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 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.
 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.
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
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
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:
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.
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
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