GRP roof overboard on existing asphalt flat roof terrace?

Difficult.. and depends on the situation. It depends on whether the roof is a cold ventilated roof OR to be upgraded to a warm flat roof. If it is a cold flat roof then you need to remove the felt (always)...if the deck below is structurally sound then you can overboard (maybe with some minor repairs). If this is being upgraded to a roof requiring an upgrade to conform to current building regulations then in (some) circumstances the base layer (felt) - could act as the VB - BUT - you would still need to strip areas to pick up the joists to adequately secure down the top deck - Rhyme and reason - generally always remove the felt. The old deck can (sometimes) be used as the first deck.... Usually picked up the intial survey..... GOING BACK - You have a void and a VB in the wrong place for cold or warm roofing - would suggest this may be inaccurate...for any flat roof KR Drew @ Fibretechs

Felt is much better to over lay on ash-felt Or even better arm deck Don’t do GRP as I don’t like it They crack after 1-2 years

No no strip the hole roof and start over never over board when using GRP, you can’t guarantee you are screwing directly into the joists. You need to screw directly into joist, and make sure sub frame is solid any movement, will cause cracking and premature failure. The manufacturers don’t guarantee an overlay system, if you can’t afford to get the asphalt stripped, providing the roof hasn’t been leaking badly for years, over laying in high performance torch on bitumen membrane will be the better option which is more for giving to any movement and won’t need a new decking to go down, you just prime the roof and torch straight on. Regards Frankie BT Roofing Services

I have recently carried out a balcony roof in grp. It was initially quarry tiles bedded onto ashphalt. Because of the dampness on ceiling below I drilled up through to find a ply base, I removed both quarry tiles and ashphalt and plywood decking. Then re-decked in 18mm osb applied grp. Had no problems. I would be concerned about overboarding. re-trapped moisture etc. I hope you find this helpfull. Ps. I would be concerned about liability with guarantee with overboarding. Furthermore If you overboard by screwing boards into asphalt it could end up puddling.