To optimise the benefits of the tube fastener system, the ideal combination is the use the maximum tube washer length, and the minimum fastener length available. This both eliminates heat loss through the metal fastener, and is more cost-effective as long length fasteners tend to cost more than long length tube washers
First, work out the tube length. To select the tube length, deduct 20mm from the total roof build-up, which allows for insulation compression, and choose the tube of that length or shorter.
Next, the fastener length. Take the total roof build-up, and deduct the length of the selected tube fasteners. Then, add the deck penetration/embedment length (see table further down this page), and then add 25mm for to allow for the length of fastener in the tube. Then select the fastener of this length or longer.