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The fixings used to attach sheets to purlins must be durable and able to withstand considerable wind pressures. The exact type required will depend on the sheet and purlin type (timber/light steel/heavy steel) however we usually recommend self-drilling TEK type screws with a 5/16th hex head.
Self-drilling TEK screws have a drill point for drilling through the sheet and purlin before tapping into the purlin. Under the head is a 19mm stainless steel washer with bonded neoprene pad that compresses to create a seal with the roof sheet. The screws can be fitted through the trough or pan sections of a sheet’s profile, or if fixing corrugated sheets, through the crest of the sheet’s profile using a longer screw and a soft BAZ washer. It can be helpful to use a centre punch to indent the sheet at the point you want the screw to go through to keep the screw on course when screwing it in.
Top tip - Make sure fixings are not over tightened as this can burn off the soft neoprene pad under the washer
Under normal conditions sheets should typically be fixed to supporting purlins at every second trough or third corrugation with fixings in every trough at the bottom / eaves to help protect against wind uplift if the roof is in an exposed position. Exactly what is required however will depend on the circumstances.
Top tip - A stretched string along the purlin line can make it easier to keep the fixings in line when the sheet is laid on the purlins.