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I will soon have to re-felt a shed roof, almost flat, not pitched. As in nearly horizontal. Are there any advantages/disadvantages in leaving the old felt in place, or should that definitely come off first?

Seems to me leaving it on is 'belt and braces', but there may be some good reason for it to be removed and replaced.

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    @isherwood - this felt is roofing felt, made for shed roofs. Green stuff. Not the underfelt one uses before fitting tiles (on a nearly flat/horizontal roof..?)
    – Tim
    Commented Dec 20, 2022 at 16:11
  • I realize now that you're talking about what I know as "mineral roofing" or "roll roofing". "Felt" is probably a legacy term, as most of it is fiberglass-based nowdays.
    – isherwood
    Commented Dec 20, 2022 at 16:26
  • @isherwood - yes, quite heavy, not paper-like, comes on rolls a metre wide, and uses black tar-like gunge to join edges. Hardly needs any nails, it's so heavy. It's over 1/2" ply to stop any sagging, and can be walked on, but I can't locate several leaks.
    – Tim
    Commented Dec 20, 2022 at 16:36
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    "Roofing felt" is still the term used in the UK. Supposedly made from modified bitumen and polyester, or sometimes fiberglass. See for instance roofingsuperstore.co.uk/help-and-advice/product-guides/… FWIW that page recommends removing the existing felt before laying the new but they don't give specific reasons for it.
    – Carl
    Commented Dec 20, 2022 at 16:52
  • Having had so many issues with shoddy roofing in my life I now insist that the roofing company remove all old materials and install precisely to the spec of the product they're using. New felt is not expensive. Problems and contractors making excuses for leaks are expensive
    – Matthew
    Commented May 19, 2023 at 18:41

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Two layers of felt is recommended for flat roofs. So if the old stuff is good, I would leave it

https://housinghow.com/2-layers-roofing-felt-on-shed-roof/

The National Roofing Contractors Association [NRCA] even suggests a double layering of roof felt underlayment.

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  • That page refers to underlayment. That's not apparently what this question is about.
    – isherwood
    Commented Dec 20, 2022 at 16:22
  • The writer seems a little confused, which has in turn confused me. And what's 15lb felt??
    – Tim
    Commented Dec 20, 2022 at 16:32
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    @Tim I think it's a US v UK thing. In the US, this is US "roofing felt". It comes in 15lb or 30lb weights (pounds per... square yard maybe?) and is used as an underlayment layer between the roof sheathing and the shingles - it's not designed for long-term exposure to weather. In the UK, it seems that "roofing felt" is a roofing material that is designed for direct weather exposure for decades.
    – FreeMan
    Commented Jan 19, 2023 at 18:02

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