Planning to replace flat roof with fibreglass


9 Sep 2013
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United Kingdom
Hi all,

I recently noticed the felt at the edges of my garage roof have split and all of the chipboard sheeting in that area has been getting damp. The roof could do with replacing (patched a few areas last winter) and as part of it covers the kitchen extension I figured time to bite the bullet and get it done.

Having looked at the different options I think fibreglass might be the best one to go for. There are some great videos on youtube (from which I have watched along with others and I think it's something I should be able to do.

Doing research in to the materials and starting to plan there are a couple of potential problem areas and I'm struggling to find documented solutions for working around them. Hoping some of the good folks on this forum may be able to guide me in the right direction as I definitely don't want to put all of the effort in but end up with a bodge job.

Problem Area 1 - Soil pipe comes out of the wall from the toilet. The bottom of the pipe is 110mm from the roof. I think the standard height that a GRP angled wall section meets the wall is around 120mm. I'm sure I could trim it down a bit but the main problem is that the pipe will still be in the way of the lead flashing (or simulated flashing).

Problem Area 2 - The gap between the mortar line currently used for flashing/felt anchoring and the flat roof surface currently ranges from 50mm at one end of the roof to 120mm at the other. This is because one end is slightly raised so that water drains off towards the front. Might need to use a higher mortar line for the flashing but then it's going to come in half way up or above the soil pipe.

Problem Area 3 - Soil pipe goes through roof. Can't find any solid info on how to go around an immovable pipe with fibreglass. I believe if prepared properly the resin will bond to the plastic pipe so I could just go around it with come bandage and resin but think there needs to be some sort of expansion gap? It's only 80mm from the wall so clearance against the wall angle trim will probably require some trimming

Problem Area 4 - Flat vertical section about 600mm high - leading up to pitched section of garage roof at the front. Currently covered in felt - presume it's ply behind. First thought is to ship off the mortar at the top, simply fibreglass this as a continuation of the flat roof and once it's dry put a new mortar line along the top to seal it. Only issue is what about expansion?

These photos might paint a thousand words: Stuff/Roof?sort=3&page=1

These might be silly concerns that are easy to work around. Hope somebody can point me in the right direction.


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Have you considered EPDM? It's easier to DIY than fibreglass, less messy, less time-critical and less risk of ending up with a pair of boots stuck in the centre of the roof :) I did a large roof with no experience and it was a doddle.

If the roof deck has been getting wet, it might need repairing or replacing, especially if it really is chipboard (not suitable) rather than OSB. Worst case, the structural timbers themselves could be affected, so plan to inspect carefully.


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