How to construct a shallow sloping roof with minimal thickness.

[Andy H]

I am building a single storey building under permitted development rights and am subject to a maximum build height of 2.5m. In order to get a good ceiling height I am digging below ground. The less space I can take up in the roof construction the less I need to dig out.

 

I would like to get a U value of 0.15 or better and am happy to have any structural beams that may be required to remain exposed rather than loose space by covering them up with plaster board,  providing it meets fire regulations. The building is to be  an ICF construction, 12m by 6m and will not have any internal walls to provide structural support.

 

I currently have 2 ideas:

 

 1 an insulated floor panel called 'themafloor' by Thermohouse which is like an insulated form that can span the 6m onto which a 100mm concrete with rebars is poured - this would give a depth of 32cm but would need a small amount of additional insulation to get to a U of 0.15 along with a waterproof topping.

 

2 Cold pressed steel beams that span the 6m to which are attached an insulated roof panel from somebody like Kingspan. I am assuming that the cold pressed steel beams could be left exposed but I may be wrong.

 

Any thoughts on these options and any other ideas would be appreciated.

[Andy H]

I had a useful conversation with a technical roofing expert from a company called Bauder https://www.bauder.co.uk/. The company focus on waterproofing systems for roofs and we spent at least half an hour discussing my options, mainly to with structural options of the roof which they themselves dont provide products for. Obviously there intention is that you hopefully use one of their products but he was not in the slightest bit pushy. He quickly followed up the call with relevant information including CAD detail drawings and technical details. He was open to me calling him again if I needed further help.

 

A possible solution from the conversation is a 'single ply warm roof system' . The details are as follows:

 

1. Structural timber beams to span the 6m building depth with a built in fall of 1:40 to be left exposed leaving height within the building

2. Plywood 18mm on top of the beams

3. Air and Vapour control layer 2mm

4 Insulation layer 200mm (stuck rather than mechanically fixed to prevent fixings showing through plywood ceiling) U-Value 0.125

5 Single ply PVC water proofing membrane 2mm, quoted 35 year life

 

This would give a depth of 22 cm which is better than I had hoped for which means less digging! I will be getting a quote for the installation of 

 

I need to confirm that leaving the wooden beams exposed and using the underside of the plywood to form my ceiling is compliant with building regulations, but my reading of them is that it would be OK.

 

 

 

[mvincentd]

Have you accounted for 1:40 furrings adding 15cm (by my maths) at the high end.

The Bauder guy hasn't suggested anything clever...that's pretty standard for single-ply systems.

You could get away with less than 200mm insulation though, i've got 160mm under Bauder.

I'm wary of your 'exposed leaving height' concept being potentially counter productive.  The visual 'clutter' of the elements may impose on your eye bringing the ceiling down in your perception whereas a flat white ceiling  is more unobtrusive.

[Mr Punter]

Is there a reason you cannot have a pitched roof?

[Andy H]

52 minutes ago, Mr Punter said:

Is there a reason you cannot have a pitched roof?

I can have a pitched roof but the ridge of the roof is still restricted to 2.5m as I am within 2m of neighbouring boundaries.

 

My thinking is that a flat roof with a very slight decline from back to front to facilitate water run off would maximise internal space -but this is my first build so I am open to suggestions.

 

Thanks

 

Andy

[Andy H]

1 hour ago, mvincentd said:

Have you accounted for 1:40 furrings adding 15cm (by my maths) at the high end.

The Bauder guy hasn't suggested anything clever...that's pretty standard for single-ply systems.

You could get away with less than 200mm insulation though, i've got 160mm under Bauder.

I'm wary of your 'exposed leaving height' concept being potentially counter productive.  The visual 'clutter' of the elements may impose on your eye bringing the ceiling down in your perception whereas a flat white ceiling  is more unobtrusive.

The idea was for the beams themselves to be laid with the 1:40 slope. 

 

Your point on visual clutter is worthy of serious consideration, I have just been mathematical about it. Your comments have also got me thinking of whether my sloping beams may look odd to the eye.

 

I am trying to work along passive house principles, hence the 200mm insulation. The foundation is going to be an insulated slab and the walls ICF. So far I cant work out how I avoid any thermal bridges at the wall/roof intersection, particularrly if I allow the beams (and roof)to extend out beyond the walls to to provide some cover for the windows from the mid summer sun

 

Thank you for your comments.

 

Andy

[Mr Punter]

Bear in mind that for each 100mm you excavate you will generate about 15 tonnes of material.  Maybe you could get planning consent for a more attractive building with pitched roof?  You could overlay the worst case PD scheme in elevation as a contrast.

[Andy H]

41 minutes ago, Mr Punter said:

Bear in mind that for each 100mm you excavate you will generate about 15 tonnes of material.  Maybe you could get planning consent for a more attractive building with pitched roof?  You could overlay the worst case PD scheme in elevation as a contrast.

Yes you are quite accurate I have had around 14 Muck away trucks already!

 

I decided against planning for cost and time delays and in any event wouldnt want a building in my garden that was much above 2.5m high as it would dominate the garden so I would have still had to dig down to get sufficient height.

[Temp]

I believe you can build a warm roof with up to 1/3rd of the insulation between the joists and 2/3rd above. However you probably need to get an interstitial condensation risk analysis done to confirm. The insulation suppliers might be able to provide one of you send them your proposed make up.

 

That would mean the underside of the rafters would be about 60mm higher but the gaps between them would be perhaps 30mm lower than a roof with all 200mm insulation above. Something like..

 

1. Structural timber beams to span the 6m building depth with a built in fall of 1:40 to be left exposed leaving height within the building

2. Plywood 18mm on top of the beams

3. Air and Vapour control layer 2mm

4  Insulation layer 140mm. 

5  Single ply PVC water proofing membrane 2mm, quoted 35 year life

6. 60mm fitted between the rafters. Age gaps around edges sealed. 

7. 20mm battens fixed to side of rafters.

8: Plasterboard fixed to battens and skim.

 

Flat roof rafters on a 6m span will be about 220mm deep I think.

PS: You might be able to use insulated plasterboard instead of insulation, battens and regular board. That would reduce the 30mm loss  between the rafters to perhaps 12mm. The insulated plasterboard would have to be screwed diagonally into the sides of the rafter or perhaps even glued to the underside of the OSB/Ply.

 

 

Edited February 4, 2021 by Temp

[Mr Punter]

Just to cause further confusion you could consider a cold roof with (from outside) torch on felt, 18mm OSB onto firrings minimum 40mm as ventilation, fully filling the rafters, 50mm PIR under rafter joined with ali tape, polythene and plasterboard.

[Temp]

I noticed in the guidance it says....

 

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/830643/190910_Tech_Guide_for_publishing.pdf

 

If any part of the building, container or enclosure is within 2 metres of the boundary of the curtilage of the house, then

the height limit for the total development is restricted to 2.5 metres if it is to be permitted development.

 

Is this a change? I thought in the past that only the bit within 2 meters had to be below 2.5m.

 

 

[Andy H]

2 hours ago, Temp said:

I noticed in the guidance it says....

 

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/830643/190910_Tech_Guide_for_publishing.pdf

 

 

 

 

Is this a change? I thought in the past that only the bit within 2 meters had to be below 2.5m.

 

 

I can't comment on the past but my interpretation of the current guidance is that it relates to the whole building.

[Andy H]

3 hours ago, Temp said:

I believe you can build a warm roof with up to 1/3rd of the insulation between the joists and 2/3rd above. However you probably need to get an interstitial condensation risk analysis done to confirm. The insulation suppliers might be able to provide one of you send them your proposed make up.

 

That would mean the underside of the rafters would be about 60mm higher but the gaps between them would be perhaps 30mm lower than a roof with all 200mm insulation above. Something like..

 

1. Structural timber beams to span the 6m building depth with a built in fall of 1:40 to be left exposed leaving height within the building

2. Plywood 18mm on top of the beams

3. Air and Vapour control layer 2mm

4  Insulation layer 140mm. 

5  Single ply PVC water proofing membrane 2mm, quoted 35 year life

6. 60mm fitted between the rafters. Age gaps around edges sealed. 

7. 20mm battens fixed to side of rafters.

8: Plasterboard fixed to battens and skim.

 

Flat roof rafters on a 6m span will be about 220mm deep I think.

PS: You might be able to use insulated plasterboard instead of insulation, battens and regular board. That would reduce the 30mm loss  between the rafters to perhaps 12mm. The insulated plasterboard would have to be screwed diagonally into the sides of the rafter or perhaps even glued to the underside of the OSB/Ply.

 

 

Thanks for the comments and clear diagrams. I will investigate this 1/3 , 2/3 split. Do you or any other member know if the issue of interstitial condensation goes away if wooden beams are not used within the roof structure?

 

What package are you using to make your diagrams in?

 

Kind regards

 

Andy

[Temp]

No it doesn't go away because you can get enough water forming for it to drain back down to the plaster board. The key is to make a good job of the vapour barrier which must be near enough to the warm side that you dont get condensation on it. Putting too much insulation on the warm side effectively "moves" the vapour barrier and ply/osb nearer the cold side of the insulation increasing the risk. 

 

For simple drawings I'm using an ancient progam called Paint.net. I think it might originate from MS Paint but around this time Microsoft were in trouble for giving away programs with Windows so they let a third party develop it.

 

https://www.getpaint.net/

 

I think its still free. There are probably better progs available but its fairly easy to use and works on Windows 10. For more advanced stuff like 3D printing I use Fusion 360 which is a full blown CAD package. You can get a free personal use license for that as well.