On 13 Aug, 08:59, harry wrote:
On 13 Aug, 08:56, 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.- Hide quoted text -

- Show quoted text -

Oh and you will need eves ventilation (behind the gutters) and
consider ridge ventilators too.

not part of trad. roof construction tho are they? why would they be
needed in this instance?

underfelts should also allow water to drain off them over the eaves
(usually lapped into troughings) and not allow water to pool or "pond"
behind the fascia boards... and rot the underfelts and drain on/into
the walls...

Jim K