Sup roof on masonry build

[Lewis88]

I am considering a SIPs roof on my masonry/cavity build however most local SIPs manufacturers are indicating that my project isn’t suitable for SIPs - having said that I haven’t found one that has fitted a SIPs roof to a masonry build before.
 

Please can any shed any light as to what the issue is likely to be? I’m assuming that it’s a difficulty with the span as the main ridge beam would need to be 13m long.

 

Is there anyone that has a masonry build with a SIPs roof, and would the span be an issue?

[Russell griffiths]

I looked into sips roof on an icf house and forgot about it because of the spans. 
sips cannot span very far on their own without intermediate structural timber. 
you either need large splines of timber between the sips which leads to cold bridging which then needs additional insulation, or you need to put additional purlins at mid span to stop the droop from eaves to ridge, these then project into the room space. 
 

I would build a cassette type roof on site using I joists and insulation then osb. 

[SBMS]

I looked at this. Realised that if the rest of the walls weren’t being done in SIPS there was little point. No SIPs manufacturer would install as they need your masonry walls to be within a very small tolerance - fine if they are putting them up in sips, otherwise they just said they’d supply only. 
 

Reason number 2 - in my research of all the places a SIPS panel could fail it’s along the ridge where the two panels meet. If this detail is imperfect you’ve got osb weetabix. 
 

Reason number 3 - long ridge spans don’t work very well with sips. A glulam ridge was looked at for ours… then they said steel…

 

Reason number 4 - I was attracted to sips because we are doing attic rooms and having them vaulted was a desirable. But many on here have said the noise is quite high when it rains.
 

Instead we are going with a 304mm posi rafter hanging off a steel Ridge, filled with warmcel blown in. Fully vaulted and should be excellent u value, and quiet in the rain. And can be assembled by a normal roofing gang. 

[torre]

You've not mentioned why you're considering this route?

I looked briefly at it as it seemed seductively easy to get a roof on but building in masonry, very tight tolerances so little adjustability and most importantly cost weighed against it (seemed a huge gap between the cost of the panels and a whole roof even supply only). I was surprised to learn it's still classed as a cold roof and so needs ventilation above meaning you need counter battens above for example so the build up isn't quite as thin as you'd think either.

[Lewis88]

Thank you all for your responses. The SIPs company have now told me that it is the span that’s the issue so it is likely that there is no way around this and it is probably better to choose standard attic trusses. 
@Russell griffithswhat is a cassette styled roof?  Is this essentially using I-joists from a main ridge beam and covered over with OSB? Would you happen to have a detail showing this? Sorry, this is my first self build so not Ai fait with the process or terminology.

[Mr Punter]

If most SIP manufacturers think your roof is not suitable it probably isn't.

 

Trusses or cut roof and maybe some steel are what is widely used.

 

Who is doing the structural design?  They should be able to help.

[SBMS]

1 hour ago, Lewis88 said:

Thank you all for your responses. The SIPs company have now told me that it is the span that’s the issue so it is likely that there is no way around this and it is probably better to choose standard attic trusses. 
@Russell griffithswhat is a cassette styled roof?  Is this essentially using I-joists from a main ridge beam and covered over with OSB? Would you happen to have a detail showing this? Sorry, this is my first self build so not Ai fait with the process or terminology.

Do you have rooms in the roof?

[Russell griffiths]

What exactly do you want to achieve 

vaulted ceilings to the bedrooms, or 1-1/2 storey with room in roof. 
some context needed really as to why you wanted a sips roof. 

[SBMS]

29 minutes ago, Russell griffiths said:

What exactly do you want to achieve 

vaulted ceilings to the bedrooms, or 1-1/2 storey with room in roof. 
some context needed really as to why you wanted a sips roof. 

That’s why I was recommending posi rafters if vaulted ceilings. I discounted attic trusses because I didn’t want to insulate with PIR and couldn’t get trusses deep enough for cellulose blown insulation. 

[Lewis88]

So the property will be 3 floor, the attic being the third floor which will be utilised as living space. I don’t currently have a structural designer for the roof but there are basic plans drawn out by a retired architect and outlines ‘to structural designers specification’.
 

I was hoping that this design service would be provided when purchasing the attic trusses or SIPs roof from a supply and fit basis. That said I do have a structural engineer that designed the foundation and would no doubt assist for a fee.

[Lewis88]

Sorry, my reference to third floor is if the ground floor is counted as floor 1. In essence it’s 2 story with a habitable attic space.

[Russell griffiths]

Attic trusses craned into place. 
insulated between and over the top with some form of semi rigid insulation, not pir, it’s a nightmare to fit and offers very little in the way of sound absorption. 

[Lewis88]

Thanks for the advice @Russell griffiths I’ve had a quote for attic trusses and they seem quite reasonable. If PIR isn’t recommended what would you say is a good option to look at? Does that mean the detail would be:

 

Inside

*wooden attic trusses with insulation between (inside)
*OSB

*another layer of insulation (rigid)

*another layer of OSB

*weatherproof membrane

*battens

*slates

Outside

 

 

[Russell griffiths]

What’s all the osb for. 

[SBMS]

1 hour ago, Lewis88 said:

Thanks for the advice @Russell griffiths I’ve had a quote for attic trusses and they seem quite reasonable. If PIR isn’t recommended what would you say is a good option to look at? Does that mean the detail would be:

 

Inside

*wooden attic trusses with insulation between (inside)
*OSB

*another layer of insulation (rigid)

*another layer of OSB

*weatherproof membrane

*battens

*slates

Outside

 

 

You will struggle to get a good value with attic trusses without using PIR - which I wouldn’t. 

Posi rafters will go right to your wallplate

you don’t even need a dwarf wall if you don’t want. You can do a 304mm posi rafter and blown cellulose insulation. That’s my makeup for our habitable attic rooms:

 

 

[SBMS]

This makeup as well with steico boards as sarking. And propassiv inside for air tightness and vapour permeability. Note that the timber rafters are the posi rafters. 
 

Edited January 15 by SBMS

[Russell griffiths]

@Lewis88

follow what has been drawn up by sbms, that’s probably one of the most clearly laid out drawings you could want. 

[lizzieuk1]

On 15/01/2025 at 21:05, SBMS said:

Posi rafters will go right to your wallplate

you don’t even need a dwarf wall if you don’t want. You can do a 304mm posi rafter and blown cellulose insulation. That’s my makeup for our habitable attic rooms:

That's exactly what we are hoping to do with our roof so really useful. Could I ask:

Is there a max span for the 304mm rafter?

How does the cost compare to say, an attic truss or cut roof with pir?

[SBMS]

1 hour ago, lizzieuk1 said:

That's exactly what we are hoping to do with our roof so really useful. Could I ask:

Is there a max span for the 304mm rafter?

How does the cost compare to say, an attic truss or cut roof with pir?

I think some posi joists span up to 8m but not sure about posi rafters. I can’t see many roofs they wouldn’t work on. 
 

Our roof design below was around 7k, I had an attic truss design done as well

which was 6k. it’s probably cheaper to install trusses but you cant get trusses deeper than few hundred mm.  so by the time you extended the rafters to get 300 or so mm depth for cellulose it’s probably a wash. But we do have ridge steels which you wouldn’t have with trusses which added to the cost.
 

About the same again price wise for the cellulose. I think that’s comparable to PIR with the cutting and fitting costs but a better job with cellulose from What I understand. 
 

 

[lizzieuk1]

8 hours ago, SBMS said:

I think some posi joists span up to 8m but not sure about posi rafters. I can’t see many roofs they wouldn’t work on. 
 

Our roof design below was around 7k, I had an attic truss design done as well

which was 6k. it’s probably cheaper to install trusses but you cant get trusses deeper than few hundred mm.  so by the time you extended the rafters to get 300 or so mm depth for cellulose it’s probably a wash. But we do have ridge steels which you wouldn’t have with trusses which added to the cost.
 

About the same again price wise for the cellulose. I think that’s comparable to PIR with the cutting and fitting costs but a better job with cellulose from What I understand. 
 

 

Thanks sbms,

Did you manage to solve the issue of cellulose install providers?

[SBMS]

25 minutes ago, lizzieuk1 said:

Thanks sbms,

Did you manage to solve the issue of cellulose install providers?

I did yes, we were put in touch with a local installer.  Where are you based?

[lizzieuk1]

On 01/03/2025 at 09:06, SBMS said:

I did yes, we were put in touch with a local installer.  Where are you based?

Ah that's good, we're in Lincs