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Helping my son and daughter-in-law with a roofing problem on their "patio' which leaks like a sieve due to poor roof installation.
Problem: It was originally intended to be more like a simple, open pergola with no roof. The structure's rafters are dead flat, ie there is no run built into the patio, but the previous homeowner put a polycarbonate roof on it with no purlins / battens and therefore no slope for drainage. It's attached to the house along its Northern and Western sides, so when I re-roof it I need to make a North to South run in the roof so rain can run off into a gutter or into the garden at its southern end.
Apparently (per the Suntuf installation guide) this should be a 5 degree slope. Which means a 235 cm drop along its 2670mm length.
MY MAIN QUESTIONS:
1) Do I really need so much of a slope it seems excessive!? If not what is the smallest I can get away with?
2) What is the best method or achieving it:
Do I add "purlins" of the appropriate heights perpendicular to the existing rafters (rafters run north : south)...this sounds challenging because the purlins would need to be very wide, ie the northern most purlin would need to be 23.5cm higher than the top of the patio's rafters (!!), then successively narrower purlins to achieve the slope needed until I reach the southern end of the patio?
OR
Do would I be better off adding a tapered beams running along the top of the existing rafters to create the slope I need, then add standard purlins on top of those beams to attach the roofing sheets to?
I don't love the sound of either of those options so maybe someone with far more knowledge than I have can give me a better idea!!!
Any advice welcomed! Photo taken from the South East corner of the structure attached
Cheers
Tim
Hi @TandT,
That butyl strip size should be perfectly sufficient for this application, especially if you’re aiming to keep things neat visually. A 1.5 mm thick by 20 mm wide strip will compress nicely once the flashing is fixed in place and will do the job of sealing out water without being overly bulky.
The key thing is surface prep. Make sure both the wall and the flashing contact areas are clean, dry, and free of dust or loose material before you apply the strip. I’d suggest running the butyl along the very top edge of where the flashing will sit, and again along the lower contact point with the roof sheetin, so there’s no path for water or debris to track in behind the flashing.
Also worth keeping in mind that while it’s an adhesive strip, it shouldn’t be relied on as the primary fixing. Think of it as a gasket rather than glue. Once the flashing is positioned, make sure you mechanically fix it with enough fasteners to properly compress the butyl. When you’re installing it, press the flashing firmly against the wall, then fix it off while maintaining that pressure.
Mitchell
