Fixings & Installation for Timber Cladding
An overview of the requirements to install timber cladding on your project.
How to Install Timber Cladding
If you are unsure on how to install, please either get in touch for advice, or have your timber cladding fitted by a professional installer.
Prior to installation ensure the cladding profile you have purchased is suitable for your intended application. Some profiles are specifically recommended to be used either vertically or horizontally. Various profiles can be used in either orientation.
When installing horizontal boarding, start at the base of the elevation, with the tongue facing up, and securely fix this starter piece – this will ensure moisture runs down the boards and cannot sit in the groove. Using an appropriate spacer, install subsequent boards on the elevation, checking the expansion gaps are not missed, and that boards are being installed level.
With vertical boards, the same process should be followed, however there is the freedom to work from either side of the elevation to the other.
Choosing the Right Timber Battens
Timber cladding is traditionally installed either on UC3 preservative treated softwood battens (Use Class 3 level of protection (ref. BS 8417) to ensure adequate durability.)
Treated softwood battens are most commonly used. However, due to dimensional movement we advise thermally modified Redwood battens for enhanced stability, ensuring easier and faster installation. We advise using Thermowood battens on all timber but especially when using quick grown species. Should you use a standard treated batten, which is prone to movement, issues can occur. If the battens move dimensionally one way, the cladding will want to pull in the other direction. This can lead to the cladding buckling on the façade and pulling fixings out.
Batten Installation and Specifications
In vertical applications, when using a cross batten system, chamfered horizontal profiles may be required as part of the batten system. This will help with water run off and can also be used signally, depending on the profile, when cross battens are not always required. However, this is subject to profile of the cladding specified.
Commonly used batten sizes are Ex 38 x 50, or 47 x 50mm, ensuring they have been regularised to provide uniformity along the length.
Thermowood D Battens do not require preservative treatment.
Click to download information on timber battens.
Batten centres are typically between 400 – 600mm for most applications. When installing vertical cladding, there is likely to be a requirement for a batten and counter batten to ensure airflow can enter the cavity and moisture can exit through the base of the cavity. Chamfered horizontal battens will help to encourage excess moisture to leave the void.
| Minimum Batten Sizes mm (w x t) | |
| Vertical batten | |
| with allowance for side jointing* | 50×38 |
| no allowance for side jointing* | 50×25 |
| Horizontal batten | 50×38 |
| Counter batten | 50×25 |
*where a short batten length is fixed alongside a batten to enable boards to be butt jointed.
Fixings For Timber Cladding
Overview
It is vital to make sure that each board is independently fixed. In the case of any movement, the boards are then able to expand and contract into the gaps left between them.
Depending on species and board size, the recommendation to either face fix or secret fix can differ. If face fixing, allow 2 face fixings at quarter points per board, per batten centre. Ensure that the fixing sits flush with the surface of the board. Overdriven fixings may lead to moisture ingress in the timber and should be filled with a suitable external grade filler.
Nail or screw dimensions should be relative to the thickness of the board you are installing and be able to adequately secure the cladding. Annular ring shank fixings are recommended for treated softwood cladding:
Diameter: Recommended minimum 2.3mm.
Material: Stainless Steel Grade 304
Length: For a regular 20mm cladding board, approx. 45mm fixing
Positioning: Quarter points, 20mm from ends of boards to reduce splitting, on dense species, pre-drill where necessary.Do not use a lost head fixing or panel pins as the opportunity for movement and for the boards to come lose is high.
Battens & Counter Batten Fixings
Where it is necessary to fix cladding battens to counter battens, use nails or screws as below. Horizontal batten spacing is wind load dependent. Pay attention to the closer spacing required.
Design wind load (kN/m²) Below 1.5 1.5 to 2.5 Nails Diameter (mm) 3.0 to 3.8 – Length (mm) 75 to 120 – Batten centres (mm) 200 – Screws Diameter (mm) 6 6 Length (mm) 110 to 180 110 to 180 Batten centres (mm) 400 300 Fixing Back to Primary Structure
The following tables give guidance for timber cladding assemblies carried out on a timber framed or mass timber backing wall. If the backing wall is masonry, connections between the cladding support battens and backing wall need to be designed by a structural engineer.
Fastener sizes for connecting vertical battens or counter battens to the backing wall (for horizontal boards or vertical boards with flat rear face).
Design wind load (kN/m²) Below 1.5 1.5 to 2.5 Nails Diameter (mm) 2.8 to 3.8 3.4 to 3.8 Length (mm) 75 to 120 90 to 120 Batten centres (mm) 150 150 Screws Diameter (mm) 5 5 Length (mm) 90 to 120 90 to 120 Batten centres (mm) 200 150 Fastener sizes for connecting horizontal battens to the backing wall (for vertical board-on-board timber cladding)
Design wind load (kN/m²) All wind loads below 2.5 Nails Diameter (mm) 3.8 Length (mm) 100 to 120 Batten centres (mm) 600 Screws Diameter (mm) 5 Length (mm) 90 to 120 Batten centres (mm) 600
Protecting Timber Cladding from Moisture
When installing any specie or profile, it’s imperative to give the cladding the best chance of performing to its maximum capability. Therefore, we advise all cut ends are end grain sealed with an end sealer to reduce and slow down water ingress via the ends of the timber. This process will help reduce the chances of the timber “bulking” in the wetter months. When left exposed, timber absorbs moisture up to 250 times more rapidly than other wood surfaces. Unprotected end grain can also lead to interior rot and discolouration adjacent to the cut end.
As with most façade materials, water running down the face of the board can lead to dirt and contaminants being left. It is therefore best practice to ensure the design and detailing on the building, in particular the flashings and cills, will direct water away from the building.
When installing vertical cladding or fins we advise that all timber components have chamfered tops and bottoms. This will allow water to run off rather than sitting on the ends of the timber, causing water ingress. Depending on the specie, timber will hydrocephaly absorb moisture, which can cause dimensional movement known as swelling or bulking. Chamfered ends should direct the water back to the façade’s ventilation path. Should the angles direct the water forward, staining may become visible on the cut area. Cut ends should always be end sealed.
Installation Guidelines and Expansion Gaps
Ensure the base of your cladding detail is installed a minimum of 200mm above impermeable ground. This will limit splashing and the potential for staining that this may lead to. Keep the façade free of vegetation. Not only can this be detrimental for the long term performance of the timber, there is also the increased risk of staining on the façade.
When fixing boards end to end, a 2-3mm expansion gap will allow the boards to move as they take on and lose moisture. The same 3mm expansion gap should be left when installing boards. Ensuring a spacer is used to provide that 3mm gap, do not tightly push the tongue into the groove of the next board. Similarly, around the external perimeter of your timber cladding, if you leave 8-10mm to other materials, this gap will allow boards to move. The shadow created will also provide a crisp visual along the edge.
Check with your architect for project specific detailing as these guidelines are intended to offer an overview.
Page Reference – TTF The Timber Cladding Handbook for specifiers, buyers and installers.