OK...

A poser:

As I understand it, sarking is for windproofing, not water proofing a tiled
roof?

Then, if we install celotex between and under the rafters and therefore
become concerned that we maintain good ventilation above the celotex, then
why not just remove the sarking and let the roof breath through the tiling?
The celotex (at al) will give us our wind proofing...

Patches of my sarking are falling off and I see no evidence of water ingress
in those areas, but I do feel the air moving when the wind blows.

Just a thought...

Tim

Tim S wrote:
OK...

A poser:

As I understand it, sarking is for windproofing, not water proofing a tiled
roof?

On shallow pitches its definitely used for waterproofing. I cant
answer the other question though.


NT

Tim S wrote:
OK...

A poser:

As I understand it, sarking is for windproofing, not water proofing a tiled
roof?

its also a second line of defence against water. (a felted and battened
roof is reasonably water tight even before tiled).

Then, if we install celotex between and under the rafters and therefore
become concerned that we maintain good ventilation above the celotex, then
why not just remove the sarking and let the roof breath through the tiling?
The celotex (at al) will give us our wind proofing...

Patches of my sarking are falling off and I see no evidence of water ingress
in those areas, but I do feel the air moving when the wind blows.

Just a thought...

If you leave it in place, make sure there are some vents at the bottom
of the rafters, and a gap behind the celotex, then you can probably
achieve enough through draft at the top by just cutting away a 4" strip
of sarking near the ridge.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

John Rumm coughed up some electrons that declared:

Tim S wrote:
OK...

A poser:

As I understand it, sarking is for windproofing, not water proofing a
tiled roof?

its also a second line of defence against water. (a felted and battened
roof is reasonably water tight even before tiled).

Then, if we install celotex between and under the rafters and therefore
become concerned that we maintain good ventilation above the celotex,
then why not just remove the sarking and let the roof breath through the
tiling? The celotex (at al) will give us our wind proofing...

Patches of my sarking are falling off and I see no evidence of water
ingress in those areas, but I do feel the air moving when the wind blows.

Just a thought...

If you leave it in place, make sure there are some vents at the bottom
of the rafters, and a gap behind the celotex, then you can probably
achieve enough through draft at the top by just cutting away a 4" strip
of sarking near the ridge.


It could be a useful technique on a few blind areas under windows (my dormer
walls are externally tiled too).

On Fri, 03 Apr 2009 01:14:27 +0100, Tim S wrote:

As I understand it, sarking is for windproofing, not water proofing a
tiled roof?

No it's secondary waterproofing to guard against broken/slipped tiles or
heavy rain/wind blowing water in between the tiles. Don't under estimate
the power of the wind to blow water up hill...

Is cellotex/kingspan a closed cell foam? Will it absorb moisture if used
as you suggest? Remember youi can only get at the face of any joins to
seal them against wet as well. To save stripping the roof to replace the
sarking I guess you could put a membrane over the rafters tape any joins
then the insulation on top. Remembering to get overlap between rows of
membrane the right way round...

--
Cheers
Dave.

On Apr 3, 1:14*am, Tim S wrote:
OK...

A poser:

As I understand it, sarking is for windproofing, not water proofing a tiled
roof?

Then, if we install celotex between and under the rafters and therefore
become concerned that we maintain good ventilation above the celotex, then
why not just remove the sarking and let the roof breath through the tiling?
The celotex (at al) will give us our wind proofing...

Patches of my sarking are falling off and I see no evidence of water ingress
in those areas, but I do feel the air moving when the wind blows.

Just a thought...

Tim

The saking, underlay, felting or breather membrane, call it what you
like is soley a vapour barrier. Covering wind driven snow and frost
condensation. It will cut down on wind ingress but not totally unless
all horizontal and vertical joints are taped.

An air gap is important as it keeps the gap between insulation and
roof covering aired to stop mildew growth.

In article et, Dave
Liquorice writes
On Fri, 03 Apr 2009 01:14:27 +0100, Tim S wrote:

As I understand it, sarking is for windproofing, not water proofing a
tiled roof?

No it's secondary waterproofing to guard against broken/slipped tiles or
heavy rain/wind blowing water in between the tiles. Don't under estimate
the power of the wind to blow water up hill...

Well put.

When putting celotex between rafters it is even more important to
exclude water. If it creeps into the unventilated trough formed by the
celotex, between the rafters, it wont be able to escape and we all know
where that leads. This is the case even if the area above the celotex is
ventilated.

Definitely repair the sarking[1].

[1] In Scotland the roofs are fully boarded and that is the term used
for the boards rather than the felt.
--
fred
BBC3, ITV2/3/4, channels going to the DOGs

fred coughed up some electrons that declared:


When putting celotex between rafters it is even more important to
exclude water. If it creeps into the unventilated trough formed by the
celotex, between the rafters, it wont be able to escape and we all know
where that leads. This is the case even if the area above the celotex is
ventilated.

Definitely repair the sarking[1].

Since you put it like that, it makes sense.

I'll get some spare and work it into the hole so the overlaps are more or
less right, then tape it up with some duct tape.

Cheers

Tim

On Apr 3, 9:27*am, fred wrote:
In article et, Dave
Liquorice writesOn Fri, 03 Apr 2009 01:14:27 +0100, Tim S wrote:

As I understand it, sarking is for windproofing, not water proofing a
tiled roof?

No it's secondary waterproofing to guard against broken/slipped tiles or
heavy rain/wind blowing water in between the tiles. Don't under estimate
the power of the wind to blow water up hill...

Well put.

When putting celotex between rafters it is even more important to
exclude water. If it creeps into the unventilated trough formed by the
celotex, between the rafters, it wont be able to escape and we all know
where that leads.

The point is that you simply don't do it without arranging adequate
ventilation. It's not all about letting water ingress from outside
escape.

MBQ

On Apr 3, 9:32*am, Tim S wrote:
fred coughed up some electrons that declared:



When putting celotex between rafters it is even more important to
exclude water. If it creeps into the unventilated trough formed by the
celotex, between the rafters, it wont be able to escape and we all know
where that leads. This is the case even if the area above the celotex is
ventilated.

Definitely repair the sarking[1].

Since you put it like that, it makes sense.

I'll get some spare and work it into the hole so the overlaps are more or
less right, then tape it up with some duct tape.

You *still* need to arrange for ventilation of the void between the
insulation and the sarking.

MBQ

In article ,
Tim S writes:
OK...

A poser:

As I understand it, sarking is for windproofing, not water proofing a tiled
roof?

No, it's a second line of defence against moisture ingress.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On Apr 3, 1:14*am, Tim S wrote:
OK...

A poser:

As I understand it, sarking is for windproofing, not water proofing a tiled
roof?

Then, if we install celotex between and under the rafters and therefore
become concerned that we maintain good ventilation above the celotex, then
why not just remove the sarking and let the roof breath through the tiling?
The celotex (at al) will give us our wind proofing...


i put in a BC application to add celotex under an existing slate
roof. i proposed slate/waterproof_sarking /50mm_airgap/celotex/
thinsulex/plasterboard. the BCO commented that if I used a breathable
membrane instead of waterproof sarking then I only needed a 10mm
airgap.

Robert

Man at B&Q coughed up some electrons that declared:

You *still* need to arrange for ventilation of the void between the
insulation and the sarking.

MBQ

Already have a plan for that - soffit vents at the lower end and dormer
fascia vents at the top (no need for ridge vents as dormer roof is within a
foot of the ridge). Need to ensure that all voids are interconnected for
free air flow, but BCO is happy with solution, although a bit
unconventional.

50mm gap under the sarking above celotex at all points.

Cheers

Tim

In article
, Man
at B&Q writes
On Apr 3, 9:27*am, fred wrote:

When putting celotex between rafters it is even more important to
exclude water. If it creeps into the unventilated trough formed by the
celotex, between the rafters, it wont be able to escape and we all know
where that leads.

The point is that you simply don't do it without arranging adequate
ventilation. It's not all about letting water ingress from outside
escape.

Yes, the area above the Celotex needs ventilated but that trough cannot
be ventilated (otherwise the Celotex would be bypassed) so it is
imperative that moisture doesn't get there in the first place.
--
fred
BBC3, ITV2/3/4, channels going to the DOGs

In article , Tim S
writes
fred coughed up some electrons that declared:


When putting celotex between rafters it is even more important to
exclude water. If it creeps into the unventilated trough formed by the
celotex, between the rafters, it wont be able to escape and we all know
where that leads. This is the case even if the area above the celotex is
ventilated.

Definitely repair the sarking[1].

Since you put it like that, it makes sense.

I'll get some spare and work it into the hole so the overlaps are more or
less right, then tape it up with some duct tape.

Spot on in checking the overlaps are right, always to the outside at the
bottom so that any runoff is onto the top of the sheet below.

Watch out on the quality of the tape, I've seen the adhesive on the
cheaper ones fail after time, particularly in extremes of hot and cold
as you get in a roof.

Last time I did a similar, I got the patch in place, then pulled it back
to apply silicone to its upper face along the top and down the sides
(none at the bottom), then pushed it back in place, secured temporarily
with tape and put a couple of tile battens across the back of it to
support it properly. The silicone was really as an adhesive but would
also stop any sideways spread of water, possibly bypassing the patch.

It was blowing a gale at the time so I got a chance to see just what
forces play around the roofspace when the wind is blowing.
--
fred
BBC3, ITV2/3/4, channels going to the DOGs

In article
,
RobertL writes
On Apr 3, 1:14*am, Tim S wrote:
OK...

A poser:

As I understand it, sarking is for windproofing, not water proofing a tiled
roof?

Then, if we install celotex between and under the rafters and therefore
become concerned that we maintain good ventilation above the celotex, then
why not just remove the sarking and let the roof breath through the tiling?
The celotex (at al) will give us our wind proofing...


i put in a BC application to add celotex under an existing slate
roof. i proposed slate/waterproof_sarking /50mm_airgap/celotex/
thinsulex/plasterboard. the BCO commented that if I used a breathable
membrane instead of waterproof sarking then I only needed a 10mm
airgap.

10mm, interesting to know, thank you.

Watch out using breathable membranes over boarded roofs, they don't like
getting scuffed and will leak if they do.

Btw, I'd say skip the thinsulex and save some cash.
--
fred
BBC3, ITV2/3/4, channels going to the DOGs

fred coughed up some electrons that declared:

In article , Tim S
writes
fred coughed up some electrons that declared:


When putting celotex between rafters it is even more important to
exclude water. If it creeps into the unventilated trough formed by the
celotex, between the rafters, it wont be able to escape and we all know
where that leads. This is the case even if the area above the celotex is
ventilated.

Definitely repair the sarking[1].

Since you put it like that, it makes sense.

I'll get some spare and work it into the hole so the overlaps are more or
less right, then tape it up with some duct tape.

Spot on in checking the overlaps are right, always to the outside at the
bottom so that any runoff is onto the top of the sheet below.

Watch out on the quality of the tape, I've seen the adhesive on the
cheaper ones fail after time, particularly in extremes of hot and cold
as you get in a roof.

Last time I did a similar, I got the patch in place, then pulled it back
to apply silicone to its upper face along the top and down the sides
(none at the bottom), then pushed it back in place, secured temporarily
with tape and put a couple of tile battens across the back of it to
support it properly. The silicone was really as an adhesive but would
also stop any sideways spread of water, possibly bypassing the patch.

It was blowing a gale at the time so I got a chance to see just what
forces play around the roofspace when the wind is blowing.

Sounds an excellent method - thanks!

Tim

On Apr 3, 10:12*am, fred wrote:
In article
, Man
at B&Q writesOn Apr 3, 9:27*am, fred wrote:

When putting celotex between rafters it is even more important to
exclude water. If it creeps into the unventilated trough formed by the
celotex, between the rafters, it wont be able to escape and we all know
where that leads.

The point is that you simply don't do it without arranging adequate
ventilation. It's not all about letting water ingress from outside
escape.

Yes, the area above the Celotex needs ventilated but that trough cannot
be ventilated (otherwise the Celotex would be bypassed)

What exactly is this "trough"?

MBQ

In article
s.com, Man at B&Q writes
On Apr 3, 10:12*am, fred wrote:
In article
, Man
at B&Q writesOn Apr 3, 9:27*am, fred
wrote:

When putting celotex between rafters it is even more important to
exclude water. If it creeps into the unventilated trough formed by the
celotex, between the rafters, it wont be able to escape and we all know
where that leads.

The point is that you simply don't do it without arranging adequate
ventilation. It's not all about letting water ingress from outside
escape.

Yes, the area above the Celotex needs ventilated but that trough cannot
be ventilated (otherwise the Celotex would be bypassed)

What exactly is this "trough"?


Fixed width font reqd:

From the top:
T Tiles
S Sarking
R Rafters
C Celotex
PB Plasterboard

Outside

TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT TTTTTTT
SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS SSSSSSS
RRR RRR
RRR Ventilated Air Space RRR
CCCCCCCCCCCRRRCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCRRRC CCCCCCC
CCCCCCCCCCCRRRCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCRRRC CCCCCCC
CCCCCCCCCCCRRRCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCRRRC CCCCCCC
PBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPB PBPBPBP

Inside

The trough is the area occupied by the rafter but shielded from
ventilation by the celotex. If the wood in this area gets damp for any
reason then it will not clear easily as it is unventilated. Any moisture
egress would need to be through the wood to the upper ventilated part
which likely leave it damp longer and more susceptible to rot.

If the Celotex is placed below the rafters then there is no trough and
residual damp from moisture ingress can be cleared more easily by the
ventilation but of course this has the huge disadvantage of loss of
internal space.

Outside

TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT TTTTTTT
SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS SSSSSSS
RRR RRR
RRR Ventilated Air Space RRR
RRR RRR
RRR RRR
RRR RRR
CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC CCCCCCC
CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC CCCCCCC
CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC CCCCCCC
PBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPB PBPBPBP

Inside

I wouldn't use the first option unless I was sure the roof was drum
tight water wise.
--
fred
BBC3, ITV2/3/4, channels going to the DOGs

On Apr 3, 11:19*am, fred wrote:
In article
s.com, Man at B&Q writes



On Apr 3, 10:12*am, fred wrote:
In article
, Man
at B&Q writesOn Apr 3, 9:27*am, fred
wrote:

When putting celotex between rafters it is even more important to
exclude water. If it creeps into the unventilated trough formed by the
celotex, between the rafters, it wont be able to escape and we all know
where that leads.

The point is that you simply don't do it without arranging adequate
ventilation. It's not all about letting water ingress from outside
escape.

Yes, the area above the Celotex needs ventilated but that trough cannot
be ventilated (otherwise the Celotex would be bypassed)

What exactly is this "trough"?

Fixed width font reqd:

From the top:
T Tiles
S Sarking
R Rafters
C Celotex
PB Plasterboard

* * * * * * * * * * * * Outside

* TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT TTTTTTT
* SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS SSSSSSS
* * * * * * *RRR * * * * * * * * * * * * * * * *RRR * * * *
* * * * * * *RRR * Ventilated Air Space * * * * RRR * * * *
* CCCCCCCCCCCRRRCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCRRRC CCCCCCC
* CCCCCCCCCCCRRRCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCRRRC CCCCCCC
* CCCCCCCCCCCRRRCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCRRRC CCCCCCC
* PBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPB PBPBPBP

* * * * * * * * * * * * *Inside * * * * * * * * * * * * * * * *

The trough is the area occupied by the rafter but shielded from
ventilation by the celotex. If the wood in this area gets damp for any
reason then it will not clear easily as it is unventilated. Any moisture
egress would need to be through the wood to the upper ventilated part
which likely leave it damp longer and more susceptible to rot.

OK. I disagree. Anyone more knowledgeable care to comment?

MBQ

In article ,
fred writes:

Watch out using breathable membranes over boarded roofs, they don't like
getting scuffed and will leak if they do.

Breathable membranes don't like the back being touched by anything,
or they can become leakable membranes. I didn't understand how that's
supposed to work when they rest on a rafter (except that the natural
sag between rafters will direct any water away from them), so I used
non-breathable when I needed to replace some of the old tar ones (which
were non-breathable until they wore out). In my case, there's a vent
formed at every sarking overlap, so there's no shortage of loft
ventilation.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On 3 Apr, 01:14, Tim S wrote:

why not just remove the sarking and let the roof breath through the tiling?

Breathing isn't the same as (or the opposite of) windproofing.
Breathing allows a low flow rate with only a small pressure
differential, windproofing prevents the flow rate becoming excessive
even when the pressure difference is high (i.e. the wind is blowing).
A breathable roof still shouldn't be draughty. Although an unsarked
roof would be breathable, it will also be unacceptably draughty.

Be careful with ventilation above roof insulation. Condensation in
that insulation layer will cause rot, rot you don't notice, and by the
time you do the repair costs get pretty serious.

Andy Dingley coughed up some electrons that declared:

On 3 Apr, 01:14, Tim S wrote:

why not just remove the sarking and let the roof breath through the
tiling?

Breathing isn't the same as (or the opposite of) windproofing.
Breathing allows a low flow rate with only a small pressure
differential, windproofing prevents the flow rate becoming excessive
even when the pressure difference is high (i.e. the wind is blowing).
A breathable roof still shouldn't be draughty. Although an unsarked
roof would be breathable, it will also be unacceptably draughty.

Ah, but the celotex and taped joints would prevent the wind from entering
the loft/room space, surely. Not discounting what others have said
abou****er blowing in, but this was my original line of thought...

Be careful with ventilation above roof insulation. Condensation in
that insulation layer will cause rot, rot you don't notice, and by the
time you do the repair costs get pretty serious.

Indeed. Given the abysmal lack of effort to ventilate anything there right
now, I'm surprised I haven't more rot than there is... But a roof rafter
would be very difficult to replace.

When I eventually get round to this (it's in phase 2, much later...) I will
put some little probe holes (like a short bit of 8mm pipe glued in) and
plugs in various locations in the ceiling so I can stick an RH probe in for
occasional monitoring.

Cheers

Tim

In article
, Man
at B&Q writes
On Apr 3, 11:19*am, fred wrote:

The trough is the area occupied by the rafter but shielded from
ventilation by the celotex. If the wood in this area gets damp for any
reason then it will not clear easily as it is unventilated. Any moisture
egress would need to be through the wood to the upper ventilated part
which likely leave it damp longer and more susceptible to rot.

OK. I disagree.

OK. What are you disagreeing about?
--
fred
BBC3, ITV2/3/4, channels going to the DOGs

In article , Andrew Gabriel
writes
In article ,
fred writes:

Watch out using breathable membranes over boarded roofs, they don't like
getting scuffed and will leak if they do.

Breathable membranes don't like the back being touched by anything,
or they can become leakable membranes. I didn't understand how that's
supposed to work when they rest on a rafter (except that the natural
sag between rafters will direct any water away from them), so I used
non-breathable when I needed to replace some of the old tar ones (which
were non-breathable until they wore out). In my case, there's a vent
formed at every sarking overlap, so there's no shortage of loft
ventilation.

A relative who is a building surveyor (working in Scotland) has reported
that some contractors have expressed a preference for using tougher, non
breathable sarking even though it requires a greater ventilation space.
--
fred
BBC3, ITV2/3/4, channels going to the DOGs