Russell griffiths

Roof sarking boards. Treated, untreated.

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Hi all, our roof will be metal raised seam 

it is supposed to sit on 18mm osb which I don’t plan on doing, I’m planning on using a sarking board just like our Scottish neighbours do

would you use treated timber or just rough sawn. 

I have two views on this but thought I would get a collective opinion. 

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What is wrong with OSB?  Cheap and solid.

 

But our box profile rood sits on top of that on battens and counter battens, not directly onto the OSB.

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Our sarking is 18mm OSB3.  As @ProDave says, cheap and solid.  We also have counterbattens on top of the sarking, the membrane laid over those, then the battens nailed on top as normal.  Makes for a nice solid roof. IMHO.

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I have always used 22mm treated sarking because I can’t get anything elts locally. If you can get rougth sawn then I think it should be grand as it will never get wet. My house roof has 100 year old untreated sarking without any membrane and it’s still remarkably good. Couple of points, if you use sarking board that is wet then it shrinks considerably.... I got up to 10mm gaps after tightly fitting 150mm wide boards. not a problem on a traditional Scottish slate roof as the slates can withstand that movement BUT if you lay the sarking and then attach your tin and then the sarking shrinks it could distort the fixings slightly as the wood shrinks, it’s minimal and I have not had a problem but on a swanky new house roof I would at least take this into consideration. Sarking is great for slates as it takes and holds direct nailing very well, also I am much happier with a roof that can breath and the gaps in the boards are  great for keeping everything nice and vented. I have done my shed roof in structural ply as at the time osb was out of stock and they gave me the ply for the same price, it was a big disaster as the membrane froze and then water leaked in and soaked the ply which immediately got black mould growing on the inside...... I vowed never to use anything but sarking boards again as they are so robust. 

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The only advantage I can see in using planks rather than sheets is that they are individually easier to work with, and you can miss one out every so often to make it easier to climb up the roof whilst building.

Sheets are cheaper, quicker, and give considerably better racking strength.

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I probably missed out an item that could distort the answers

if I use a sarking board I will do it hit n miss fashion so only half as many boards to cover the same area. 

 

If I had a smaller roof I would probably go osb straight away, but being I have 450m square to do I’m looking at alternative products. 

 

 

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Sarking boards look great then and seeing how the roof is fixed, a bit of shrinking won’t do any harm. I can’t see how using untreated would be a bad thing. 

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On 08/01/2019 at 18:38, JSHarris said:

We also have counterbattens on top of the sarking, the membrane laid over those, then the battens nailed on top as normal.

 So Roof up, osb covering it, battens running top to bottom, then membrane over the top then battens running side to side. Yes/no??

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Also, whilst I think I know what sarking is, why is it not always used? Is it just a belt and braces thing?

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27 minutes ago, Big Neil said:

Also, whilst I think I know what sarking is, why is it not always used? Is it just a belt and braces thing?

 

It's very much a regional thing.  Sarking boards were always used with slate in Scotland, and it's become standard practice there to use sarking on any roof now.  Our new-build bungalow in Scotland had a tiled roof, bit still had plywood sarking.  I really like the idea of sarking boards, for a few reasons.  They stiffen up the roof structure a fair bit, help to prevent wind-wash through the membrane and also reduce rain noise a bit.  In addition they make life easier for the roofers, as there is a solid roof under their feet and they aren't reliant on just the strength of the battens when working up there.

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2 minutes ago, JSHarris said:

It's very much a regional thing.  Sarking boards were always used with slate in Scotland, and it's become standard practice there to use sarking on any roof now.  Our new-build bungalow in Scotland had a tiled roof, bit still had plywood sarking.  I really like the idea of sarking boards, for a few reasons.  They stiffen up the roof structure a fair bit, help to prevent wind-wash through the membrane and also reduce rain noise a bit.  In addition they make life easier for the roofers, as there is a solid roof under their feet and they aren't reliant on just the strength of the battens when working up there.

 

I tend to like the idea of a belt and braces approach anyway so i suppose it suits that philosophy plus adds some benefits. Was my process above correct? Sark - batten - membrane - batten?

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3 minutes ago, Big Neil said:

 

I tend to like the idea of a belt and braces approach anyway so i suppose it suits that philosophy plus adds some benefits. Was my process above correct? Sark - batten - membrane - batten?

 

Yes, that's what we have.  OSB3 sarking boards, then 50 x 25 counterbattens nailed through along the line of the rafters, then non-tenting membrane laid over the counterbattens, then 50 x 25 slate battens nailed to the counterbattens over the membrane.  It makes for a very solid roof, and because we have very deep (400mm) rafters, hung from a ridge beam, with no purlins, it also adds a lot of cross bracing and stiffness to the whole roof structure.

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ahhhhh - I see. no insulation between the battens at all? Would this be a reasonable option? You know, given that there's a gap there anyway, or just pointless?

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Membrane can be under the counterbattens. As designed mine would be:

 

1) Rafters (JJI I-beams)

2) 150x22 treated sarking boards.

3) Membrane.

4) 12x38 treated counterbattens up and down the roof, over the rafters and nailed through to them.

5) 38x50 treated battens across the roof.

6) Profile steel sheeting.

 

(Actually, 1-4 are done but the membrane has failed so I'm planning another layer of membrane over the counterbattens).

 

Putting insulation between the battens would be a bad idea. The purpose of the gaps is to allow ventilation and to allow any water that gets under the outer rainscreen (tiles, steel or whatever) to run down. Water can get there by being blown by wind from odd angles or by condensation on the back of the rainscreen. Apart from marking where the rafters are for future nailing the counterbattens' main purpose is to allow air to get up past the battens and water drops to run down the roof. If the battens were nailed direct to the sarking or the gap between the counterbatten but under the battens was filled with insulation then water running down the roof would be dammed by the battens leaving them permanently soaked. Even treated wood won't stand for that.

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so is the need for movement of moisture the reason for double battening even when a roof has sarking boards on it? Otherwise why not just horizontal battens as I see on most new roofs (although admittedly volume built)?

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23 minutes ago, Big Neil said:

so is the need for movement of moisture the reason for double battening even when a roof has sarking boards on it? Otherwise why not just horizontal battens as I see on most new roofs (although admittedly volume built)?

 

You can't assume that the final roof covering will remain 100% watertight- you have to plan for what happens when water gets under it.

A continuous horizontal batten laid directly onto the sarking would not allow for any drainage, so the purpose of the counter-batten is simply to space the tile batten off the roof, and give a drainage gap. 25mm seems the usual size but building regs (in Scotland at least) allow for 12mm minimum, IIRC.

If you really wanted to save a few pennies you could use little squares of ply or something as a spacer at each fixing point, but even someone as parsimonious as me would find that a bit stingey!

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