Green roofed shed - structure advice please

[KevGillan]

Hi folks

I'm planning to build a 3x4m timber shed/workshop in the garden. I am keen to avoid concrete and also very keen on giving it a green roof. I think the saturated weight of roof is likely to be in the region of 2-3 tonnes. (Does that sound ridiculous?)

I have some Sketchup screenshots below of the kind of structure I have in mind. Looking at it though I'm worried about the whole structure raking from the tall side to the short side.

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• KevGillan
• 14 Mar 2013
• 1

Posts sunk about 1m into hard clay ground

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• KevGillan
• 14 Mar 2013
• 1

Double 50x200mm beams on both long sides.

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• KevGillan
• 14 Mar 2013
• 1

Slope will be around 8-9 deg. (There's a post cut off in the middle of the short side there for a large window.)

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• KevGillan
• 14 Mar 2013
• 1

Lots of rafters - probably 50x200mm at 400mm centres across 3.2m span

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• KevGillan
• 14 Mar 2013
• 1

Obviously there's room for some stud work there, but really relying on six posts to keep everything upright.

Any advice on this would be very much appreciated. I've never built anything bigger than, er, a workbench so a bit in the dark.

Cheers
Kev

 

[tony1851]

The rafters are not the problem; it's the posts.

I wouldn't fancy working in that shed with two or three tons of slutch swaying about above my head.

 

[KevGillan]

Okay, cheers Tony.

Is it compression strength you'd be worried about (solve with massive posts?) or the swaying? I wonder if diagonal bracing could solve that?

 

[^woody^]

It depends what you are cladding it with, and whether that will give the required racking resistance

 

[KevGillan]

I was thinking 9mm ply internally.

Externally I had in mind simple horizontal overlapping boards (clapboard?) as a friend can source some cedar cheaply. I hadn't really thought of the external cladding as structural though.

Plus, of course, some studwork. Perhaps like this...

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• KevGillan
• 14 Mar 2013
• 1

Thanks for your thoughts.

 

[^woody^]

It looks substantial, bordering on over-design, but I can't see any problems with it in terms of stability and load bearing.

Have you researched the sedum or green roof, as they should be free draining and so will it be as heavy as you mentioned?

Personally, I don't like timber posts set into the ground as they never seem to last long, unless treated oak, or whatever they used to make telephone poles from

 

[freddiemercurystwin]

Aye, timber posts into the ground is daft, its not a deck you're building. They will rot.

 

[KevGillan]

Okay, thanks guys.

Yes, can see your points on the rot problem. I guess even the best pressure treated stuff will at some point and its not like there's a sensible way to do maintenance on it. (Uncharacteristically dry weather over the last month probably made me forget how much it rains in Manchester too!)

Perhaps its back to the drawing board and thoughts of a low masonry base to build a timber frame up from. Perhaps I'll start a blog on the projects section once I've got more sense of direction.

On the roof weight, Woody, yes sedum can build a lighter roof as you can get away with much less substrate. Its entirely an aesthetic thing for me but if I can I'd like to get a greater depth so I can grow a wider range of plants.

Thought maybe not as much as this:

(From http://bit.ly/Yel4iW)

 

[tony1851]

made me forget how much it rains in Manchester too!)

Oh come on, Kev, you know it dosn't rain in M/c that much!!

 

[maltaron]

I would think that your slope is too much, I can see all the soil running off.
You should also consider possible planning issues, some councils may consider it a raised deck.

 

[xdave]

With no access onto the roof it is unlikely any council would consider a building to be a raised deck because it has a green roof. As long as it fits in with your PD rights then you won't need permission.

As a general guide intensive green roofs can weigh over 400kg/m2. Extensive green roofs are about 100kg/m2. I have never designed/built an intensive roof but the basic principles are the same. You would use a plastic retaining mesh laid over the roof covering to retain the drainage medium, lay a geofabric mebrane, then use the same mesh to retain the growing medium. Where depths of medium over 100mm are required I would stagger mutliple layers of mesh and medium pegged every 300mm to reduce the risk of slips. Once the green covering has established the risk will be very minimal. Your proposed slope will be fine. You may want to consider installing reservoirs in the drainage layer to retain some of the water during dry periods. You can get proprietory plastic trays that are installed over the drainage medium but under the growing medium which can retain up to 30 litres/m2.

Out of interest what are you intending to grow on there?

 

[Gazman16]

For The timber in the ground you could look into Accoya. I think it is garenteed not to warp, twist or rot for upto 25 years underground.

You can read a bit about it here:
http://forum.expertexpert.com/viewtopic.php?f=9&t=47

 

[pinenot]

Hi Kev, from your post I deduce you are keen on sustainable building, in which I Mastered at uni.

You say hard clay? to be even more sustainable why not use this to your advantage, and build a rammed earth shell, say with timber rafters??

http://www.youtube.com/watch?

v=3zKv0f6hXaMhttp://www.thegreenroofcentre.co.uk/Library/Default/Documents/Green%20Roof%20Pocket%20Guide%20V3.pdf


good luck...pinenot

 

[tony1851]

to be even more sustainable why not use this to your advantage, and build a rammed earth shell,

How thick would his walls have to be? And how would he fix shelving etc to the walls?

 

[^woody^]

Why not build it out of recycled coke bottles?

Perhaps he just wanted a timber building. And timber is just as sustainable as anything, and far more practical for building than a rammed earth monstrosity

Back OT

There would be other options for post supports, either choice of timber or careful design to allow future repair/replacement