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Under Slate felt , whats the story?



 
 
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  #1  
Old August 13th 10, 04:14 AM posted to uk.d-i-y
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Posts: 870
Default Under Slate felt , whats the story?

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

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  #2  
Old August 13th 10, 08:56 AM posted to uk.d-i-y
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Posts: 9,216
Default Under Slate felt , whats the story?

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.
  #3  
Old August 13th 10, 08:59 AM posted to uk.d-i-y
external usenet poster
 
Posts: 9,216
Default Under Slate felt , whats the story?

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.
  #4  
Old August 13th 10, 09:16 AM posted to uk.d-i-y
external usenet poster
 
Posts: 511
Default Under Slate felt , whats the story?

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
  #5  
Old August 13th 10, 09:23 AM posted to uk.d-i-y
external usenet poster
 
Posts: 9,586
Default Under Slate felt , whats the story?

On Thu, 12 Aug 2010 20:14:29 -0700 (PDT), Adam Aglionby wrote:

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


Had the modern breathable membrane stuff put in when we had our slate
roofs redone a few years back on the basis that the normal sarking
rots eventually and doesn't withstand exposure at all well. ie you
loose a slate and the sarking fails where day light (and water) can
get in between the slates... Along the gutter line our roofers put
18" (maybe 2') wide DPM with the membrane stopping 6" or so short of
the slate edge on top of it. DPM doesn't degrade...

Inside the roof it is dry, there is occasional condensation but it
does dry out no vents as such but it's still pretty drafty up
there... The lofts is much brighter as well, membrane is a light
colour rather than
dark.

--
Cheers
Dave.



  #6  
Old August 13th 10, 09:30 AM posted to uk.d-i-y
external usenet poster
 
Posts: 18,600
Default Under Slate felt , whats the story?

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.

  #7  
Old August 13th 10, 11:47 AM posted to uk.d-i-y
external usenet poster
 
Posts: 511
Default Under Slate felt , whats the story?

On 13 Aug, 09:30, The Natural Philosopher
wrote:
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.



??

it was a 60s invention to make life easier for re-roofers

Jim K
  #8  
Old August 13th 10, 12:11 PM posted to uk.d-i-y
external usenet poster
 
Posts: 2,367
Default Under Slate felt , whats the story?


Had the modern breathable membrane stuff put in when we had our slate
roofs redone a few years back on the basis that the normal sarking
rots eventually and doesn't withstand exposure at all well. ie you
loose a slate and the sarking fails where day light (and water) can
get in between the slates... Along the gutter line our roofers put
18" (maybe 2') wide DPM with the membrane stopping 6" or so short of
the slate edge on top of it. DPM doesn't degrade...


Had my roof reslated 2 years ago, with hand-finished Spanish slate -
very happy with it all round.

Similarly had Tyvek throughout the whole roof, with additional
waterproof felt/DPM on the lowest 18-24" and lapped into the gutters.

Inside the roof it is dry, there is occasional condensation but it
does dry out no vents as such but it's still pretty drafty up
there... The lofts is much brighter as well, membrane is a light
colour rather than
dark.


My roofspace is bone dry, all year round, no condensation. No roof
vents, though there is an original (1873) arrow-slit type vent in the
brickwork on one gable end.

My additional tip would be don't skimp on the detailing - leadwork,
chimney & other flashings, chimney maintainence, gutters, fascias &
soffits etc - get the lot done at the same time.

Get it all specified in the contract, and accept there may be cost
overruns if it becomes apparent additional remedial work is required.
Swallow the cost and get it all done.
  #9  
Old August 13th 10, 03:25 PM posted to uk.d-i-y
external usenet poster
 
Posts: 565
Default Under Slate felt , whats the story?


"Adam Aglionby" wrote in message
...
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.



Welsh Slate is the best in the world ... and no carbon offset bringing
across Europe.

  #10  
Old August 13th 10, 04:39 PM posted to uk.d-i-y
external usenet poster
 
Posts: 9,216
Default Under Slate felt , whats the story?

On 13 Aug, 15:25, "Rick Hughes"
wrote:
"Adam Aglionby" wrote in message

...

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.


Welsh Slate is the best in the world ... *and no carbon offset bringing
across Europe.


Welsh slate IS the best. You can have thinner slates and a lighter
roof and they weather significantly better, better appearance and last
longer. The trouble is the Taffs don't work as cheap as the spics,
the chinks or the portuguese.
 




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