DIY Doctor

Options on Replacing Flat Roof After Poor Work

Postby Rohit15 » Fri Mar 06, 2020 4:14 pm

I need advice on flat roof replacement. A cowboy builder built a fibre glass flat roof with Velux window and incorrect slope about 6 months ago. The problem I am having with the roof is minor leaking especially around eaves and Velux due to incorrect slope and job was done poorly.

Recently a roofer suggest following:

Option 1: Rip off existing roof, correct slope and change Velux window to roof dome and redo fibre glass.

Option 2: . Re do a new structure
timber deck roof to 15 degrees
pitch roof. Apply 18mm osb
board and GRP fibreglass
system with a top coat in grey.
Sides of the extension timber
deck it and supply a plastic fascia

Which option would be better? Can someone please advise me?
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Simply Build It

Postby thedoctor » Wed Mar 11, 2020 8:36 pm


On looking, both options are pretty much the same. I would rip it all off and start again with a minimum fall of ½ an inch per 12 inches of run. IE 1 in 24. The minimum recommended by Building Regulations is 1 in 40.

I would use 3mm rubber rather than GRP as it’s guaranteed for 40 years and will move if the roof moves, which they always do.

I would create upstands for a window to guarantee water-tightness using windows like this ... rb-55x78cm

Hope this helps
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Joined: Sat Apr 08, 2006 12:15 pm

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