Is roof felt waterproof?

Shaskey · 27 Oct 2025

An adjoining neighbour is having an extension to the rear which has meant that part of their roof has been removed. Ignoring the raised flashing, which I've since put back into place, the removed tiles go beyond the party wall meaning that the only thing protecting the cavity of the party wall is the roofing felt. It's been exposed like this for a week and could be like it for many more as the weather turns more wet and colder. A message was sent to the builder, via the home owner, who has said there's no problem. I think at the very least some DPM should be put over the cavity.

I have a couple of questions.

1) Am I correct to assume that water getting into that cavity, which has a decorated wall on the other side, is a problem?
2) I was always told that it's the tiles that make a roof weather proof and whilst the felt can offer some temporary waterproofing it shouldn't be exposed for more than a few days. Especially given its age. Am I correct?

Both of these things seem obvious to me. If they are true, how should I address them with the builder who isn't mine and doesn't believe there is a problem?

Thanks in advance, Steve

noseall · 27 Oct 2025

Shaskey said:
An adjoining neighbour is having an extension to the rear which has meant that part of their roof has been removed. Ignoring the raised flashing, which I've since put back into place, the removed tiles go beyond the party wall meaning that the only thing protecting the cavity of the party wall is the roofing felt. It's been exposed like this for a week and could be like it for many more as the weather turns more wet and colder. A message was sent to the builder, via the home owner, who has said there's no problem. I think at the very least some DPM should be put over the cavity.

I have a couple of questions.

1) Am I correct to assume that water getting into that cavity, which has a decorated wall on the other side, is a problem?
2) I was always told that it's the tiles that make a roof weather proof and whilst the felt can offer some temporary waterproofing it shouldn't be exposed for more than a few days. Especially given its age. Am I correct?

Both of these things seem obvious to me. If they are true, how should I address them with the builder who isn't mine and doesn't believe there is a problem?

Thanks in advance, Steve

The (in tact?) sarking felt is robust enough to keep the water out as long as it is draped over the masonry and is prevented from flapping about in the wind. I'm not sure what additional polythene would do here other than become a nuisance, unless the felt layers are torn/damaged?

Deluks · 27 Oct 2025

The flappage would bother me, I'd place a length of timber underneath the flying battens, up against the wall to trap the felt against the wall. Then screw in from the top.
Use another batten or bit of 4x1 or 2.

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Is roof felt waterproof?

Shaskey

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noseall

Deluks

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