Roofing Felt

moan 77

Hi All,

Just got my list from my roofer buying this next week, however the question is he has down protect A1 roofing underlay which is fully waterproof but not breathable should i not be using breathable underlay on the roof or does it matter. I hope to fit a airtight membrane on the underside of rafter counterbatten this for services and then and insulated slab to finish it off, the house is a story and half domer made from concrete block, if this matters with a precast first floor.

Any help would be great just so i'll understand the discussion and i'll have learned another item.:D

RKQ

There are many felts on the market, varying in price. Personally I'd use a fully breathable felt - well worth the money. I'll PM my recommendation.

A breathable felt is very important in Warm roof construction but equally important in Cold roof construction on a dormer or storey & half roof. See manufacturers details & agregate certificates for insulating between the roof rafters. A 50mm air gap is required between ordinary bitumen felt and insulation.

Discuss this with your Roofer & your Certifier. Ventilation of the roof is extremly important.
It should be noted that most breathable felts are UV sensitive and require bitumen felt circa 300mm long at the eaves. (Thats 300 to 450mm up the roof slope, the full length of the eaves).

creedp

RKQ - I was not aware of the UV issue with breathable felt. My roof is currently being felted but the builder is using the same felt throughout. Is the UV issue critical - should I insist on him using a bitumen felt 300mm up the roof? As per usual, my drawings make no reference to this issue. This building process is really fraught with uncertainty, as without the assistance of contributors to forums such as this I would be copmpletetly dependant on guys who have no interest in providing me with any information.

Thanks to all

Hidalgo

I'd definitely recommend using a breathable felt. A friend of mine used regular felt and is now having major problems during winter time if there's a sustained period of frost. The front of the house is fine but the back is shaded so the frost doesn't thaw. This has led to dampness coming onto the slab and cracking the skimcoat/hardwall. You also need to ensure that any insulation between the rafters does not touch the felt. This can lead to more problems. For the extra cost of breathable felt it's worth it. Do it once and do it right.

RKQ

Creedp UV rays can be a problem. A lot of I A certs refer to this issue.
Naturally its not an issue for the roof once its covered with slate but the piece that laps into the rain-water gutter will be damaged over time.

Fitting the 300mm layer under the breathable felt, along the eaves acts as a back up. If the last few inches of breathable decays the bitumen felt underneath continues to throw water into the gutter.

Many breathable felts also recommend counterbattening on top of the felt.
Hidalgo sweating of bitumen felt is common in poorly ventilated roof spaces - last winter was abnormaly cold for a long period of time. The shaded part of many roof experienced condensation in the roof, particularly during the snow / frost.

slattsteen

sorry for changing the subject slightly,but has anyone used a breathable membrane called <snip>? i can buy rolls of it for 50 eu,would be using it on timber frame

Poor Uncle Tom

slattsteen wrote: »

sorry for changing the subject slightly,but has anyone used a breathable membrane called <snip>? i can buy rolls of it for 50 eu,would be using it on timber frame

Please read the forum charter about naming products on-thread.

kboc

slattsteen wrote: »

sorry for changing the subject slightly,but has anyone used a breathable membrane called <snip>? i can buy rolls of it for 50 eu,would be using it on timber frame

what ever product it is might not be what you want at that money. Pay cheap get cheap. Any of those fully breathable membranes are nearly 200 euro/roll

Be carefull, penny wise, pound foolish!

Good luck