Installation and Weatherproofing

Installation and Weatherproofing


Prior to fixing, the installer should undertake the following:

  • Check that the purlins and girts are in plane and that the purlins are not canting.
  • Check the spacing of purlins or girts relative to the specification or the maximum span permissible from the load span chart for the particular class and profile of FRP sheet being used and the site conditions.
  • Check that, where required, safety mesh has been installed and that protector strips or approved alternative protection has been installed over safety mesh at all fixing points.
  • Check that the sheeting supplied is the correct class and grade, has the correct coating thickness and is the correct profile, cover width and colour.



Where possible the FRP sheet should be installed concurrently with the main cladding material. Where this is not possible the opening or temporary sheet being installed must be checked to ensure the correct opening cover width dimension has been provided.


The FRP sheet should be laid overlapping the main cladding on both edges. All fixing holes must be predrilled oversize as specified for the sheet length and fastener being used. Fasteners must be fitted with approved washers. Fixings should be tightened to ensure a weatherproof seal but they must not be over tightened as this could result in distortion or fracture of the FRP sheet and restriction of natural expansion and contraction of the FRP sheet.


The number of fixings used, should be in accordance with the specification for the profile being used.


Side-lap fasteners must also be installed at specified centres with the FRP sheet having clearance holes provided as per the main fasteners.




Flashing Points

Weatherproofing shall be provided at all flashing points under the normal flashing by the fitting of a foam closure strip to match the profile being used. Closure strips will be sealed to the sheet with an appropriate nonsetting silicone sealant. Refer to the Parapet Flashing and Eave Closure diagrams located within this Section.


End Laps

Where permitted, all end laps to similar or dissimilar materials must be sealed by the provision of two lines of compressive foam strip or non-setting silicone placed 150mm apart and for the full width of the FRP sheet. Care should be taken to ensure the seal is not too thick which would result in a large gap at the joint.

Where lapping is required take particular note of the following situations:

  • Low roof pitch
  • Sheet class exceeding 2400 gsm
  • High wind area
  • Roof flooding likely at any time.


In the overleaf situations a special sealing should be provided as detailed in the diagram. The edges of the underlapping sheet should be cut back as shown.


Special Weatherproofing Situations

Special care should be taken to provide a weatherproof seal in areas where water can remain on the roof and can be blown or sucked over the ribs or under the flashings.


FRP sheet is unable to be formed to the intricate shapes of metal and may require a special seal to be fitted on the side overlap joints, or the use of double laps at the edges. The following situations may allow the penetration of water through side-laps in some circumstances.

  • Low pitched roofs particularly where high winds are prevalent.
  • At the high point of the roof particularly if the roof finishes at the parapet, or a raised construction where blown water can build up and flood the roof profile
  • Roofs subject to a high rate of movement on large roof expanses where water can build up on the roof in some situations. In these situations, consider double lapping at overlaps, special side flashing or use of sealants should be considered.


Bird, Vermin and Dust Proofing

To assist in preventing the entry of birds, vermin and dust, closure strips can be fitted under the FRP sheet at a gutter, roof step or similar location.

The closure strips should be fitted at the position of the fixing screw to the main support beams.