Old roofing felts used a carrier of either paper or rag, or it could
be asbestos.
The cheapest sort in the Sheds is known as 1B type which has weak
paper carrier.
2nd problem is that the coating is oxidised bitumen, which through UV
degradation becomes brittle and cracks.
This then allows water into carrier which if paper, is bad news.
Use a high performance felt, normally based on polyester these days.
Secondly look for one coating with a "modified" bitumen, typically SBS
or APP.
These have good elongation properties and so wont crack.
This combined with a strong polyester base carrier will give you up
to 25 years life. Normally you have at least 2 layers, with top layer
having
granular coating to protect from sun/uv damage.
You can get product that is torched on ie with adhesive bitumen
provided
on the roll.
Or use a cold adhesive, or good old hot bitumen, but as with Torch On
be careful. Yes there are also butyl and other types laid cold; you
can also get customised blankets of the stuff; this means you do not
have all the joins to make yourself when laying product from a roll;
thats where water can get in.
There are alos some self adhesive types.
See http://www.ruberoid.co.uk/

"Richard Sterry" wrote in message ...
I'm sure this must be a regular chestnut on this forum, in which case
perhaps there are some useful sites and FAQs which people would be kind
enough to point me at.

I have a very large lounge, with a flat roof. It looks rather like it's an
extension to the main house, but is in fact part of the standard design and
the joists are properly linked in with the main roof. It's in effect a
bungalow with bedrooms within the roof space, with windows on the gable
ends, and from the 'spare' roof space on one side of the first floor I can
peep through and see above the lounge ceiling. Because the joists are the
same thickness as those in the main house, they are perhaps a little deeper
than you might expect in the circumstances - 9" in fact. The relevance of
this will become apparent later.

The main problem is that the roof is done in the traditional 'tar-and-felt'
method, and of course now that it's a few years old, nice big blister have
appeared and it's only a matter of time before the darned thing starts to
leak and spoils the decor. I aim to act before that stage is reached, but am
reluctant to have the roof redone in the same hideously old-fashioned way.
When you come to sell a house with a tar-and-felt flat roof, no matter how
good condition it's in, the surveyors have an absolute field day. I'm sure
they have a little word processor macro specifically to spit out a few
paragraphs of doom and gloom about flat roofs! (Needs redoing imminently,
will cost lots of money, blah blah blah, rhubarb rhubarb rhubarb.)

So, I am looking for a much more elegant and long-term covering, even if it
is going to cost considerably more. At least I'd have peace of mind, if
nothing else. Conversion to a pitched roof is not really practicable for a
variety of reasons, so I'm looking at more modern methods of recovering the
existing flat roof.

One of my neighbours appears to have a sheet metal covering on his, though I
have yet to approach him to allow me to examine it at close quarters. I have
also heard of custom-made rubber covers, rather like a fitted bottom sheet
on a bed, which is made to size and then fitted on site. I suppose it would
resemble a very heavy duty garden pond liner.

Finally, there is the question of roof insulation - there isn't any! I am
wondering whether to get the wooden boarding removed from the roof as well
as the old covering, and get the contractors to lay rolls of insulation.
(This is the method the aforementioned neighbour opted for some years ago.)
With 9" joists there would be room for, say 6" (150mm) of insulation, with a
3" air gap on top to ventilate the joists and boards. Would that be enough
of a gap?

Alternatively, I could do what I did at our last house, and that is insulate
on top of the boarding before the new covering goes on. It was something
like 50mm Kingspan if you know the stuff, only instead of being dense foam
sandwiched in foil it was sandwiched in roofing felt. It worked very well,
but the weak point was the conventional roof covering above it, which did
blister in time. Having said that, it would probably still have had to
deteriorate a lot further before it actually leaked.

Sorry this was rather verbose - please feel free to clip most if it out in
replies.

Ideas pleas?