Waterproof roofing for shed?

On 30/10/2015 07:31, Terry Pinnell wrote:

Appreciate that there is a wide differences between different "qualities" of felt roof...
You basic thin "shed" felt applied as a single layer and nailed on, its unlikely to offer more than a few years of reliable service (if at all).
Whereas a hessian re-enforced 2mm thick SBR[1] nail prep sheet glued or nailed on, then covered with 2mm SBR under felt, and finally finished with a 4mm thick SBR cap sheet with mineral finish will last decades as long as the substrate its fixed to is decent and stable.
[1] One of a number of trade names for a bitumen material modified with synthetic rubber or latex compounds to make it tougher and more flexible with age. Often available as "torch on" felts which are highly effective and very quick and easy to apply.
--
Cheers,

John.
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Thanks John. The only local man I've chosen so far to visit for a quote sounded enthusiastic about the 'torch on' felt. He also raised the alternative of rubber sheeting. I'll ask him about Onduline (or Wicke's equivalent) when he comes.
Although speaking from the depths of ignorance, I'd have thought that any method that did NOT involve seams and nails had a head start? Surely nails (or screws) are potential leak points, apart from the extra labour. After my latest inspection, I'm fairly sure that's where my current leaks are coming through.
Also, is there no spray-on stuff that dries over the top of the existing roof to a tough waterproof finish. A sort of industrial equivalent to these tins of paint-on covering I've been buying from Homebase?
--
Terry, East Grinstead, UK

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On 30/10/2015 13:58, Terry Pinnell wrote:

Well if you are going to use nails you use them to fix the first layer only. A so called "nail prep" layer. This is a felt with hessian or similar fibres in it that make it much harder to pull the nails through. You fix that down with clout nails using a random pattern all over. Then you torch the underlay directly onto that. Once that is on there are no *visible* nails.
You normally run of felt across the roof. starting from the bottom of the slope at the eves, and work up. The next strip overlaps the previous by abut 4" to 6" and directs any water over the previous strip. The top strip needs to saddle the ridge such that it covers the top of the strips of felt on both sides. (there are times where you can run an underlay perpendicular to the final layer).
When you fix the cap sheet you arrange for the overlap to be in a different place from the layer below. The cap sheet also has one "sticky" edge about 4" wide on the mineral finish side, That means that the two layers actively bond to each other at the join.
Have a look at:

https://www.youtube.com/watch?vLsTbXdERSU

for an example of use.

Indeed - if the nails penetrate from the surface they can be leak points. Having them just fix an undersheet however avoids that.

You could just stick a torch on cap sheet over the lot.
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Cheers,

John.
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On Fri, 30 Oct 2015 13:58:06 +0000, Terry Pinnell wrote:

Might be but I wouldn't bank on it. Hot dipped galvanised clouts have farly large diameter shanks and a big flat head. The bitumen displaced by the shanks is compressed back around the shank by the head, in effect they are self sealing. But is the spacing is too wide and or movement the flet can fatigue and fail around the head of the nail.
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Cheers
Dave.
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On Friday, 30 October 2015 07:31:25 UTC, Terry Pinnell wrote:

If you want a rapid fix, just another layer of felt will do it. Easily diyed.
NT
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It must have been very badly done to only last 6 years. Even a poor felt roof should last 20 or so - especially on a shed with presumably a pitched roof.
--
*If a pig loses its voice, is it disgruntled? *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Fri, 30 Oct 2015 13:55:50 +0000 (GMT), Dave Plowman (News) wrote:

first

Get about that on our felted shed roof. But that's using "shed roof felt" which is very lightweight compared to proper roofing felt.
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Dave.
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