General Design

The general guidance and schematic details presented in Sections 4 and 5 (General Design and Installation) of this manual have been prepared in accordance with best practice guidelines recommended by TRADA Technology and are specific to Finnforest ThermoWood®.

The purpose of ThermoWood® cladding is to provide a waterproof design which gives long service life and low maintenance costs.These instructions are offered as general recommendations.They do not provide structural design.The compatibility of these guidelines to the specific structure should be verified on case by case basis. In short the basic principles for the installation of ThermoWood® cladding may be summarised below.

1. Take into account the surrounding buildings, local vegetation and ground as well as the territorial claims when designing the cladding.
2. Always incorporate a drained and ventilated cavity between the ThermoWood® cladding and external walls whether they are of timber frame or masonry construction.This will allow airflow and moisture management behind the ThermoWood® cladding and will reduce the potential for mould growth on the inside faces of external walls.
3. The drained and ventilated cavity should extend from the lower edge to the upper edge of the wall and be open from both edges to allow vertical circulation of air (See figures 9 and 10).
4. Make sure that water flows away from the cladding boards and structure and that sufficient ventilation is provided to allow the cladding boards to dry.
5. Chamfer all horizontal surfaces.
6. Make sure that appropriate flashings are applied at intersections between cladding boards, windows and doors, horizontal trim and at roof/cladding intersections. Flashings should be installed so that they direct water away from the cladding boards.
7. Install gutters so that water flows away from the cladding boards and structure.
8. Ensure that the cladding and structure is protected from indirect wetting, as a result of splashing off the ground, by stopping any wood cladding 200mm above ground level or abutting roofs to prevent this form of wetting.
9. Do not plant bushes or other plants in direct contact with the cladding.
10. Always seal the end grains of cladding boards when installed. Especially if cladding boards are to be finished with a low build stain or opaque paint system. End sealing reduces the potential of any moisture ingress.

It is important to avoid direct contact between ThermoWood® and porous or wetted non porous surfaces.This is particularly important where the end grain is exposed because it is very absorbent. End grain protection can be provided by either a damp proof membrane or flashing.Vertical boards should always be kept clear of any flashings below by at least 10mm and their performance can be enhanced by chamfering the bottom edges which improves the ability of the board to discharge rain
water to the ground. Also have the top edges well protected. Horizontal boards should stop short of any vertical members by 8mm to allow ventilation to the end grain of the surfaces. Butt joints between cladding boards are installed without a gap.

Battens
For the purpose of allowing efficient ventilation behind the ThermoWood® cladding, the cavity behind the cladding should not be less than 19mm wide.The width of the ventilation gap is determined by the size of battens necessary to fix the boards.

The following rules of thumb regarding the batten sizes should be followed:

• When using improved nails (e.g. annular ring shank) a batten twice the thickness of the board is adequate. Vertical ThermoWood® battens of thickness 21mm x 92mm are suitable if fixed to timber frame walls and are coincident with wall studs.
• When using standard nails the battens should be at least 2.5 times the thickness of the boards to be fixed.Vertical ThermoWood® battens of thickness 21mm are suitable if fixed to timber frame walls and are coincident with wall studs.
• If 42 x 42mm battens are used, it will be necessary to double batten the position where cladding boards meet i.e. produce an 84mm surface to fix to.

Figure 3. Where joints occur the batten width should be a minimum of 84mm. Nail fixing should be at least 25mm from the ends and edges of the boards. Boards under 120mm can be fixed with one nail. The nail should be placed to the lower part of the board.

When timber cladding is exposed on elevations of taller buildings or when battens are not fixed coincident with timber studs or when timber cladding is fixed to masonry building increased thickness of battens is recommended.This ensures that cladding is fixed robustly to the building.

• When using standard nails the batten of thickness 50mm is suitable.
• When using improved nails (e.g. annular ring shank) the batten should be a minimum thickness of 42mm.

Timber frame walls
When designing horizontal ThermoWood® cladding, the battens must be coincident with the timber studs and these are usually at 600mm centres.

Figure 4. The battens must be coincident with studs when fixing to timber frame wall.

Masonry walls
To avoid driving nails into masonry walls battens should be thick enough to provide the necessary penetration of the fixing.

Figure 5. Battens should be thick enough to avoid driving the board fixing nails into masonry.

Maintaining a drained and ventilated cavity
If vertical boards are fixed to horizontal battens it will be necessary to introduce vertical counter battens behind the horizontal battens to provide drainage and air circulation. If the horizontal battens are to be fixed to counter battens, the counter battens must be of sufficient thickness and follow the guidance detailed above. If board-on-board cladding is used it is not necessary to
provide counter-battens. In this case it is preferable if the horizontal battens are chamfered on the top edge to shed any water outwards.
Note – the battens must be 2 or 2.5 times the board thickness (depending whether improved or smooth shank nails are used) unless the battens are coincident with timber studs.

To avoid infestation by insects any openings at the top or bottom of close jointed boards should be protected by an insect mesh.

Openings
Most areas of timber cladding will either contain openings for windows or doors, or be contained within a dimensional framework determined by areas of glazing or a regular pattern of windows. Ideally any openings should be in multiples of the chosen board width to avoid the need to notch or split boards. Having to notch or split boards around openings can lead to poor appearance. It is also important to consider the flashing, sills and dpc’s around any openings to ensure that any water is
drained away to the outside of the wall.

Corner details
The same corner details can be followed when installing cladding to a masonry wall. Always use thicker battens 42/50mm when fixing cladding to a masonry wall (See figure 5).

External corners for horizontal cladding - timber frame wall

Figure 6. Outer corners for horizontal cladding.
Battens at max. 600mm centres.
Leave 8mm expansion gap between end of cladding board and corner trim.

Internal corners for horizontal cladding - timber frame wall

Figure 7. Internal corners for horizontal cladding.

External and internal corners for vertical cladding - timber frame wall

Figure 8. External and Internal corner detail for vertical cladding. Showing batten and trim detail using 21x92mm and 42x42mm ThermoWood

Upper edge and lower edge details
Recommendation for upper edge and lower edge details are presented in figures 9 and 10.Take into account the surroundings, local vegetation, ground and climate as well as the territorial requirements and conditions when designing the details.

	Figure 9. Upper edge detail showing ventilation gap above top board.
	Figure 10. Lower edge detail showing 200mm gap between lower edge and the ground. Note the chamfered bottom edge allowing water run off. The drained and ventilated cavity should be open from both lower and upper edges to allow vertical circulation of air.