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I am planning to build a new shed that’s about 10 by 14 foot. I want to put a green roof on and plant out with wild flowers/sedum. I was going to put the rafters across the shorter length. The green roof suppliers have said a loading of 120kg per m2 is required.
Anyone have any experience of green roofing a shed and the size of rafters I may need.
Ta
I wouldn't to be honest, weight + damp will be quite an issue.
Could you not EPDM the roof, and put a few planter boxes up on top?.
From expeirience, it was ok for about 2 years, then the top edges of the walls began to soften and bow. Rebuilt half the walls and roofed with shingle instead
It wont just be the rafters, it will likely need to be the entire frame supporting them.
The rafters in your average house can be 8" deep, by 3" thick, but those can take an extraordinary amount of weight, so just a small shed then probably 2x4 would be more than adequate.
It would be better to tie each rafter to the next by way of noggins(or dwangs, if youre that way inclined), which will give you a completely solid structure where the entire roof is tied to all parts.
But its the frame it sits on which will also have the full weight bearing on it so that should be very solid, so 2x4 framing at maybe 14" centers, with 4x4 corner posts, and lots of noggins(or dwangs if you're that way inclined)
The earth is one thing, but in winter snow and being waterlogged will add a lot of weight, and it will likely be that extra you're accounting for.
Noggins(or dwangs if you're that way inclined) will prevent it from twisting or parallelogram'ng
Thanks
Was going to use tantalised timber, edpm then one of the green roof suppliers systems so think the damp should be ok. My main concern was the loading. Keeping below the 2.5m height with a small single pitch may end up with head/rafter incidents 😁
It's not just roof structure its entire structure.
Mines 6ft wide * 9ft long flat roof and half height. -just a bike shed
Made my corner posts out of 4 inch fenceposts with 4*2 framing in-between and 4*2 rafters with dwangs and big nails all clad in 18mm timber and ply with dpc/fibreglass mat/gravel/weed suppressant then 20/40mm compost/substrate mix depending on which area of the roof-gone for a hybrid extensive (sedums and herbs) and semi intensive leading edge where I have planted strawberries.
Only been in a week but already showing strong signs of growth
That said this is all on a flat roof. Bit harder on a sloped roof as you need to section it and it's probably better to use established mat than grow from single plants and seeds as I have.
Here's a completely useless photo showing none of the structure nor the fully planted roof but does show I need to paint it haha
Edit. Looks like it's an even more useless
There’s no need to guess. And you don’t need a structural engineer either. You just need a timber span table. Google it.
You know your required loading is 120kg/m2, most tables will describe this as 1.2kN/m2.
The tables will specify the spacing (centres) and timber size and type (eg c16 strength grade) that will support this load over a given span.
You’ll probably only find a table for 1 and 1.5 kN, so take a look and split the difference. Err on bigger.
First google result for me: https://www.timberbeamcalculator.co.uk/en-gb/span-table/flat-roof-joists?class=C16&slope=0&load=1.5
4 x 2 will not be any here near adequate for that loading. You'll be needing 7 x 2.
Search for trada span tables. C24 is stronger than c16 too
What Jim25 says 7x2...