The length of the fixing is calculated by the total thickness of material build-up above the rafter plus an additional 35mm for softwood rafters or 25mm for hardwood rafters. Additional length must be allowed for if the rafters have a bow - as is common in barn conversions. The Skew Fast is driven like an ordinary nail, but screws through the counterbatten and into the rafter as it is driven. For lengths over 125mm, a Support Tool should be used, which is a sleeve that goes over the top of the Skew Fast, and - with the Hand Support Tool - hit with a hammer and inserted in the normal way, or - with the Power Support Tool - inserted into an SDS hammer drill on hammer action that can be used to input the fixing. This is to prevent bending of the fixing as it is inserted.
The thickness of the counterbatten will need to be carefully considered. If the thickness of 37mm+ is selected the tile batten may be fixed using an ordinary clout nail. For 25mm to 36mm thickness an improved nail (e.g. 'ring shank') must be used. For counterbattens less than 24mm thick consideration should be given to fixing through the tile batten and counterbatten in one single fixing operation using longer Skew Fast.
The tendency for any sliding load may be decreased by the introduction of a stop batten, usually at eaves level. The stop batten would need to be the same thickness as the insulation material, and is inserted in place of the insulation and is the structurally fixed to the rafters. The counterbattens are then fixed directly on top of the stop-batten. If the roof is long, over 8m, from eaves to ridge, an additional stop-batten should be introduced at the mid-point.
A stop-batten allows the use of much thicker insulation materials with the need for a greatly increased fixing density.