Gabled timber frame extension to solid stone house

[Tom Barton]

Hello,

I'm planning a gabled extension to a solid limestone/rubble infill house using timber frame. The whole house is to undergo refurbishment and I'm aiming for Silver Standard.

There is one point among many others that is proving to be a headache:

The proposed extension is approximately 5m(ridge) x 4.7m(span) @ 22.5deg, my structural engineer has specified 152mm x 152mm RSJ as a ridge beam (one end in stone wall) with 125 x 50 rafters. This only leaves 150mm from the point at which the rafters meet at the ridge to the underside of the 1st floor window cill. Also, with a 200mm projection past the wall finish the eaves are just 1900mm from ground level. In other words it's a tight spot.

The available gap of 150mm from where the rafters join to the cill will quickly shrink to 75mm/50mm when ridge tile/slate/batten/counter batten is added.
Therefore will I only be able to insulate between(125mm) and below(125+mm)? Builder has insisted Triso will satisfy but old threads on the forum seem to discredit it.

Is it important to keep the steel ridge beam on the airtight side? Wouldn't this leave it too close to the outer third of insulation?
I am intending to add 'ties' to board to in order to leave a cavity for MVHR ducting, can I provide airtightness here?

I would be very grateful for any help,

Sorry if I've missed loads of info,

Tom

[Anonymous]

An alternative construction method would be to build a portal timber I-beam frame which would be 'free standing' and would negate the need for the steel ridge beam. It would also allow you to fit as much insulation as you like which would be dependant on the width of the I-beam.

[Tom Barton]

Thanks Peter,

I was keen to use I-beams and scissor trusses for the roof, but if I were to use a 300mm I-beam, even with a modest 150mm projection externally you would have to duck under the eaves (1730mm). Unless I'm misunderstanding the joint detail between I-beam and wall plate?

[Anonymous]

Tom, there is no wall plate because it's a portal frame with the walls and roof structure made of timber I-beams. The I-beams are joined at the ridge and eaves with timber plates between the I-beam flanges. I am having trouble visualising the problem, any chance of a sketch?

[Tom Barton]

I'm struggling to add an image. In the meantime I've done a bit of swotting about portal frames/I-beams etc. I think I get it now.
Assuming the joints between wall and roof are mitred, is there an optimum angle? Do you know of any good resources online for junction details?
So no need for trusses either I suppose?

The Silver Standard guidance in the Carbon Lite section shows a I-beam roof on a standard timber frame wall plate, which was leading me astray. It does have some useful details such as tapering fake rafter doubling up as a sort of tilt fillet.

[Anonymous]

Tom, these might help to give you an idea of what I'm talking about.

[Tom Barton]

Thanks Peter, very clear. Definitely sold on this method, looks beautifully simple.
Are you able to advise on foundation requirements? Single storey, slate roof.
I'm keen to use jablite with a screed throughout the house including the extension and I'll probably use 240mm x 45mm i-joists with with a further 60mm render board.
I've passed on the Steico I-joist data sheets to my structural engineer for his thought regarding footings. He's not familiar with this method so will roll his eyes a bit.
Thanks

[Anonymous]

Tom, I'm not an architect nor structural engineer so I'm not able to advise but I can tell you what I used for my build. I designed the PH myself and used PH certified components. The raft foundations are Isoquick with 300mm insulation underneath and 200mm upstand which contains a 200mm thick reinforced concrete slab. The walls and roof are 350mm Steico I-beams with 15mm OSB3 external racking. There are 50mm thick high density Rockwool batts on the outside of the OSB and 350mm Icynene insulation between the I-beams. Ground floor ceiling is constructed of metal web joists with the first floor ceiling vaulted.
If I can be of any further help, it's no problem.

[Tom Barton]

Thanks Peter, I assume the photos are of your build, very smart.
I'll be able to get a full frame from one 7m length with 400mm to spare, very satisfying.
Could you tell me what material was used to join the I-joists across the web and what the fixings are?
Thanks, Tom.

[Tom Barton]

I-joists arrive next week! Thanks Peter.

[Anonymous]

The method of construction was decided upon by the structural engineer in conjunction with the timber frame company. The frames were constructed in the factory, delivered to site by lorry and lifted into position by crane. The I-beams and plates were cut using CNC machines.

[Anonymous]

The frames were constructed in the factory

Ah, that explains it, thanks.

[Author]

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