Cladding Fixings

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  • Nail and screw length shall be at least 2.5 times the thickness of the board
  • Timber cladding shall have a moisture content of 16% ± 4% at time of installation (unless using green oak)
  • On dense species, pre-drill all fixing points 2mm oversize
  • For Green Oak, prepare fixing points 4mm oversize and use fixings with washers or large heads

 

 

Heat treated Platowood Frake hardwood. Fixing with large headed stainless steel screws into 2mm oversize holes allows for any movement.  Photo: Howarth Timber.

Screws or nails?

Normal practice is to install timber cladding using face fixed nails or screws. For most cladding softwoods, small headed ring shank nails are used but for Western Red Cedar larger heads are recommended to avoid the potential for nails being pulled through this low density species.

On all hardwoods and some higher density softwoods e.g. Siberian Larch, Douglas fir the use of screws is recommended. For all dense species, the pre-drilling of pilot holes 2mm oversize should be carried out to allow for any movement in the board that may occur after installation. On green oak, it is important to use oversize (4mm minimum) fixing points with washers or fixings with larger heads to take up the shrinkage that will occur.

Pneumatic fixing guns should be used with care to ensure the surface is not affected by impact damage or the fixings driven below the surface. Many fixings supplied for gun application come with off centre D shaped heads and designers should satisfy themselves that this is aesthetically acceptable. For screw fixing, hand held “impact” type screw drivers are best as these are less likely to damage the wood or the drive recess and will insert the screw at optimum speed and prevent the coating burn associated with high speed power drivers.

All fixings should finish flush with the surface and should not be punched or countersunk because this can result in splits and surface staining. As such it is important to ensure that the cladding moisture content is around 16% at time of installation to avoid shrinkage that would leave fixings proud of the surface.

Where concealed fixing is required then boards may be back fixed to counter battens or installed using a proprietary metal fastening bracket or support system. Concealed fixing systems are best used in the prefabrication of cladding panels.

Metal types

Suitable metals

  • Stainless steel-Austenitic grade
  • Silicone bronze
  • High performance coated steel
  • Hot dipped galvanised (BS7371:6 min) cooper

Unsuitable metals

  • Aluminum
  • Electro plated steel
  • Brass

All metal fixings – nails, screws, concealed support systems, bolts and accessories shall be made from corrosion resistant materials. Austenitic stainless steel or corrosion resistant coated components shall be used throughout whether the surface is to be finished with a coating or left unfinished to weather.

The use of dissimilar metals on the same fixing point should be avoided to minimise the risk of galvanic corrosion. When using galvanised fixings care must be taken to prevent the coated surface being damaged by hammers or driver bits as this can lead to corrosion of the mild steel below.

When using species with high tannin or corrosive oil content such as Western Red Cedar, Douglas Fir and some hardwoods, only use stainless steel fixings otherwise the reaction to iron can cause permanent black spotting and corrosion staining of the surface.

Cladding fixing positions

  • Use double battens to support abutting boards. Batten should extend the full width of the boards either side.
  • Pre-drill fixing points at board ends to prevent splitting.
  • For boards wider than 100mm, use two fixings per board at every batten.
  • Fixings (nails and screws) should be 2.5 times the thickness of the board.
  • Use two fixing points per board where they cross a support batten.
  • On boards under 100mm in width, use one centrally place fixing.
  • Where boards abut, use a second batten to provide adequate support.
  • Position fixing 25% in from each side and a minimum of 20mm in at board ends and pre-drill to avoid splitting.
  • Where the board on board style is used, the fixing used to install the top board must never pass through the underside board. This is because any movement in the top and bottom boards caused by seasonal moisture changes and expansion and contraction will result in splitting.
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