Building a Green Shed Roof

by Paul
(London UK)

Hi John, I'm constructing a shed 5 metres x 2.8 metres with a sloping roof of 8 degrees, high point back to low front. The roof will be an extensive green roof with a substrate thickness of approx 100mm. The walls will be framed from 50mm x 50mm (2" x 2") uprights with a 48mm x 100mm (2" x 4") top sill and cladding to the sides. The roof construction will be 48mm x 100mm (2" x 4") spanning approx 3 metres at 400mm centers

My question is does anyone have any experience with green roof construction and loadings? Will my design take the loadings or is there another construction method I should consider.

Any assistance would be appreciated

Thanks in anticipation



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Jun 24, 2009
 Green Roof Construction
by: John - Admin

Hi Paul,

Thanks for your question. Firstly well done for attempting a green shed roof, I hope that you will share the results with us when it is complete.

In relation to the roof, allowing a loading of 200kg/m² for the 100mm thick layer of soil/growing medium and a 75 kg/m² access load ( so that you don't fall through while building it) I calculate that you need 47/170 C24 timbers at 400 centres. There are two commonly available grades of timber at timber merchants, grades C16 and C24, there is a small price difference but C24 is the one you require here. We could go to a larger size in the lower grade if necessary.

For the walls I think that using 50x50mm is too small I think that the minimum that you should look at sould be a 47x75 and preferably a 47x100. Once again in C24.

You mentioned cladding if this is indiviuual boards such as shiplap or featherboard then you will need additional diagonal braces to resist wind loads and loads arising from any lack of verticality in the supports over the years. If you are using plywood or OSB then when this is nailed to the wall studs this will act as bracing in itself so the addditional bracing will not be required.

I hope that this is of help, let me know if you have any wmore questions. Please send in some photos and a report of how you get on.

Regards

John
Jun 24, 2009
 Reply
by: Paul

John

Thanks for the prompt reply, the information you provided was where my construction thoughts began, but suspecting I was being over cautious scaled back on stud sizes!

Construction is moving on and I'm about to order the wall materials, and now I will beef up the timbers; yes the cladding to two sides is shiplap, with the back and one end in ply as these are against the property boundary and not visible. As a DIY builder I did not appreciate the structural grading of timber, always learning.

I decided to take the green roof option becuase the garden is relatively small and I wanted to impact on the garden as little as possible, certainly from a visual aspect, also the increased atraction to wild birds and of course the challenge.

Thanks again, if I have anymore queries, I know where to come. Of course when I complete I will post some photo’s

Regards
Jul 05, 2009
 Further Questions
by: Paul

John

Re: Shed Green Roof Construction

I'm taking your earlier advise including using 7" x 2" roof timbers, however if possible, I would appreciate a couple of clarifications;

Where the roof timbers meet the front wall and rear wall, I will have to notch out approx 0.5" at the front and 1" inch at the back to enable the timber to sit square on the wall top. Could this reduction in width, especially at the back, compromise the structural strength of the timber?



I know this may sound a bit of a strange question, but with the potential weight loadings present, I need to double check all aspects before construction.

Thanks again
Jul 08, 2009
 Notches in timber
by: John -Admin

Hi Paul,
Good question. Notching the underside of a timber at the support does reduce its strength. In technical termis it reduces its shear strength.

I have had a look at the parameters for your particular case and having a notch of that size for the size of timber and loading that we have discussed does not reduce the strength below that allowed by the design code (BS5268).

It looks OK to me!

cheers

John
Jul 08, 2009
 Reply
by: Paul

John

Once again thanks for your confirmaation and support. This allows me to go forward with confidence.

I will let you know how I get on as the shed progresses

I'm off to Hampton Court Flower show on Thursday, hopefully to get some inspiration with regards to the green roof

Thanks again
Aug 06, 2009
 Drainage layer for green roof
by: Paul

Hi John, my shed is nearly there and I'm now looking at the green roof installation. I have a waterproof layer, pond liner, and now want to install a drainage layer, but I am unsure what to use. I would rather not buy a manufactured green roof system if I can help it and have seen blogs suggesting polystyrene peanuts (less weight) or thin layer of pea gravel (will pea gravel not slip) to hortag or simular. Do you have any ideas? Thanks
Aug 10, 2009
 Green roof drainage layer
by: Anonymous

Hi Paul,
Sorry for the delay in getting back to you, just got back from holiday!

In terms of the weight considerations there are no real concerns whatever drainage medium you choose. The roof design we did earlier will encompass the worst case.

I think (and it is only personal opinion) that pea gravel would be the best it is a natural material and also I think that you have a fairly shallow roof (less than 15 degrees) so slumping should not be a concern. I would think that you are going to be installing a filter membrane above this drainage layer to keep the drainage layer and the growing medium separate?

How did you get on with choosing the plants on your trip to Hampton Court Flower Show? It would be great to see the latest progress shots and find out the varieties of plants that you selected.

All the best

John
Aug 10, 2009
 Green Roof Drainage
by: Paul

John

Delay was not a problem as I too was having some time off although generally spent on pushing the shed forward. Hope you had a great holiday and if you stayed at home the weather was kind?

You are correct the pitch of the roof is circa 9 degrees and I will have a filter layer over the drainage, so shingle it is; I do prefer the use of shingle rather than polysterene or other; simple and natural product! The roof will be sub-divided into squares to limit the potential for slippage, the timber dividers being under cut and filtered to allow drainage. Also the sub-division will assist during the laying out of the growing area so I do not have to walk over the laid roof too much, it's a large area.

I do have lots of pics on the build developement, but have not sorted through them yet, I will see if I can pick a few and upload them?

Hampton Court was a great show, but also disappointing from a green roof aspect. Although plenty of exibitors included retro green roofs to shed displays, none could provide information on the design and build. I was not able to locate any other exhibitors (none commercial) that could provide information on green roofs, the commercial boys (not many) were there to drum up business, not provide free support!

The show had clearly been effected by the current finacial climate and was smaller than previous years; as I said still a great show but with very limited information for my current project.

So I'm back to using the trusty internet for my planting research, but have not yet managed a planting list, still a little time to go, I probably don't want to plant up until the hot sun (when it shows) has pasted on by for another season

Thanks again for the support

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