Lateral thought - sarking

[Tim S]

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

[meow2222]

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

NT

[John Rumm]

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

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.

[Tim S]

John Rumm coughed up some electrons that declared:

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

[Dave Liquorice]

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...

[Kipper at sea]

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.

[fred]

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.

[Tim S]

fred coughed up some electrons that declared:

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

[Man at B&Q]

14:27 +0100, Tim S wrote:

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

[Man at B&Q]

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

MBQ

[Andrew Gabriel]

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

[RobertL]

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

[Tim S]

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

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

[fred]

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]

In article , Tim S writes

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]

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.

[Tim S]

fred coughed up some electrons that declared:

Sounds an excellent method - thanks!

Tim

[Man at B&Q]

What exactly is this "trough"?

MBQ

[fred]

Fixed width font reqd:

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

Outside

TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS RRR RRR RRR Ventilated Air Space RRR CCCCCCCCCCCRRRCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCRRRCCCCCCCC CCCCCCCCCCCRRRCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCRRRCCCCCCCC CCCCCCCCCCCRRRCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCRRRCCCCCCCC PBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBP

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

TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS RRR RRR RRR Ventilated Air Space RRR RRR RRR RRR RRR RRR RRR CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC PBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBP

Inside

I wouldn't use the first option unless I was sure the roof was drum tight water wise.

[Man at B&Q]

n

=A0 =A0 =A0 =A0 =A0 =A0RRR =A0 =A0 =A0 =A0

RR =A0 =A0 =A0 =A0

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0

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

MBQ