This grp flat roof design in Sale was a flat roof, wrapped around the rear elevation of the property, it had a sunken rainwater gully to the front section and a roof lantern for natural light positioned just off centre of the flat roof.

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This grp design was a flat roof, wrapped around the rear elevation of the property, it had a sunken rainwater gully to the front section and a roof lantern for natural light positioned just off centre of the flat roof.
 
The roof was constructed as a warm roof, a timber joist system was built with furrers fixed on top to create a fall towards the front edge of the flat roof. A sunken rainwater gully was formed approx' 400mm in from the front edge and sides with a fall from right to left. 100mm Kingspan was then installed to the flat roof area around the gully section before 18mm OSB3 interlocking boards were fitted to the flat roof area and the now formed rainwater gully, and screwed to the timber joists beneath.

The gully was basically a right angled box so we cut strips of OSB3 boarding and installed them along the sides and end to create slopes the to box gully. This is a tip I would advise anyone doing a grp roof. Get rid of as many right angles as you can, if not all. whether its boxing out soil pipes or lining gullys, where you cant get a preformed trim to do it for you, then cut boards and create your own angles and slopes. The problem with a grp install and right angles is, once you've got the matting saturated in resin and you're trying to work it into a right angled corner with a paint brush the matting starts to stretch out and separate. Its akin to wetting a tissue and trying to do the same thing. To me itweakens the integrity of the matting so we avoid it and take the time to form angles that the matting can just roll down and off with ease, it makes the job so much simpler.

Once all the prep' work was completed with the boarding we fitted all the preformed grp trims to the inner and outer perimeters of the roof and around the boxing out for the roof lantern, We also fitted a lead welded baseplate/pipe (100mm) to the left hand end of the gully for rainwater drainage before bandaging all corners and laps to trims in activated resin and bandage matting.

While this area was curing we pre-cut all the various lengths of of fibre glass matting, ready for the main flat roof area to be done. Once this was this was completed we resined the boards in sections, rolled the matting over the top, resined again on top of the matting and used a metal pad roller to push both resins through the matting, any areas we noticed lacking we added more resin and pad rolled til a uniformed finish was achieved.

After finishing the initial 1st layer of the grp system and allowing it to harden, we then sanded all the bandaging we had done earlier, bringing it to a nice smooth finish. A layer of activated topcoat was then applied to the entire flat roof surface and all the preformed trims, before a further process of sanding was then undertaken to tidy up any defects to the flat roof area.

Finally, we gave the roof another coat of activated topcoat before installing lead flashings to the internal perimeter wall, completing the watertight finish to this new extension roof.