Straw bale garden room - roof options?

Hi hi,

Currently in the process of putting together a garden room with load-bearing straw walls as a precursor to bigger things. Here's the plan:





and here's a view of where I am so far:



The sequence goes: gravel pits (I'm on very hard clay subsoil with very expansive made ground on top); blocks on top of those (stone was too expensive, and I'm very much not a fan of the tyre approach); box beam on top of those; baseplate on top of that; and now the straw (barley, managed to find a farmer basically next door).

Not visible in the pics, but I'm three bales high along the west and I've got the doors hung, so hooray for that.

The plan is to lime render three of the sides and internally, but we're late in the season for that so it's probably going to stay covered until spring.

The fourth side butts up against a brick retaining wall at the edge of the property, which is all sorts of fun. I've ended up dipping the bales in lime render (for fire and critter protection) before setting them in the wall:





Currently I'm trying to design the roof. Bit weird to start without having an idea of that rather important piece, but I'm a little hamstrung by planning here in the UK - I can only go to 2.5M above ground level without getting permission. With the retaining wall, I might technically have started building underground (they take ground level as the highest bit of ground immediately adjacent to the building); if so, I have perhaps 800mm to "spend" on the roof, which might barely be enough to get a 5/12 pitch in, permitting cedar shingles which I'm quite keen on.

Aside from showing off the work so far, I'm interested in thoughts on the roof design, if anyone cares to share some. I'm struggling to reason about it with the "curved" wall (it's actually just coming off at 105° rather than 90° either side; I plan to fake the curve in the straw and render) in the mix.

One thought I had was a fairly basic hip - here's some very basic CAD -



Presumably the different rafter lengths can be worked out just by (very accurately) cutting different angles along each of the common rafters, allowing me to keep just a single plane along the curved wall? Or is it more complicated than that? The nightmare scenario would be needing to break it up into 3 or 4 planes for some as-yet-unanticipated reason.

Anything that requires less accurate measuring would be great, though, as my joinery really isn't up to much! I've been looking at pictures and plans of roofs until I've gone cross-eyed, recently, but applying them to this D shape is turning out to be a struggle for me. Bonus points if it's something that permits an open roof space - having a ceiling at 2M high isn't super amazing.

(The floor will be suspended wood, and I've got the detail of that mostly worked out, at least... I just can't put it in until I'm finished with the walls, for access reasons).

Despite the straw, thermal performance isn't super-important in this build; it's more about the acoustic properties, and working out whether I'd actually like to build like this on bigger projects.

That looks like you are going to have a nice garden room.

I that was my plan I would do a slanted roof with roofing panels.

Hi Nick - congratulations on your build so far. I'm assuming you've checked any building regulations requirements - I'm a bit worried since you seem to be close to the adjacent building which might affect things.

Nick Thomas wrote:
Presumably the different rafter lengths can be worked out just by (very accurately) cutting different angles along each of the common rafters, allowing me to keep just a single plane along the curved wall? Or is it more complicated than that? The nightmare scenario would be needing to break it up into 3 or 4 planes for some as-yet-unanticipated reason.

Anything that requires less accurate measuring would be great, though, as my joinery really isn't up to much! I've been looking at pictures and plans of roofs until I've gone cross-eyed, recently, but applying them to this D shape is turning out to be a struggle for me. Bonus points if it's something that permits an open roof space - having a ceiling at 2M high isn't super amazing.

I think that you may need to put a curve in the front wall in height as well (lower in the middle) if you want the roof to be flat rather than curved. That would make the wall plates a bit more complex - you may get away with a couple of parallel ones at two or three different positions perhaps.
However, if you are thinking of wooden shingles, you may be able to accommodate a slight curve in the roof and this could look quite charming.


treehouse roof

Alternatively, since you will probably want a good overhang anyway to keep the walls drier, just make the roof square with a bigger overhang at the corners than the middle.

Nancy Reading wrote:Hi Nick - congratulations on your build so far. I'm assuming you've checked any building regulations requirements - I'm a bit worried since you seem to be close to the adjacent building which might affect things.

Thanks! Yup - I'm working quite hard to keep it exempt from both building regs and planning, hence the 2.5M height envelope. The requirements are only partially overlapping, and don't even use the same definitions for things like height and area, so that's been all sorts of fun. I could probably get planning if I absolutely had to, but really wanting to avoid building regs. Until recently, I was resigned to some sort of flat roof.

Nancy Reading wrote:I think that you may need to put a curve in the front wall in height as well (lower in the middle) if you want the roof to be flat rather than curved. That would make the wall plates a bit more complex - you may get away with a couple of parallel ones at two or three different positions perhaps.

I think I've worked this out, but I had to make a little model out of paper to get there. Makes sense now, thanks!

I wouldn't mind a curved roof, but with the door in the middle of that wall it should really be convex rather than concave, to avoid concentrating water above it. Varying the height of the wallplate to get a flat plane along there makes perfect sense, and I can always put a curve on the overhang, if that doesn't look rubbish.

The shape of your curved front wall, with a single horizontal top plate, would make for a gently convex roof surface which would disperse water from the center over the door, not concentrate it. You would still want a gutter to eliminate drips in front of the door.

If you set the rafters on the plate with their varying angles, then bend plywood sheathing or continuous thin purlins over those, you would end up with a very attractive curved shingle roof.