB has to be treated with special chemical preservation additives.

EGGER can offer termite treated OSB, which is produced successfully for Australian market since 2005.
The boards carry the New South Wales brand certificate for H2 treatment.

According to EN 335 (wood preservation), OSB is not endangered by insects in the usage class 1 & 2.
No preservative treatment against insects decay (except termites) by chemical additives is required.


What is the density of OSB boards?

The product standard for OSB (EN 300) does not provide any requirement value for density.

The densities of OSB can be found in the product data-sheets.

Their range: OSB 2 ≥ 580 kg/m3, OSB 3 ≥ 600 kg/m3, OSB 4 TOP = 600-640 kg/m3

Is the density of the OSB variable with the thickness of the boards?

Yes. The smaller the thickness, the higher the density, provided however that the minimum density for each board type complies with the product data-sheet.

What is the benefit of a higher density OSB board?

Density is a property which is related to many other properties of OSB, such as:

  • Strength and stiffness
  • Embedding strength of fasteners
  • Fire resistance
  • Air permeability
  • Water vapour diffusion resistance
  • Sound insulation

All these properties improve with higher density.


Can the formaldehyde (HCHO) contained in the boards affect human health?

Solid wood naturally contains a small amount of formaldehyde, in average < 0,03ppm.

Low-content formaldehyde boards (E1 class, as defined by EN 300), are not hazardous products for human health.

The formaldehyde content of such panels is limited to maximum 8mg/100g of oven-dry board (single value). Additionally, FPC statistics have to prove that the rolling half-year average of the perforator value (EN 120) does not exceed 6,5mg/100g.

If the chamber method is used for the determination of the formaldehyde content (according to EN 717-1), then the steady state emission value of formaldehyde must be ≤ 0,124mg/m3 of surrounding air (or < 0,1ppm). Boards with formaldehyde contents that exceed these values belong to formaldehyde class E2.

EGGER has committed voluntarily to deliver only E1 boards to the market.

What does formaldehyde-free bonded OSB mean?

Formaldehyde-free bonded OSB panels are produced using resins that do not add any formaldehyde to the product, such as PMDI (polymeric diphenylmethane diisocyanate).

Although the existing product standard EN 13986 does not officially recognize such a formaldehyde class and refers only to E1 and E2 classes, most manufacturers are "marketing" their formaldehyde-free bonded OSB panels under own brands (ex: E0, TOP, ECO, F****), having the formaldehyde content < 0,03ppm acc. EN 717-1, that complies with the natural formaldehyde content of solid wood.


Is it true that the floors of timber frame houses usually squeak?

No, squeaking can be easily avoided, following the given recommendations:

  • Correctly choosing the boards, regarding type and thickness
  • Properly conditioning the boards prior to installation
  • Using the recommended type of fasteners
  • Providing sufficient expansion gaps
  • Choosing mainly T&G panels with glued edges
  • Using kiln-dried wood for the supporting beams (u . 20%)
  • Using beams with the correct cross-sections and spanning resulted from the static design

What causes a squeaking noise?

Squeaking is usually generated by:

  • The stress-strain of fasteners produced by the swelling (increase in thickness) of the boards due to a rise in their moisture content.
  • Insufficient expansion gaps left between the edges of adjacent panels when fixing them on the supports. This can lead to friction between the panels as a result of board expansion in length and width, associated with the increase in moisture content.