To counter batten, or just batten?

[cwr]

Our drawings states that the whole slate covered roof should be battened and counter battened. This was the advice from an architect friend as best practice.

However, the builder has announced that he only intends to counter batten the warm roof sections as there's no need for the cold roof parts.

What are the thoughts on this? Should i insist on counter batons everywhere, over both cold and warm roofs? Thanks.

[Thorfun]

Depending on the roofing membrane counter battens may allow you to fully fill the rafters with insulation. Single battens will mean you’ll need an air gap between insulation and membrane. 

 

check with building control first though as I believe some will want an air gap regardless. 

[nod]

As above 

But your builder is correct 

[SuperJohnG]

What kind of roof is it? Just a normal trussed roof? 

[Iceverge]

2 hours ago, cwr said:

Our drawings states that the whole slate covered roof should be battened and counter battened. This was the advice from an architect friend as best practice.

However, the builder has announced that he only intends to counter batten the warm roof sections as there's no need for the cold roof parts.

What are the thoughts on this? Should i insist on counter batons everywhere, over both cold and warm roofs? Thanks.

You should insist on it.

 

If you have counter battens everywhere you can omit ventilation through your cold roof section, breathable membrane dependant of course, and ventilate above. 

 

This will make your cold roof perform properly as the wind won't be blowing through the insulation and there'll be a saving on soffit vents etc.

 

Venting an attic below the membrane is poor practice thermally and a hangover from days of yore . 

 

I like battening and counterbattening  in a practical sense in that the batten running  parallel to the rafters pinches the membrane fully making it much less likely to tear. 

 The counter battens then make a nice ladder to climb the roof within sticking your boot through the membrane. 

 

 

 

 

[Dave Jones]

builder is correct.

[Temp]

On 03/09/2021 at 20:48, cwr said:

However, the builder has announced that he only intends to counter batten the warm roof sections as there's no need for the cold roof parts.

 

Counter battens might be needed for several different reasons but... The roof should be designed so that water cannot pool on the top of the roof battens. This can happen when something stops the membrane from draping between the rafters. For example when insulation or sarking boards press it against the underside of the battens. In this case counter battens raise the tile battens allowing any water blown under the tiles to run down.

 

 

 

 

[Iceverge]

You're better off keeping the wind out of your cold loft. 

 

Breathable membrane taped at all joints. 

 

Batten and counterbatten and ventilate above the membrane. 

 

Attached are diagrams from proclima and tyvek and a greenbuildingstore video that from 3:30 discusses Thermal Bypass. 

 

 Pro Clima

[cwr]

Thanks for the replies. Enough to convince me to counter batten.

 

Perhaps in the vid (which I'll watch this eve), what is the best detail for taping the membrane at its lower edge? Does the membrane not needed to somehow dispense any water into the gutter, but needs to seal to the inner leaf of the cavity wall?

[Patrick]

On 03/09/2021 at 20:48, cwr said:

Our drawings states that the whole slate covered roof should be battened and counter battened. This was the advice from an architect friend as best practice.

However, the builder has announced that he only intends to counter batten the warm roof sections as there's no need for the cold roof parts.

What are the thoughts on this? Should i insist on counter batons everywhere, over both cold and warm roofs? Thanks.

In a roof . as a rule of thumb, without further looking into it, one can say:

ventialtion ventilation ventialtion.

There can be a lot of calculations made about where dew points are , about condensation risk assessments....

But at the end:

More ventialtion has 0 disadvantages

Less ventialtion has the potential to ruin your roof

(rule of thumb)

 

 

[cwr]

So, I spoke to the roofing sub contractor, who uses Proctor Roofsheild which is a breathable membrane which will be fitted with no sofit vents. So in effect giving a slightly ventilated cold roof, in as much as the ventilation is through the membrane. He also insists the counter battens are not needed. 

[Iceverge]

8 minutes ago, cwr said:

So in effect giving a slightly ventilated cold roof, in as much as the ventilation is through the membrane

I'm not sure this will pass regs as you either need air passing below your membrane via soffit vents or a clear 50mm space above and over membrane ventilation. What kind of insulation are you using in your cold roof? 

 

On 07/09/2021 at 11:22, Patrick said:

rule of thumb, without further looking into it, one can say:

ventialtion ventilation ventialtion

 

Timber needs to be able to dry but there's difference between that and letting a gale blow through the structure of the building. 

 

 

[Patrick]

2 minutes ago, Iceverge said:

Timber needs to be able to dry but there's difference between that and letting a gale blow through the structure of the building.

Maybe- but there is no harm done having "a gale blow through the structure of the building" as long as it s on the outside of the insualtion and VCL . But there is a lot of harm when not ventilating (or too little) , therefore i stand by that statement.

[Thorfun]

1 hour ago, cwr said:

So, I spoke to the roofing sub contractor, who uses Proctor Roofsheild which is a breathable membrane which will be fitted with no sofit vents. So in effect giving a slightly ventilated cold roof, in as much as the ventilation is through the membrane. He also insists the counter battens are not needed. 

counter battens aren't required *if* you have a gap between the membrane and the insulation. it doesn't have to be 50mm with Roofshield though, just enough so that any water than runs down the membrane isn't in contact with the insulation (I think Proctor say 15mm but believe my BCO said 25mm iirc).

 

I believe this is obviously only an issue if you're insulating between the rafters. if you're insulating at joist level then no counter-battening is required I guess

 

https://www.proctorgroup.com/products/roofshield

 

this page has detailing for different roof types for the product.

Edited September 9, 2021 by Thorfun
added link to Roofshield webpage