Roof construction help

[Vijay]

Morning,

 

Really need some advice and thoughts on my roof construction. It's not the simplest of roofs and is quite large. There's 2 levels, over to the left is on top of single storey and the middle/right is on top of 2 storey. They are all habitable rooms so were designed as room in roof attic trusses and are designed as a cold roof. The roof is to have 12mm OSB sarking.

 

I guess my question is would you go with this construction method? Is there a better of of building this roof or a better bang for your buck method?

 

I also contacted Thermoroof after spotting it on the latest series of Grand Designs, but they never bothered replying (I've just emailed them again).

 

Is SIPS worth considering for a roof? My guess is it would work out flipping expensive but maybe balance out by the time I've bought all the trusses/insulation/sarking?

 

Are there any other systems worth considering?

 

 

This is the layout 

[Simplysimon]

44 minutes ago, Vijay said:

They are all habitable rooms so were designed as room in roof attic trusses and are designed as a cold roof

so what is the insulation to be and u-value?

[scottishjohn]

1 hour ago, Vijay said:

Is SIPS worth considering for a roof?

limited span without extra support and no trusses with sips roof 

about 4.5-5m from memory ,

is often though used 

Edited May 4, 2019 by scottishjohn

[PeterW]

What year building regs are you using ..? and what’s the u-Value in the DER/SAP..?

 

For ease and simplicity I would go with Attic trusses as you get both the floor and the roof in one hit. Easy and simple to manage, any crew can do it. You also get a full set of plans and the design done for you - most if not all will do a site survey prior to manufacture so you get an accurate roof. 

 

Some now also use Wolf/Posi systems for the trusses so you can get very deep roof construction that is very light. 

 

I wouldn’t go with  any sort of panel system with that design - too many odd angles and you can’t tweak them the same way you can move a piece of OSB etc. 

[Vijay]

2 hours ago, Simplysimon said:

so what is the insulation to be and u-value?

Roof construction on my plans is:

 

12mm OSB sarking

150mm truesses

100mm Celotex GA4000 between rafters

100mm Celotex GA4000 below rafters

 

U value 0.11

[Vijay]

1 hour ago, PeterW said:

What year building regs are you using ..? and what’s the u-Value in the DER/SAP..?

 

For ease and simplicity I would go with Attic trusses as you get both the floor and the roof in one hit. Easy and simple to manage, any crew can do it. You also get a full set of plans and the design done for you - most if not all will do a site survey prior to manufacture so you get an accurate roof. 

 

Some now also use Wolf/Posi systems for the trusses so you can get very deep roof construction that is very light. 

 

I wouldn’t go with  any sort of panel system with that design - too many odd angles and you can’t tweak them the same way you can move a piece of OSB etc. 

 

The last drawings were end 2016/beginning of 2017.  

 

DER for dwelling was 10.81.

Fabric U value for roof was 0.10 weighted average and 0.12 highest

 

I think that's exact where my A.T was going, with simplicity on this particular job.

 

Good point about not being able to tweak panel systems and I can't see an ICF house being completely accurate to plans with walls slightly moving/settling when concrete is poured.

[Simplysimon]

definitely stick with trusses

[Vijay]

3 minutes ago, Simplysimon said:

definitely stick with trusses

 

Why do you say that, for simplicity?

[Simplysimon]

as stated previously design done for you, you could get a joiner to build it taditionally but engineers don't like that. it could be done with i beams but would need re-engineered for ridge loading but would allow you to blow insulation as opposed to cutting and fitting.

so for simplicity as you say, stick with trusses. buy your insulation from seconds and co. if you have the insulation prior to trusses arriving then have them cut nice and straight and to size and if fillets cut and fitted to side of trusses, then the insulation can be dropped in as roof erected prior to sheeting.

[AliG]

Could you use 150mm of rock wool between the rafters with 100mm of PIR below? This would give approx the same U-value.

 

If you can use rock wool it is a much cheaper way to get the same U-value as PIR but it is thicker. It needs a lot less labour also.

 

I am not sure if you can full fill between the rafters or will need a ventilation gap, also it would have to be supported which might eliminate any savings. Has it been designed with a ventilation gap above the PIR? We have full fill PIR and no ventilation, this requires the use of permeable membrane above the roof. Not sure about the relative cost but it does mean that we don't have any eaves ventilation which is much neater.

[Vijay]

On 04/05/2019 at 17:15, Simplysimon said:

as stated previously design done for you, you could get a joiner to build it taditionally but engineers don't like that. it could be done with i beams but would need re-engineered for ridge loading but would allow you to blow insulation as opposed to cutting and fitting.

so for simplicity as you say, stick with trusses. buy your insulation from seconds and co. if you have the insulation prior to trusses arriving then have them cut nice and straight and to size and if fillets cut and fitted to side of trusses, then the insulation can be dropped in as roof erected prior to sheeting.

Thank you

[Vijay]

On 05/05/2019 at 16:36, AliG said:

Could you use 150mm of rock wool between the rafters with 100mm of PIR below? This would give approx the same U-value.

 

If you can use rock wool it is a much cheaper way to get the same U-value as PIR but it is thicker. It needs a lot less labour also.

 

I am not sure if you can full fill between the rafters or will need a ventilation gap, also it would have to be supported which might eliminate any savings. Has it been designed with a ventilation gap above the PIR? We have full fill PIR and no ventilation, this requires the use of permeable membrane above the roof. Not sure about the relative cost but it does mean that we don't have any eaves ventilation which is much neater.

 

More than happy to save some money on insulation if I can get the same spec  Will look at that nearer the time as prices for everything will probably have changed by the time I get a roof on lol

 

It has indeed been designed with a 50mm ventilation gap. This is my roof buildup:

[AliG]

As you have a VCL and breathable membrane and tiles on battens I do not believe that the 50mm ventilation gap is necessary, but I would ask whoever wrote the spec.

[ADLIan]

As a general rule if OSB or ply sarking is used this is classed as having a high vapour resistance so 50mm vent air space required below the sarking/above insulation (even if breathable membrane used above sarking). See BS 5250.

[scottishjohn]

17 minutes ago, ADLIan said:

As a general rule if OSB or ply sarking is used this is classed as having a high vapour resistance so 50mm vent air space required below the sarking/above insulation (even if breathable membrane used above sarking). See BS 5250.

I take what you say as true 

but if roof is sealed and you have an airtight barrier on inside below insulation where is this 50mm ventilation void going to vent to ?

[ADLIan]

Vents required at high and low level - normally eaves and ridge.

[AliG]

We have the sarking installed in strips with small gaps in between for breathing. If you installed it without this then the 50mm may be necessary.

 

This piece from Sandtoft refers to BS5250

 

https://wienerberger.co.uk/downloads/20160505112912/sandtoft-technical-notes-roof-space-ventilation.pdf

 

If you have a warm roof, which they define as having insulation between or above the rafters and a "well sealed ceiling" both of which @Vijay appears to have and a air and moisture permeable membrane then ventilation is not required.

 

If you think about it, if the ceiling is "well sealed" where would moisture be coming from that would have to evaporate through the roof.

Edited May 7, 2019 by AliG

[Ed Davies]

3 hours ago, AliG said:

We have the sarking installed in strips with small gaps in between for breathing. If you installed it without this then the 50mm may be necessary.

 

I have 150×22 sawn treated sarking which will have mineral wool immediately below. When I argued that all of the mineral wool on the outside would be within 75 mm of a gap and mineral wool is pretty vapour open my BCO was happy to accept that ventilation below the sarking wasn't needed. There will be ventilation above the sarking and the breather membrane, of course.

[Vijay]

I’m trying to get the wall levels up to wall plate heights and just need to ask about the attic trusses for my single storey roof.  I Believe I will need “Bob tail” attic trusses and I wondered if these can be supported off joist hangers? I have attached a picture where I have arrowed where I mean would sit on joist hangers. It’s not as straight forward as I’m building in ICF and would install a vertical wall plate to fix the hangers.

[PeterW]

Nope - has to sit on a proper wall plate.  

 

If you are cladding in brick then you just build up the outside of the stub wall. Ask the manufacturer to put stubs at ICF thickness and you can insulate within that. 

[Vijay]

So the wall plate would be under "A" and not "B"?

 

I'm cladding in brick slip. If the stubs were built out, would that not be a bit of a cold bridge? I'd much prefer to keep the walls ICF.

[PeterW]

Wall plates on an bobtail truss always go under A. 

 

There is only a cold bridge if you don’t insulate the ends. What carrier system are you using for the slips ..? 

[jamiehamy]

2 hours ago, Vijay said:

I’m trying to get the wall levels up to wall plate heights and just need to ask about the attic trusses for my single storey roof.  I Believe I will need “Bob tail” attic trusses and I wondered if these can be supported off joist hangers? I have attached a picture where I have arrowed where I mean would sit on joist hangers. It’s not as straight forward as I’m building in ICF and would install a vertical wall plate to fix the hangers.

Why would you not rest the joists on the concrete core of the ICF? When you pour the concrete, you leave it shy of the top by around 55mm - and then pace in your wall plate with pre-assembled wall plates with J-bolts through then. If you do it smartly, then you'll measure the joists to be 20mm in fron the outer edge of the ICF core - then ply right along the edge 'B'. And then you can continue up the outer layer of Polarwall to meet the roof pitch. I'll draw something up and post. 

[Vijay]

21 hours ago, PeterW said:

Wall plates on an bobtail truss always go under A. 

 

There is only a cold bridge if you don’t insulate the ends. What carrier system are you using for the slips ..? 

Carrier system? I was advised I only need to bond them on.

[Vijay]

21 hours ago, jamiehamy said:

Why would you not rest the joists on the concrete core of the ICF? When you pour the concrete, you leave it shy of the top by around 55mm - and then pace in your wall plate with pre-assembled wall plates with J-bolts through then. If you do it smartly, then you'll measure the joists to be 20mm in fron the outer edge of the ICF core - then ply right along the edge 'B'. And then you can continue up the outer layer of Polarwall to meet the roof pitch. I'll draw something up and post. 

 

The only reason was it would mean lowering the height of the wall plate to suit and then finishing the top part of the wall in either another method (timber I guess) or possibly in ICF.

 

So did you fix your wall plates inside the ICF or on top of it?

 

I wondered if I could carry up the wall in ICF (once the trusses are in) and then fill with concrete mixed in a cement mixer. That would mean the original wall plate and then the vertical part of the attic trusses are buried in the concrete core of the ICF - but it would mean the wall is as much of a continuation of the ICF walls as I can get it