Flat roof Rubber v Fibre glass?

paddypaws101

Anyone had experience of either of the above systems for flat roof, or can offer advice on pros and cons of either. First quote for area 19msq in fibre glass is £2700 which is more than the whole house cost to be re-roofed ( ok, it was a few years ago! )
I understand that both these systems are much longer lasting than old style felt...but which is best???

AdrianW2

Are you expecting the answers to be any different to when you asked on Sunday?

paddypaws101

Oh excuse me for repeating myself!
Actually my previous question was just about fibre glass. Having done a little more research on the subject I have now heard about rubber sheeting which seems to be less expensive.

FRC_2

If you can give me a little more info I can help you.

Is the flat roof going to be newly installed or are you planning on "overlaying" a new flat roof over an excising one?

If a new roof, are you planning on insulating the roof under or above the roof membrane?

What is the construction of your building? Where is this flat roof going to be? Garage roof or on your house/property. And will you be requiring to use the flat roof for access (ie balcony).

The rubber membrane you have spoken of, is this an EPDM membrane, PVC membrane or TP/TPO Membrane?

Regards

paddypaws101

Ok, let me see if I can answer all those questions!
It is a flat roof extension kitchen and bathroom on the back of my house, no regular access required. Originally built circa 1975 I replaced the roof 15 years ago with felt overpainted with reflective coating.
I presume the joists are ok, but have been told that ply is blown around area where boiler flue was fitted.
I have seen that some systems can be overlaid on existing structures but do not know how sound this would be - advice needed!
Having had the house double glazed and decent loft insulation installed I am of course interested in insulating this flat roof space as well - subject to cost!
Whilst cost is always an issue, I am not interested in cutting costs so much that I end up with a shoddy job, and as I am planning on holding onto this house long term am interested in a long term solution.
I eagerly await your reply!:rolleyes:

FRC_2

Thanks for the information.

First off, the Boiler Flue is a common problem, warm air from the flue will be making pockets of warm air under the membrane, because there is no insulation to stop the warm air bridging to the cold air outside you are getting condensation buildup (cold bridging) under the membrane. I would try and have a good look at the ply in the surrounding areas. It is usually the case that the moisture will spread over time, as water is a good conductor of heat it will excelerate the cold bridging process, the only real way to stop this is to either remove the damp ply or dry it if its not rotten(the hard option tbh). Then you will need to either install some sort of ventilation next to the flue or insulate the whole roof.

If you are planning to overlay your excising roof please be aware if the rubber membrane you are considering is a PVC you could face problems in the future as bitumen(felt) is known to eat the rubbers in PVC making it brittle and likely to split under expansion. I would presume you would be using a fully bonded system using adhesives rather than fixing the membrane to the roof with screws, this would mean close contact of the PVC and Bitumen and may be an issue.
Although saying that, I have only seen this issue when felt has been overlaid onto PVC. Not the other way round.

I have seen many fiber glass roofs and see no real issues with them as such. The only issues I could see is they don't seem to have the expansion needed in some building construction types(metal structure or wood structure). Were as rubber or PVC roofs have expansion of up to 400%.

There are issues with overlaying onto old roofs but they can be done if done right.

Make sure all moisture is removed before any new system goes down. If any moisture is entrapped between the membrane layers it will expand under thermal expansion, this will cause what we call a blistering effect on the roof, if you were to use a material that had little or no expansion you could end up with spits. And it doesnt look very nice.

If you are planning to insulate the overlay, make sure your structure has the load-baring capabilities. While the insulation is relatively light, once you add snow/rain/hail onto the roof you might find it to be too much for your structure ending up with a caved in roof. Especially if you have rotten ply underneath.

Obviously I cannot give you any real detailed advice without seeing the roof myself but these are things to consider. If you want to send me some photos of the roof I can give you a much more accurate judgement.

Also, if you want to make things cheaper without problems, ask around and see if anyone has a friend in the roofing inustustry. Its quite common that roofers will take off cuts or left over materials from big projects to use on their own contracts in the weekend(garage roofs etc). While you may not save a huge amount of money, you will end up with someone experienced and possibly using a high performance membrane (trocal, Sarna, LPL).

And just an extra note, if you are unable to see the ply under the membrane there is an option to have a thermal-graphic survey carried out. This will be able to tell you where the ply is saturated giving you a more detailed view of what needs to be done. This is not a cheap option but maybe cheaper than having the whole roof re-roofed.

paddypaws101

Thanks for giving me so much detailed advice. It seems I have opened a can of worms for myself with this one. The builder who plastered my kitchen says he has worked with fibre glass and could do the roof....but obviously this wont give me any guarantee which I would get from a bigger firm. Sometimes money saving has it's drawbacks!
I have organised for 3 other firms to come and have a look and quote, so will go from there.