Roof build up

[ianfish]

I will be looking at getting joists into place soon going for c24 225x47mm, we had considered a cold roof.

 

I recently came across Steico products, which got me thinking, then I came across M Wye site, with further thoughts on how to then effectively fill the 225mm depth in the joist. I like the Sarking board idea which you then vent over this and do your choice of ply or osb. I have noted how wools can sag away from joists but if they were placed in and made captive, I am thinking this could negate any unwanted voids around joists.

 

Now I'm thinking as this is a new build and want the joists wrapped as effectively as possible and done as cost effective, has anyone completed their roof from the outside? 

 

I am thinking of a hardboard or a suitable breathable board "ceiling" which would then be battened to do the membrane and service layer. We could then stuff a cheaper or most budget friendly wool as a first layer right into the bottom of the ceiling using steico in successive layers then add the dry board over the joists to then allow the rest of the construction to progress.

 

Thoughts?

[ianfish]

65 views

[Iceverge]

Any pics/sketches/diagram's of what you propose?

 

It sounds like a hybrid approach.

 

If you could ensure a robust airtight/vapour control layer and a fully breathable build up outboard with a  ventilation layer between the final roofing and the sarking I would see no problems. I'm sure the "it'll rot to pieces" brigade will be along soon to correct my erroneous ways. 

 

Have you considered blown cellulose between the joists? Perfect fill, cheap and excellent in almost every way otherwise. 

 

I think Siga have some details. I'll check......

 

 

Edited August 6, 2022 by Iceverge

[Iceverge]

 

There's one. 

[Iceverge]

I've re-read your post....

 

 

My idea for something cheap and good.

 

EDPM 

OSB/PLY,

50mm battens for ventilation layer,

Fully taped breather membrane.

OSB

225mm joists filled with blown cellulose. 

Vapour control layer. 

47mm battens at 600cc for service cavity with 50mm batt insulation. 

15mm plasterboard. 

U-value about 0.15-0.16.

 

 

To upgrade the U-value counter batten above the VCL with another 47mm batten and increase the layer of cellulose. Should be a U value of about 0.14 with excellent airtightness and  decrement decay. Thermal bridging etc  All nail gun friendly too and no exotic expensive materials. 

 

I would avoid the faff of furring strips and slant the whole roof if you could live with the aesthetic of a slanty ceiling. 

 

 

[Adsibob]

11 hours ago, Iceverge said:

I've re-read your post....

 

 

My idea for something cheap and good.

 

EDPM 

OSB/PLY,

50mm battens for ventilation layer,

Fully taped breather membrane.

OSB

225mm joists filled with blown cellulose. 

Vapour control layer. 

47mm battens at 600cc for service cavity with 50mm batt insulation. 

15mm plasterboard. 

U-value about 0.15-0.16.

 

 

To upgrade the U-value counter batten above the VCL with another 47mm batten and increase the layer of cellulose. Should be a U value of about 0.14 with excellent airtightness and  decrement decay. Thermal bridging etc  All nail gun friendly too and no exotic expensive materials. 

 

I would avoid the faff of furring strips and slant the whole roof if you could live with the aesthetic of a slanty ceiling. 

 

 

This is a good build up, but I think wood fibre has a better decrement delay than blown cellulose. At least according to ecomerchant.co.uk:

 

insulation materials offering a high decrement ‘factor’ include cellulose fibre (7.3 hr), wood fibre insulation board (11.3 hr); whereas materials with a low decrement factor would include low-density mineral fibre (3.7 hr) and polyurethane/ polystyrene.

 

 

[SimonD]

13 minutes ago, Adsibob said:

This is a good build up, but I think wood fibre has a better decrement delay than blown cellulose. At least according to ecomerchant.co.uk:

 

insulation materials offering a high decrement ‘factor’ include cellulose fibre (7.3 hr), wood fibre insulation board (11.3 hr); whereas materials with a low decrement factor would include low-density mineral fibre (3.7 hr) and polyurethane/ polystyrene.

 

 

 

No, the wood fibre insulation board referred to by ecomerchant is a different product compared to the between rafter wood fibre bats. In this case, blown cellulose is by far the better option. The high decrement delay woodfibre boards are those that are usually installed above the rafters and form a sarking so you end up with a hybrid warm roof - very good for reducing repeated cold bridges too.

Edited August 7, 2022 by SimonD

[Iceverge]

 

 

 

[Simplysimon]

On 06/08/2022 at 21:55, Iceverge said:

I'm sure the "it'll rot to pieces" brigade will be along soon to correct my erroneous ways.

there must be a reason cold flat roofs are not allowed in scotland, it may be down to ensuring bad habits and poor detailing is eradicated or may be other reasons. i'm not sure you can full fill a flat roof with blown cellulose

[Gus Potter]

On 27/07/2022 at 18:06, ianfish said:

Thoughts?

Go for the simple stupid. Classic warm roof, the furring pieces are the least of your worries.

 

Before you get too technical try and work out how you will detail the interfaces ( you need to keep the SE happy) and if you can find someone (builder) who will be able to follow the drawings and build it at reasonable cost. Also, you'll need good drawings unless you want to be on site every day.

 

Get a feel for these costs and then look at what it will cost you to make the insulation thicker with a simple design. Your first challenge is going to be to find a builder who will take this on. Next you'll have more labour cost, a detailing cost and site supervision plus a few other things to think about, all on top of all the other day to day stuff.

 

You mentioned C24 timber. What centres are your joists? If 400 centres have you priced / asked your SE if 225 x 75 C16 at 600 ctrs will work, maybe 200 x 75 C16. Often we look at tables, assume a certain joist size at a certain spacing.. partly because we also need to look at repeating bridges for the insulation. With a true warm roof you have much more scope to play with the joist depth width and spacing and see what is going cheep at the time.. who said it was easy!