Roof Construction and Insulation

Villain

Ok so coming from this thread I wanted opinions on roof types, i.e. cold ventilated or warm unventilated.

I have attached a picture which shows two types of warm roof, the second picture on the right is what I'm thinking, insulating above and between the rafters is not something I want for various reasons.

So either I go for a normal cold roof with 50mm air space left under slates and insulate 125mm between rafters and then use drylining plaster board under rafter.

Or I go for the detail shown on the right in the picture attached, which means I insulate fully between rafter and sheet the roof with PLY or OSD put on a good breathable membrane and then slate.

Is there any benefit when using slates with going with the detail on the right i.e. battening again so there is space between the slate and the sheets of ply?

RKQ

Warm roof construction is recommended in new build.

The counter-batten has two aims:-

1. Ventilation of roof battens / felt.
2. Quick throw off of water.

Battens directly across the felt would impeed water, should a slate fail or be removed. Water will eventually soak thru a breather felt. So the counter-batten lifts all the battens off the felt, preventing water ponding on the felt.

Its a great system, it works, why make excuses not to use it?

Villain

Cheers for the reply, is better to put the breather felt over or under the counter batten?

topcatcbr

Slig wrote: »

I'm not too sure about that.
I dont see a problem in filling the cavity with insulation and sheeting with ply. then fixing your breather membrane, battening and cross battening.
From a practical point of view it is exactly the same as a timber frame wall as long as there is a vapour check on the inside.

This is true and i see the kingspan details shows this however as a roof is more exposed than an external wall im not convinced that the two details are interchangeable. the timber will act as a cold bridge and be prone to interstatial conedensation and would be a difficult detail to get right on site. (timberframe wall are made in a factory environment)

Personally i would be more confident of the warm deck layout.

(ps I im not that confident that the detailing for timber frame is correct but only time will tell.)

RKQ

Manufacturers recommend under the counter-batten, for speed of erection - roll out flat, staple.

I put it over mine as I wanted to ensure water couldn't penetrate staples. It was more awkward and time consuming. You also have to be careful at eaves, to prevent wind getting under felt in stormy weather - rattling felt can be noisey and scarey!

We didn't put down ply backing, probably wood if I was doing it again. Ours works fine. The hardest part was getting the Carpenter to do it - some people hate change!

Its alot easier to insulate over than under rafters. Its hard enough to slab rafters! Drill, screw, slab with two hands and your forehead!

Slates

I'd recommend the following build up for over rafter insulation

Slates on
50x25mm battens on
50x25mm Counterbattens on
Breather membrane
On Xmm rigid insulation
On Rafters infilled with Xmm insulation
Vapour check to underside of rafters
Plasterboard

Counterbattens will be fixed through the rigid insulation into the rafters with structural fixings (ie Helifix)

By using this system, there is no need for the Ply/osb. The detail on the right is a Scottish that no builder in Ireland would go near

Villain

Slates wrote: »

By using this system, there is no need for the Ply/osb. The detail on the right is a Scottish that no builder in Ireland would go near

Why is that slates?

topcatcbr

Slates wrote: »

I'd recommend the following build up for over rafter insulation

Slates on
50x25mm battens on
50x25mm Counterbattens on
Breather membrane
On Xmm rigid insulation
On Rafters infilled with Xmm insulation
Vapour check to underside of rafters
Plasterboard

Counterbattens will be fixed through the rigid insulation into the rafters with structural fixings (ie Helifix)

By using this system, there is no need for the Ply/osb. The detail on the right is a Scottish that no builder in Ireland would go near

There is no need for ply/osb but it cant do any harm. and should stop workers putting foot through insulation when battening out.

Agree 100% with everthing else.

Slates

topcatcbr wrote: »

There is no need for ply/osb but it cant do any harm. and should stop workers putting foot through insulation when battening out.

Agree 100% with everthing else.

If you are using counterbattens there will be no foot traffic on the insulation, using a sarking board on this type of roof (while a nice detail) is not needed, if you really wanted to go the whole hog on it you could stick some butyl rubber bead(50mm wide radon tape) to the underside of the counterbattens to seal around the fixings.

Also the sarking board would need a high level of vapour permeability, if not you would have to use passive ventilation on the warm side of the sarking board which negates what the detail was for in the first place.

Villain

Ok so having thought about this a bit more I think a proper warm roof might be worth the extra cost and effort.

So which of the designs in the picture attached would people think is better for a roof with slates, 45 degrees pitch on a dormer house.

Or would I be better again to go with the design on the right but batten on top of the ply as well?

topcatcbr

The insulation boards are not suitable to be walked on, as defined in HSE 33, except when covered with supporting timbers.

A timber stop rail is fixed to the ends of the rafters which are then overlayed with the 75mm Top Hat section located over and between the rafters via the rebated edges. The insulation thickness over the rafters is 55mm.

Breather Membrane is pulled taught and laid directly over the insulation.

50 x 32mm counter battens are nailed through the Breather Membrane and Top Hat board and into the rafters using spiral fixings such as Helifix Inskew 600 or Buildex RG, with minimum penetration into the rafter of 35mm.

(in order to counter batten it will be hard to avoid standing on the roof insulation)

Tiling battens are laid across the roof and nailed into the counter battens. Tiles should be stacked so that they bear directly over the rafters. Finally, the roofing tiles or slates are fixed, to the manufacturer’s recommendations.

Working from the underside of the roof, the Raftersqueeze infill board is placed in position between the rafters. Raftersqueeze is fitted starting from the eaves with all boards tightly butted.

The underside of rafters to be lined with plain or foil backed plasterboard, as appropriate. If desired, the Raftersqueeze board can be fitted from above the new roof before the Top Hat section. Install timber supporting battens, taking care to leave 20mm between the Raftersqueeze and the top of the rafter for the rebated section of the Top Hat board.

topcatcbr

Villain wrote: »

Ok so having thought about this a bit more I think a proper warm roof might be worth the extra cost and effort.

So which of the designs in the picture attached would people think is better for a roof with slates, 45 degrees pitch on a dormer house.

Or would I be better again to go with the design on the right but batten on top of the ply as well?

The detail on the left.

While a layer of ply/osb (sarking board) is not necessary it is not a bad idea

ardara1

Your counterbatten will basically become the rafter to support the weight of the roof - so fixings are critical. Google INSKEW fixings - cost around 25c a piece @ about 8? per m2.

But I'd saying going above will be more common once better U-values are asked for.

have a look at this http://www.xtratherm.com/interactive/interactive.php?video=vaultedroof

Villain

Great link thanks