Roof Felting

[Gabe]

Any views on the value of felting when re-roofing. We have a section of our roof that needs to be done, currently old canal tiles, rest of roof tuile mechanique. I have temporarily patched leaks to get through winter and will be looking to replace in Spring. Previously I have heard that felt does not last due to extremes of temperature, we are in the Gers (32) is this true ? The new part of the roof does not have felting, but I wondered as I am going to all the trouble of  stripping this section (tiles and battens), should I not add felt or similar (any advice) as extra protection. Having spent several nights moving buckets around during storms, I am now a bit paranoid.

Gabe

ps Intending to use tuile mechanique to match rest of roof, is this a waste ?

[jonboy]

Roofing felt's main purpose is to counteract condensation. The main cause of condensation is central heating. You can now get breathable felts which are not tar based (in fact the French invented them with Tyvec), these are semi expensive but worthwhile as they negate the need for other (often more expensive) ways of ventilating a roof. Hope this helps.

[Anton Redman]

For me the maths is easy if I was going to lift all the roof then even if I value my labour at 1 Euro an hour it is worth felting before I reroof.

However I would not lift a water tight roof in order to felt 

[Ernie]

Gabe, you don't say what kind of tiles you've got. If you've got 'tuiles courbes' ie the old (very, very old, Roman in fact) style of tile which is basically about one third of a cylinder cut longways, then they are never totally watertight against all weathers. The old way of laying these (over and under) has been updated with these flat 'unders'. This improves the leakage problem but doesn't totally solve it. As far as I can make out, you have two solutions: 1) a company called 'Onduline' make a product that is in effect sheets of corrugated plastic. If you want to keep to one hundred per cent traditional look, then you fit your under-overs on to Onduline. The snag (say some) is that you will get condensation. However, there are ways of avoiding condensation! Ventilation and a product I use in the UK which circulates the air. It's very cheap and has worked one hundred per cent in an outhouse I've got. 2) Use some kind of underfelt. Traditionally and cheaply and used all over the UK and in France is the great roofing felt beloved of pigeon fanciers, rabbit hutch makers for decades. It does have a lifespan, and will age and crumble after a while. It can always be patched. Alternatively, you can use any version of what builders put under concrete floors to stop rising damp.

My builder fitted a roof with trad tuiles courbes 'over' and 'flats' underneath, he didn't fit anything underneath them and it sprung leaks all over the place. His first response was to say that it was because we hadn't 'maintained' it. This was after about nine months and there was a pebble in one of the canals. However, I popped over the road to ask the woman who has a long low pitch roof just like ours whether hers had leaked in the same storm, because her roof is covered in moss, full of pebbles. 'Mais non, monsieur', she said, 'mon mari l'a goudronne.' He had literally 'tarred' it, in other words fitted roofing felt underneath it, ten years previously. Same roof, no leaks. I put that to our builder who moved to position two: 'Ah,' he said, 'you didn't say that you wanted roofing felt.' We tried to point out to him that we didn't know anything at all about rooves but if he had asked us if we wanted a roof that leaked, we would probably have said no!

Final tip: make sure that at the point where your unders feed into the gutter, that they protrude further than the overs. This way, they will deliver the water into the gutter. This is something practised by builders all over France but somehow escaped the notice of my builder.

[Ernie]

Sorry Gabe, I see now that you talk of 'canal tiles', which are of course 'tuiles courbes'. Many apologies for not reading your post properly.

[Gabe]

Thanks chaps,

The bit of roof that I still need to do has to be stripped as the battens need replacing. Can any of you recommend a named felt product, I have a Bricodepot catalogue somewhere if that helps. Just want to get an idea on costs/m2.

Gabe

[mark24]

The type of felt should be looked at with great care

You don’t wont to go to all the bother of doing the roof to stop the water ingress ,

To then find that you have condensation problems  .

To me after 30 years roofing, if I was to do my roof I would use one of the better types of breathable felt.

Tyvek is one of the best but then just as good are Monarperm 700 and Roofshield

All dependent on what the space below is used for or may be used for in years to come,

Is it open to the roof timbers ,Or is there a loft space ?

And on how much insulation there is or how much heat there is .

More heat with less insulation will mean condensation this will form on the back of non breathable  felts and slowly rot the timbers  .Where a breather felt will let the condensation pass through  and away .

One thing to think about is that the over hang at the bottom of the roof (eaves) will be subject to UV rays and this will make the felt brake down , so you should use eaves carriers or the like.  

[Ernie]

Hi Mark, And how would I know if I've got the right or the wrong timber-rotting type stuff on my roof now? It's black, and seems to have a thread running through it in squares. I've felt the bit of it that's protruding out from under the tiles and over into the gutter and it feels like plastic (not tarred felt) I don't know the make of it, though. Ring any bells? Or is it just a sheet of DPC? There isn't a one hundred per cent seal between the top of the walls and the roof, there are few gaps; but would it be wise to enlarge those (having asked and succeeded in getting the builder to seal them up!)?