Hello everyone, this is my first post, though I have read hundreds of threads and learned a great deal from the forum in the past.

I am planning an extension to my listed cottage, and think I have just about overcome the planning & listed building consent hurdles, but now face the killer - building regs!

In order to get listed building consent we agreed to demolish the existing cold felt roofed extension which eats into the side of the 200 year old granite cottage. A new extension is proposed, a few metres away, with a low, glazed link room connecting them. The challenge is this linking roof must sit fully below the eaves of the existing single story slate roof (around 2.4m high). Targeting a very modest 2m ceiling height leaves not much space for a roof, but its possible.

However the good old "warm deck" roof issue has really thrown it. My plan had been 3 glu-lam beams spanning between old and new, with joists between them, filled with kingspan. This is a no-go, it seems the structure must sit fully within the warm zone, i.e. insulation all sitting completely on top. This is great where there is space, but there isnt here.

I've followed all the usual routes, begging the building controller, googling magical thin foam insulations, etc.

I can get away with this part of the roof having a U-value of around 0.3. I have some pictures I'll try and attach.

My best idea at the moment is to still use glulam, but have the structural elements breaking throught the deck, and locally clad in super duper aerogel blankets. The other idea I have (being a mechanical engineer) is making a very slim custom steel fabricated roof structure with kingspan on top. It would be a challenge even with steel.

Basically I want to bounce the idea around and hear your thoughts. I'm also pondering how they lay standing seam zinc on top of a foam roof - surely if they put a ply skin down then that is subject to the condensation that they are trying to avoid?

Look forward to hearing your thoughts
Thomas
Aberdeenshire

and the planning drawing to show what i'm on about...

could you/would you accept a lower floor level?

could you use prefabricated laminated roof panels, still standing seam but with insulation in the sandwich?

how far out can the roof height get higher? 200mm, 500mm, 1000mm ,2000mm?

Hi Tony

1.
A lower floor level could be accepted, although the kitchen is in the old bit, so there are other regulations about the accessibility between kitchen/bathroom/bedroom zones. I think we could get away with ramping gently down as we went through from old into the link, but it would have to be minor and I can't seeing it giving us the 150mm needed to simplify the issue.

2.
I have no idea if I could use the pre-fab panels, could be an interesting option. I'm guessing they'd still need some structural supports that spanned from old wall to new wall, and they would sit on top of that? I also considered SIPS, but in my brain at least 50% of SIPS is cold, so might not be allowed in Scotland (as a flat roof).

3.
I think as long as the roof doesn't visually cut into the old roof it would be fine - you thinking rotating the pitch so that the ridge runs parallel to the wall of the old building?

Thanks
Thomas

Thanks for the thoughts, qeipl

I really like the idea of a single stiff panel, with a constant arc across it. for listed-building we'd definitely need to clad with zinc/lead - but that needn't be too much of an issue.

Does anyone know if sandwich constructions (such as described) are acceptable in terms of warm deck roofing, given that a fair portion of the structure (the upper skin) is cold. Is it ok for the upper skin to be cold if it is a material unaffected by condensation?

I am tempted to start playing around with compensatory insulation, but I also plan to live there forever so want it to feel warm. It gets pretty cold in winter round these parts.

Finally, the unheated space approach would solve all my problems, except for having to go through (at least) 2 external spec doors every time I needed a pee.

I'll definitely ponder the boat-hull approach some more...

Cheers
Thomas

qeipl has the right idea here, I think. A laminated curved structure could be made quite simply, with the necessary insulation forming an intrinsic part of the sandwich structure.

I'd be very inclined to look at using laminated ply for the inner skin (laminated up from thin sheets of ply in a curve, over a male form). Bond PU foam sheet on top of this, using PU adhesive (the foaming stuff) filling any gaps with can foam. It'll need weighting down evenly to get it to follow the curve and resist the foaming action of the PU adhesive curing. Finally, lay glass fibre up as the outer skin. This will bond exceptionally well to the PU foam (it's the way sandwich boat hulls are made) and give you a combined structural and watertight outer skin. If you don't feel confident about doing this yourself, then a boat builder or glass fibre roofing company could probably do it fairly easily.

As qeiple says, you may need to bond in some stiffening timbers at the wall plate junction, although I'm inclined to think that you would be OK with timbers bonded to the underside of the ply layer.

The net result would be a pretty tough roof that was strong yet thin.

Building in situ should be OK, but would mean making some temporary curved forms to fit between the walls to laminate the ply sections up. I'd suggest using an underside ply skin made up from three or maybe four layers of 4 or maybe even 6mm ply, with all the joints overlapping (cut sheets in half lengthways/widthways to stagger all the joints by half a sheet thickness if you can).

If you can find a means of attaching the metal roofing then it should be fine. Lead would probably be easier (but undoubtedly more costly) than zinc, but a chat with one of the zinc roofing companies might give you an idea of the art of the possible. I believe that the on-site rolling machines for standing seam can roll slight curves OK, so that might be an option.

Fitting a flat ceiling underneath would be easy, as you could utilise the forms as rafters to hold it. It would add further strength as well, although to be honest I don't think you'll need it. Years ago I made a curved deck for my old yacht like this. The deck was six feet wide and had a curve that was around 3 inches up in the centre. The lower skin was two sheets of 3/16 in ply laminated up over forms on the floor of the workshop. The foam was 1 in thick PU sheet, bonded to the ply and the deck was a modest layup of glass fibre. I could walk about on this with barely any deflection at all, it proved to be very stiff indeed. With a foam core that's maybe two or three times thicker I think you'd have a very stiff structure indeed.

Hi,
Given the location dont forget to factor in snow loading. With little roof line fall it will build up quickly and be a consderable load as seen in past 2 winters.

Cheers
Mike up North

this kind of thing what you meant, JSHarris?

<blockquote><cite>Posted By: thomasjamesrhodes</cite>this kind of thing what you meant, JSHarris?</blockquote>

Yes, that's exactly it. Main structural issue to watch is the point load at the wall attachments, where the bearing load is taken on the lower skin. Some local reinforcement of this area (maybe just a bonded on load spreader lamination on the underside at the edge) might possibly be needed, depending on the total weight plus worst case roof loading.

The roof loading stress isn't going to be very high, though, even with a couple of feet of snow on it, as the span doesn't look as if it's very wide (maybe 4m?).

I can't see the temperature differential causing a significant problem, although that depends very much of the relative thermal expansion of the inner and outer faces. As the outer face is free to move laterally and longitudinally, restrained only by the slight flex in the foam core, I doubt that it would create any unwanted buckling. In terms of degradation, then UV protected GRP as the outer skin should last a long time, provided it doesn't get excessively hot in summer. If you use a partially reflective metal roof on top I doubt that it'd get warm enough to cause any degradation.

another route is perhaps this one - its a flat panel sandwich, with pitch being introduced in the metal roof only. I have no idea how feasible this might be, but it looks a lot more traditional, and looks identical to previous plans, so might not need a non-material-variation into planning/listed-building etc.

It doesn't look as warm, won't be as stiff, isn't as cool. But sometimes that's how it goes. I'll need to speak to my building standards controller and hear his thoughts.

Cheers
Thomas

Google "curved SIP" or something like that. I expect most SIP manufacturers can make them for a price. Perhaps shop around.

Yes that's what I meant about asking about cladding (and any need for ventilation under the cladding).

I'd consider buying the curved roof first and building the rest to fit it :-)

Hi Steve

I started off thinking along the same lines as you, with insulation between structural members, both of which utilised the full thickness making for a nice warm and strong roof. However my building controller told me that this was not allowed at all, and that there basically had to be a dotted line between where insulation stopped and structure began, with all structural elements being within the warm zone. I think he simplified it somewhat for my uneducated sake. I also think this is only applicable in Scotland.

Cheers
Thomas