harry wrote:
On 13 Aug, 04:14, Adam Aglionby wrote:
Looking at getting late 1880`s 4 storey tenement re-slated, exposed
position , scotch slates are slipping towards gutter at increasing
rate, lead work evaporating etc. Flat platform in middle recovered 4
years ago.

Seem to have decided on Spanish slate as against reclaimed Scotch on
balance of cost and apperance, some areas would have to be Scotch, not
here.

Seems to be varying opinions on covering bleow new slates though, some
say use a breathable fabric and no additional slate vents, others
appear to be advsing non breathable and ` a few ` slate vents.

Seen loft to habitable conversions and BC seem very keen on lots of
slate vents. Whole of this roof is above habitated space.

Whats anyones experience of undertile fabrics and the pros and cons?

Thanks
Adam

The reason for istalling fabric is it makes the building weather proof
quickly & if there is a faultly tile/slate it keeps the waterout.
Keeps draughts and driving rain out too. Years ago they used
reinforced bitumous felt. But it was easily damaged and a fire hazard.
Then they used plastic & there were problems with condensation and
rot. So vents were installed to limit this. (They sometimes don't work
well)
The latest thing (last ten years) is the breathable membrane which is
"vapour permeable" but still waterproof. Its claimed this doesn't
need vents. Early days yet. The stuff is very tough. The main
thing about installation is to leave "droops" beween the rafters so
water can drain away and the battens stay dry underneath them and air
can circulate.

The reason for felt below slates is not to make them rainproof, although
there is a grain of truth about driving drain.

Salates overlap, and an overlappped roof is rainproof. It takes a lot to
drive rain UP a slope.

The function of the fabric is to make the roof WIND proof. There are
three reasons to do that, and the most important is to prevent suction
tearing the slates of or working them loose.

The second was to reduce heat loss from the loft..but see below

The third and least important is to slow airflow down and thereby
prevent rain crawling UP the slates. But with a double overlap, it has
to crawl 2/3rds of the way up a slate to get to the top, and in general,
it wont. The air is pretty dead behind slates even without sarking.

The fabric should never get wet, and a well laid slate roof without it
is perfectly rainproof. I've lived under some.
Once it was used, it was discovered that (especially in uninsulated
houses) it led to rot, because it acted to trap the house moisture under
it. That condensed on the cold sarking, and beams, and dripped around.

So controlled air movement, by vents, eave and ridge, and/or by
breathable membranes was introduced. Personally I think its way
overkill, because you are likely also to have a deal of insulation these
days and that implies a vapour barrier in the ceiling (foil backed
plasterboard) anyway, so there is virtually no humidity escaping into a
loft, ergo it really needs a lot less ventilation than the regulations
call for. However some houses still have water header tanks and so on up
there, so it is still useful to ensure ventilation.

If you have e.g. a modern CH system without a header, and better still
if you have a mains pressure hot water supply, and loft insulation, you
won't have water in the loft. So my instinct is to use as little
ventilation as the BCO will allow.

If you are thinking of converting to living space you will necessarily
be using something like foil backed Kingspan to insulate the rafters
with an air gap at the top anyway. Its almost arguable you need no
sarking at all in this case..

Anyway the point of this all is to show that airtightness is what you
want, and breath-ability or venting is slightly a hangover from 'days
before insulation'. With a vapour barrier at the boundary of the
habitable space, there is much less need for it.

I would personally go therefore for breathable, and as few vents as the
BCO will let you get away with.