Looking for some advice. My mum has a rear extension 3m x 6m with a conventional torched on roof. After 33 years and a few patch ups it’s probably fair to say that it’s due a replacement. Had two approx quotes, one fitter says would strip off the old roof. The other fitter says would fit new OSB boards onto existing felt (screwed?) and fit GRP onto that. I can see some logic in not stripping back to rafters, but also the potential hazards of missed inspection, insulation etc. So STW whats the wisdom on upgrading semi knackered (but firm) felt roof to GRP? Any ideas on cost for 18sqm including new fascias and gutter? Ta!
Also.... what GSM of GRP is needed? have seen 450gsm and 600gsm. Questions, questions!!
No expert at all but I'd be disappointed if I bought a house that had new boards over the existing roof - seems just lazy.
I'd be looking to strip the existing roof, maybe upgrading the insulation while it's open and then new boards but possibly with rubber rather than GRP (I've seen entire GRP roofs peel off in a strong wind!)
Watching.....my garage and kitchen have a knackered old flat felt roof which had a couple of small leaks and now one big one after the recent monsoon! Not got quotes yet. GRP is fibreglass? It sounded better in my brief googling the other day but double the price of felt. The rubber ones looked good and not much more than felt but potential issues with shrinkage over time if not secured well at the edges?
I'd look at getting insulated boards put up - regret not doing it when we had our extension roof refelted. this sort of stuff: https://www.insulationshop.co/56mm_kingspan_thermaroof_tr31_pir_bonded_to_plywood.html
I'd also be wary of not having old stuff removed - there could be damp and/or rotting plywood under the leaks.
The rubber ones looked good and not much more than felt but potential issues with shrinkage over time if not secured well at the edges?
I think it should be glued down all over.... might be wrong though.
Is the logic of putting insulation over old roof covering that the old covering provides the vapour barrier?
laziness of the fitter more like. As said old roof could hide all sorts horrors underneath.
strip back to timbers etc make good any rotten areas and start from there.
You don't need the old roof as a vapour barrier. Plywood (most types) is a vapour barrier due to the glue. Some types of foam insulation (expanded poly iirc, not extruded, although I may have the bass akwards) is also its own vapour barrier. Since the foam is in intimate contact with the plywood or chipboard and the the sheathing is on the warm side of the insulation, that's enough vapour wise. Some places allow this type of roof without any venting.
Definitely get the old roof stripped off and start from a good, known foundation for the new roof. We had ours done last year and the state of the woodwork that came out was shocking. Leaving anything rotting under a nice new roof is asking for trouble later on. We went for fleece backed EPDM, guaranteed for 20 years. Slightly bigger area than yours and less than £1.5k in south of England if that helps....
I did my roof last year, about half the size of yours, think it was about 500 quid for a kit from tinternet, when I was reading up I think the heavier sheet was load bearing, I went for the lighter stuff and am pleased so far, do not try and fit it on the hottest day of the year like I had to to. I did strip everything and redo the insulation at the same time but my roof was well past its best. Best part about GRP is if it does leak in the future just mix up some resin and patch it up piece of pish. Youtube is your friend.
Thanks for replies so far! Anyone advocating laying OSB and GRP on top of existing bitumen? To be fair the old roof is not a mushy mess, but at same time it does not feel right! Keep all advice coming. Ta!
Anyone advocating laying OSB and GRP on top of existing bitumen?
No, do it properly.
When I had my garage roof done it was found to have have bitumen covered wooden slats, 5he roofer still replaced those where the leak was just in case any moisture had made it past the bitumen and penetrated the wood. getting it done properly will save potential future costs.
I also had insulation put on. The garage is unheated and it stays about 3 degrees warmer in winter and about 5 degrees cooler on a hot summers day.
As all of the above, strip it back completely, assess the Tibet’s in the existing, replace where needed and then insulated to current building reg standards. Grp or single ply membrane (sarnofil or similar) will see it good for a long, long time.
Do not be tempted to lay a new one on top - you will get problems.
If it seems solid it will be fine to go over the top, it’s not a huge job stripping the old roof of though. More than likely there will be a bit of rot, but there will be in most of the roofs in this country. Depends on your budget
To be fair the old roof is not a mushy mess
Unless you have x-ray vision you can't be sure there's no nasties in there.
We had our flat roof done recently (covering integral garage, hall and toilet) in GRP. Old felt roof stripped off, joists checked (one small area of rot found that would have been missed if the old cover had been left in situ), new insulation, new 'structural' 18mm ply, GRP and new facia boards/gutters etc. Very pleased with the result and can recommend the firm (if you are in Gloucestershire North Wiltshire area).
Thanks all. Have found a local guy who seems good. Full strip, moisture membrane, insulation boards and GRP 450gsm. I suspected that leaving old roof in place was just a lazy option, thanks for mainly confirming that!
I did my garage with a kit from an online firm. Old roof was sound so just over boarded with 18mm OSB. The EPDM is an easy enough job for a confident DIY’er.
We had the garage roof done when we moved in. Re-boarded and then GRP. GRP done properly isn’t going anywhere but I’d only trust a competent roofer to do it - there’s many shortcuts that can be made that’ll affect how long it all lasts.
Done right and it’ll easily last 10 years + and most probably much longer.
If you’re in the Sheffield area let me know and I can recommend a first rate roofer :).
