How long do felt roofs last?

[zaax]

A friends felt roof only lasts 7 years but mine 16 years. What is he doing wrong?

[PhilÃÂÂ]

he is not petting it

[Brian G]

Unless the friend is sticking holes in it - nothing!

The average life of a felt roof is around 10 years - but that depends on many things such as the quality of the materials, quality of the workmanship, whether he used it as a walkway or balcony etc, etc,etc - or just plain, old fashioned luck :-)

I've seen them fail after only four years if that's any consolation to your friend?

Brian G

[Dave]

The roofer I employed about 3 years ago said that the felt should be replaced every 25 years and that the felt that took the most stick was that bit that was just before the guttering.

Now I have never seen this happen in the areas that I frequently drive through, so perhaps other posters might be right/might be wrong. Who can tell? My roof felt looks in good condition and does not tear easily. Is that a good sign? I don't know.

Dave

[andrewpreece]

What is the point of felt? I only ask because my slate tiled roof is fine after

65 years and never had any felt to start with!

Andy.

[Andrew Gabriel]

My parents' got about 20 years out of a felt on strawboard roof before first sign of failure, and another 10 years before it all had to be replaced. There were several failure modes in different places. Initially some parts of the strawboard collapsed, probably due to condensation in the roofspace which was unventilated and uninsulated, which resulted in puddles on the roof. The puddles themselves didn't leak, but the resulting stretching of the felt pulled some of the seams which did start to leak -- fortunately they were not under the puddles and were easily repaired with a little tar. Also, some of the layers of felt started separating, and large blisters would bubble up in the top layer. After 10 years of patching the seams, they had the whole lot ripped off, insulation, marine ply (IIRC), felt, and reflective chippings replacement.

I presume you are referring to felt roof lining under state? This thread is talking about flat roofs (normally) with felt exposed surface and no tiles/slate.

[mungoh]

As others have said, materials and workmanship play a major role in lifetime.

I've read that sunlight can be a major factor in the breakdown of the material, hence some people cover the felt with a layer of light gravel. My old man always questioned such a move. He said that on a hot day if you had to walk on the surface all it would do is push the gravel into the melted surface layer which might puncture it. I've had a neighbour tell me he moved in to his house and promptly swept his flat roof clean of the gravel!

The roofers I used last year gave either a 15 or 20 year guarantee of their work. And they've been in the business for a few decades so I kind of believe that their indication is accurate.

Mungo

[The Natural Philosopher]

Cheap felt, bright sun, and bad luck..

[Brian G]

Hi Andy,

What the OP is referring to is the green mineral felt used on flat roofs (and sheds) and has a notoriously fickle life span.

With regards to the 'sarking' felt under a slate/tiled roof - it's there primarily as a 'back-up' in case you lose a few tiles during a gale (or they crack and perish) and the water is then 'supposed' to run down this felt and into the gutters rather than onto your ceilings.

From your post, I would presume that you have not yet had that experience?

Brian G

[dg]

The major factor are exposure.

Sunlight breaks down the bonds in the bitumen and makes it less flexible. The 'freeze/thaw' cycle stresses the bitumen so that cracks eventually emerge.

The so called micro and macro climate also play their part - if part of the roof is in continuos shade, and the other heated by sunlight then it gets stressed. Ponding of water and ice formation does not help either.

Proper drainage to shed water prolongs life. The heat from the room below affects it too.

Poor workmanship will normally lead to leaks from the joints, and has a lesser effect on the life expectancy as a whole. Materials, if manufactured to the relevent standards should have similar lifespans if fixed to current standards (ie number of layers and proper laps and welts).

7 years is at the lower end of the scale and is too short even for severe exposure conditions. He should check the roof design for other contributory factors.

dg

[keith_765]

CUT

Underlay felt under slated or a tiled covered roof, is there to stop driven snow and rain entering the attic area. It also acts as a vapour barriier against melting frost penitration. Before the use of bitumen reinforced felt it was the practise to touch under the slates or tiles with cow hair mortar. The use of bitumen reinforced underlay felt with non treated lath and round wire iron nail is causing a few head aches. The condensation which penetrates via frost under the outer roof cover is rotting the laths and rusting away the hold of the lath nails. There are quite a few roofs with rows of tiles all bunched together, Time will see how the new method of tantalised laths, galvanised nails and breathable underlay lasts for.

[RedOnRed]

I had my felt roof replaced with a fibre glass one. It wasn't much more but is a far superior solution with a 25 year guarantee.

It now comes with a 50 year guarantee...

[bigcat]

cheap felt, lack of slope causing pooling and freezing, lack of any means to stop the felt cooking in the sun, chipboard that sags under the felt, leading to pooling and freeze thaw cycles...

When flat felt roofs first appeared they were designed to keep a layer of water on the roof, and this water protected it against the suns action for a while, increasing roof life. But it produced much worse failures, in need of quick action.

Now chippings are one way to do it, but must never be walked on, just one walk on the chippings is sometimes enough to wreck the roof. A better approach imho is felt that comes with tiny slivers of stone prestuck on the top. The stone chippings keep the sun off, if theyre light colour, but without the puncture problem or unevenness of big chips.

I have used cheap in the past, but soon realised it was a false economy, as felt cost is a small part of the total re-covering cost. And why decide to redo it every 7 when you can do it every 15-20-25. Use a truly good felt and WBP ply, if necessary build the structure up a bit so there is a slight slope, and you should get much better life.

NT

[The Natural Philosopher]

Er no,. its there to stop the WIND getting in and lifting the slates upwards...

Bollocks.

More bollocks

True enough. No roof is perfect...

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