Garden Room - Structural Design Advice

[efunc]

Hi all,

I'm in the process of building a garden room. It's a lean-to structure in the corner of the garden against the boundary wall. There is a wall at the rear face and against the right hand side:

My sketch is pretty rudimentary, but hopefully you can see a basic studwork structure in 2x4 timber on a brick plinth and 2x6 roof timbers. Obviously it will be clad in OSB (12mm on walls, 18mm for roof deck) and then finished in T&G cedar or larchwood cladding, etc.

My basic query is this: the building is 3.3m wide at the front and 2.8m deep. The actual roof, including the overhang to the right, is 6.1m across at the front (5.4m at the rear). The building features a set of 1.5m wide UPVC french doors at the front, and two 40cm windows on either side. There is about 25cm of studwork between each of these doors/windows. My builder says that with the two windows there will not be enough wall structure to support the roof and that I should just have the french doors. I have seen a lot of garden rooms which feature full width bi-fold doors, etc, and it seems a very common design, so is he correct or being over cautious? I concede that I have a wide amount of additional roof overhang to the right of the building (over 5m) which other designs may not have, but there is a brick wall for support at the rear and to the right.

What can I do, structurally, to make this design work? I can provide my Sketchup 3D design if anyone cares to take a more detailed look. Could I just add additional 2x4s vertically between the door/window components?

Thanks very much for any advice.

(EDIT: please note, my sketch isn't entirely accurate - of course the timber at the front support the roof joists will be bigger and laid vertically, not horizontally, and probably strengthened with a second one against it)

 

[noseall]

I'd be thinking ring beam (front only poss..?) to support the roof etc. Something like a couple of 175mm x 50mm timbers bolted together, fitted immediately above the door/window head (2.1m high). Unlikely to find anything to span 6.1m so an overlapping beam may be ok with the 100 x 50mm studs beneath.....

Ring Beam

• noseall
• 12 Jun 2019

 

[Notch7]

My builder says that with the two windows there will not be enough wall structure to support the roof

That isnt logical.

With or without the windows, the unsupported span is still only the French door opening. At worst you might want to put in some double studs either side of the windows to act as structural posts.

The only problem is the design has almost no boarding across the front to act as lateral support -however the full height wall and pier will act as a buttress.

Be careful to fix the open roof with straps to the wall to resist uplift from strong winds.

 

[tony1851]

Two structural issues here; the first is vertical loading from the roof, including the dead load (roof structure itself) and live load (eg 30cm of snow).
The OSB-clad narrow bits of wall would be well-able to take care of the vertical load.
The second issue is 'racking' under horizontal wind load on the side wall of the extension. As long as the roof is firmly fixed to the main house wall and to the header over the doors/windows, it will act as a horizontal rigid plate once the osb is on. Horizontal load on the side wall will be transmitted to the rigid roof plate, which will resist deformation and so prevent 'domino-type' collapse.

 

[^woody^]

I have a steel channel section doing a similar job and span. (3000 × 3000)

It's about 90 ×125 x 8mm IIRC, so have a look in the tables for something around this size.

Fix a timber in the web for fixing the cladding to.

But whether you want to get it calculated to be sure, is up to you

 

[tony1851]

Do they make channel that size?

 

[efunc]

Thanks all for the replies. Seems like there some great info here, but I'm not a builder so some of this is a bit over my head.

That isnt logical.

With or without the windows, the unsupported span is still only the French door opening. At worst you might want to put in some double studs either side of the windows to act as structural posts.

Ah, yes, I see what you mean. The stud timber spacing would be about 40cm anyway, so the full height windows aren't really interfering with the structure, they'll be positioned between studs. BUT, there will be less OSB holding the frame together, so maybe that was my builder's point. But he was particularly identifying roof support as being the issue with this.

The only problem is the design has almost no boarding across the front to act as lateral support -however the full height wall and pier will act as a buttress.

Be careful to fix the open roof with straps to the wall to resist uplift from strong winds.

I could certainly add additional vertical timbers between the doors/windows, but would it help if I increased the OSB cladding on the front from 11mm to 18mm perhaps too?

I'm intrigued about the suggestion of roof straps to the wall, but can't picture this. Could you point me in the right direction with a link or something? I had considered installing a vertical post in the middle of the front beam of the roof overhang on the right, but would rather not because it would restrict access to trhe storage area.

Thanks very much!

Straps?
https://www.preventivesupport.com/services/general-contractor/hurricane-straps/

http://www.lacartes.com/business/Loftech-Conversions-Ltd/35776/photos?album=110562&pid=269832

 

[efunc]

I have a steel channel section doing a similar job and span. (3000 × 3000)

It's about 90 ×125 x 8mm IIRC, so have a look in the tables for something around this size.

Fix a timber in the web for fixing the cladding to.

But whether you want to get it calculated to be sure, is up to you

Thanks for your input Woody. I'm not clued up enough to to know what this is. Does the steel channel form a roof support and run along the top or something? Do you have a link to something similar? I couldn't find much by searching online. Thanks again.

 

[^woody^]

The channel will span across the top of the frames as a beam, and the roof joists sit on it.

 

[Notch7]

But he was particularly identifying roof support as being the issue with this

Yes but you will be having a wall plate / ring beam / headplate whatever you want to call it, which will spread the point load of the rafters.

Since there is a bit of studwork between window and door frames, the headplate will have adequate support.

These are restraint straps -which can fix down where the open section of roof meets the wall. In very strong winds there could be significant uplift pressure -it may not be an issue if there is plenty of wall above. Its an important thing to add on a housr because the roof wall plate sits on the top course of brickwork -and the straps spread the fixings across lots of brickwork, not just the top course.

Maybe if you have plenty of brickwork above the roof, you could just fix some resin studding to hold on the wall plate of the lean to end.

As Tony said, you need the end of the long front headplate fixed very securely to the wall as that is your lateral restraint, locking tge whole structure to tge wall

 

[tony1851]

@ OP, IMO you're over-thinking this.
The header arrangement you have drawn over the doors would be fine if done in 4x2 - you've effectively got two separate 4 x 2 beams across the 1.5m opening. If the header/wallplate - call it what you will - runs continuously across the windows and doors, it will have the effect of stiffening the beam and reducing the stress in it.
The actual area of roof it will be supporting is about 1.5m x 1.5m, so a maximum of 2.8Kn load shared between two 4x2s = no problem, particularly if clad in osb.
You would need a more substantial beam, eg two 6x2s fixed together over the gap on the right.
As others have said, wind uplift can be an issue, so a few metal straps to tie the roof down to the brickwork.

 

[efunc]

Thank you, that's very useful information. I think I'll keep the design more or less as it is then and investigate these metal straps further. Once I've understood how they would work a bit better I might add them into the design and report back to check if I'm on track or not.

Thanks again!

 

[JP_]

Will there be a damp risk if using the garden walls? Or is this not important in a garden room?

I guess if big bi-folds are really desired then maybe a steel structure? Although I guess that makes it much more expensive!

 

[efunc]

Will there be a damp risk if using the garden walls? Or is this not important in a garden room?

I guess if big bi-folds are really desired then maybe a steel structure? Although I guess that makes it much more expensive!

The left and right side of the room are fully insulated and externally clad stud walls. The rear wall is the only one that places the frame against a garden wall, and the other side of that wall is the neighbouring house. That's the white wall behind the brick garden wall. There are three parallel walls here.

And bi-folds are not what I'm planning, but I was just surprised that my builder was concerned about having so much 'glass' at the front when I thought my design was quite modest compared to all the other's out there with huge wrap around glass doors.

 

[tomfe]

If you where to take the windows out I bet you'd not have any more studs or even different spacing.
Pack your windows glass well will help with racking. I can't see a problem with what you are doing unless you live in an exposed location where wind uplift might be a problem.