Waterproof Glue/mastic suggestions needed for joining flexible plastic sheet.

I have a hole in the plastic sheet that lies just under the roof tiles and forms a waterproof surface.
There's a bit of spare plastic sheet I can snip off as a patch, so I need a glue or mastic that is waterproof, can take high temperatures that occur in the loft in summer and that sticks this thin, flexible plastic sheet together.
Any ideas?
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On 11/07/18 16:08, me wrote:

old fashined solvent based evostick is about the only thing that reliably bonds polythene style sheet
Some of the 'bostik' glues are similar.
Also shoe repair glues - think those are polyurethanes - may work. Try on scrap plastic first.
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On Wednesday, 11 July 2018 16:20:27 UTC+1, The Natural Philosopher wrote:

What about ordinary sanitary sealant? It'd definitely be flexible and waterproof but I'm not sure if it'd take the temperatures just under the tiles.
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On 11/07/18 16:55, me wrote:

Doesnt stick well to olefin style plastics
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On 11/07/2018 16:20, The Natural Philosopher wrote:

Shoe glue: more likely to be PVC than PU?
Agree, otherwise. Also some double sided tapes are quite good (but may not survive the temperatures for long).
I suppose it *is* polythene? If not, you have a reasonable chance for PVC with solvent weld.
Bitumen mastics stick to most things and last well out of sunlight (and even in sunlight).
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On 11/07/18 17:20, newshound wrote:

Mmm. true enuff

Yeah. had forgotten those
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On 11/07/2018 16:20, The Natural Philosopher wrote:

Reliably? My experience with "traditional" Bostic that it works extremely well when first applied but after around 10 years in a normal indoor house environment it becomes brittle and the two glued surfaces just fall apart. I would expect this brittleness to occur much sooner in a hot environment such as the underside of roof tiles in the summer.

Could Solvent Weld, as used on plastic plumbing pipes be used? Definitely try on scrap plastic sheeting first as it works by "dissolving" the joining layers of the plastic and it may be too aggressive in that it creates holes in the existing sound bit of sheeting already under the tiles..
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I mended our inflatable "swimming pool" with "Sticks Like Shit" and that worked a bloody treat.
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I've got a tube of this stuff for repairing gutters https://www.toolstation.com/shop/Adhesives+%26+Sealants/d180/Building+%26+Roofing+Sealants/sd3351/Roof+%26+Gutter+Sealant+300ml/p11002
Do you think that would work?
By the way I don't know what type of plastic it is. It's not like the damp membrane that's used for concrete floors for example. And there's no manufacturer's markings so I just don't know.
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ID it first! -- as there are a few plastic that won't glue at all, at least as an installed roof membrane. Snip a bit off, and google "burn test"...
Thomas Prufer
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On Thursday, 12 July 2018 10:08:22 UTC+1, Thomas Prufer wrote:> ID it first! -- as there are a few plastic that won't glue at all, at least as

Great idea Thomas. I found this web page so I'll be burning a bit later on: https://boedeker.com/Technical-Resources/Technical-Library/Plastic-Identification
Also it's definitely not a felt, it's definitely a plastic. When it hasn't got a hoile in it it does its job really well and keeps the rain out but I'm not convinced it is made for the purpose of being used as a roof membrane.
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Is this felt sarking? Do you actually need the waterproofing? My understanding of sarking is it's designed to be waterproof while the roof is being constructed. When it is, the roof tiles do that job. The sarking is then serving no purpose, except possibly as a dust barrier and a surface for damp air to condense on (not in a good way). At this point, fixing holes in the sarking does no good.
Theo
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On 11/07/18 23:11, Theo wrote:

No. It serves a far more important purpose. A wind barrier. It helps stop tiles being lifted off by strong winds

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On Thursday, 12 July 2018 08:22:47 UTC+1, The Natural Philosopher wrote:

also acts as the 2nd waterproof layer, which is sometimes needed.

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snipped-for-privacy@gmail.com writes

Particularly for dry blowing snow!
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On 11/07/2018 16:08, me wrote:

Define 'plastic sheet' .
Do you mean the modern breathable membrane used instead of roofing felt, or an additional vapour barrier fixed to the underside of the rafters ?.
Or is this the multi-foil insulation that is common in Southern Europe but less so here ?.
Unless there is a defect with the tiles, you won't get water inside so why 'fix' it.
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