Flat roof advice please!

niallmoran

Hi folks

We've just bought an old cottage with a bigish (approx 35m2) flat roof extension that needs replacement (sagging boards, old felt covering etc) and the roofing contractor that did a pre purchase check quoted £5k + vat. for a new 5 layer felt roof with coolag insulation.

I thought that this seemed quite high so am planning on replacing it myself in the summer. After lots of reading i've decided on a fibreglass roof (mainly cause i'm used to GRP boats!) and want to build a warm roof construction.

My shopping list includes all the fibreglass material (approx 500 quids worth), CSR3 18mm decking but i'm struggling to find out what insulation board i should use (and where to source it!) for the warm roof construction as it can get a bit chilly in Dundee!

I've come across composite board that includes a 6mm ply top but i'm unsure how to use this....do i still need my 18mm deck? How do i attach it to the joists? Do i need noggins?

Can anybody point me in the right direction with this? I'd be most grateful for any help

Cheers
Niall

George_Bray

Niall

My flat roof is roughly the same size. I had professional quotes of under half yours to replace existing felt with new, top quality felt. The existing felt has lasted 25 years so I fully intend to go with felt again. It's the best material. I would not use fibreglass or anything else. Modern felts will be even better than my existing felt and should last well over 25 years. I may do it myself for the cost of the materials - about £300 or £400. If I replaced the wooden decking that would add a bit to the cost but not much. You will need to declare the job to your local authority because of the Government's rules on energy saving, insulation upgrades, etc. Be sure to speak to Building Control.

Regards
George

niallmoran

Cheers George! Maybe i should have another look at the felt option and if it was about 2-3k i'd probably get it done professionally.

Any thoughts in insulation to make it a "warm" roof construction?

Cheers
Niall

niallmoran

had a look at the rubber solution and although it's cheap i'd be a bit worried about it getting damaged by falling roof tiles etc....it seems to be better suited to roofs that have sound decking and just need re-waterproofing where as we need a new deck and want to incorporate insulation in the structure so we might as well go the whole hog!

I guess we should get a few more quotes and ideas from roofers once we move in...any other thoughts though would be most appreciated

Cheers
Niall

badmumof1

definatly would not go with felt
glassfibre will last a lot longer and is much more stronger than felt

had garage and walk on balcony done last year and we havent got to worry for 25 odd years whereas felt you have the added worry of it lifting in the high winds, cracking when you get freezing weather, damp when it rains oh the list just goes on and on

edited to add : it also wont crack even if a roof tile falls on it
infact it will probably outlive us

niallmoran

interesting hearing everybodies views....seems to be a topic where there are lots of different opinions

badmumof1

lol well we looked into it long and hard believe me
we compared prices and the man with the hotpot and felt came out only £150 cheaper.
we had all the beams, trusses and boards replaced infact we had no roof at all at one point but 7 hours later we had a brand spanking, completely water tight and neat looking roof.
even through the winter it has stayed perfect and not one bit of moss growing on it as theres nothing for it to grip to.
yes the old roof had lasted but a few holes did appear but we couldnt afford to replace it so we were then left with rotton beams that had to be replaced.
nothing can break what we have now as hell they make boats out of the stuff and they go through worse than our roof does

niallmoran

what kinda cost was your fibreglass roof? And what's the approx area? I'd like to get a good feel for what other people have paid so that i can see if quotes we get are reasonable....

badmumof1

i think it was roughly around £500 for a area of 21ft by 10ft

George_Bray

niallmoran wrote: »

Cheers George! Maybe i should have another look at the felt option and if it was about 2-3k i'd probably get it done professionally. Any thoughts in insulation to make it a "warm" roof construction?

I guess anyone who's been persuaded to go the fibreglass route is going to try and sell the idea on to others, aren't they. But felt is the de-facto high quality, long term (25 or 30 years +) solution for flat roofs, whether it be for homes or large public facilities, like leisure centres. Ridicule can be poured on like bitumen, by picturing someone with a tar brush, I guess! But I wouldn't touch fibreglass with a barge pole - especially if it costs more. Check the website of the Federation, or whatever it's called, of roofing contractors, and other sites which confirm the high quality nature of good quality felt.

I think warm roofs are where the insulation is on top and cold roofs are where some insulation is stuffed between the room ceiling and the roof boards, with felt on top. The Local Council/Government tried to force me to make the change but I appealed on the basis that the warm roof insulation blocks would cost too much (£ hundreds) and there was no financial payback. If I'd had to do it, I'd have needed to take up the existing plyboard, remove existing insulation and fit new, insulated boards. It's these new boards which would have cost quite a lot.

Regards
George

dweeby

George_Bray wrote: »

I think warm roofs are where the insulation is on top and cold roofs are where some insulation is stuffed between the room ceiling and the roof boards, with felt on top.

Good link here.

It's a Wickes leaflet about flat felt roofing, and explains (with pictures!) the various build-ups and warm/cold roof constructions. It also explains building control requirements in terms of insulation etc.

After reading it, you might decide to "do it yourself"