Joist size for green roof

[shaundiy]

Hi,

Soon I am going to build a shed 4.2 by 2.4 mtrs.
I am planning to include an intensive green roof which when wet weighs about 18llbs per square foot.

which If I have calculated it correctly is a total weight of 881kg

I am planning to use 2"x4" at 16" centres, for the whole structure. Is that sufficient or will I need larger joists for the roof or shall I reduce the spacing?

Thanks for your help

 

[noseall]

You will not only need deeper joists but to also stiffen the walls with some ply sheathing to prevent top heavy swaying.

 

[shaundiy]

I was going to sheath the walls with osb?
What size joists do you think I need?

 

[noseall]

150mm x 47mm.
probably.

 

[Static]

150mm x 47mm.
probably.

Sounds good.

 

[tony1851]

Hi,


I am planning to include an intensive green roof which when wet weighs about

Why bother with a green roof? What's wrong with ordinary mineral-finished felt - it's only a shed.
Green roofs are a complete waste of time - a passing fad. One day they will be like lava lamps and flared trousers.

 

[theoldun]

Personally we think you are crazy, but then again most people think we are, so why worry.
We have never done a green roof and have only briefly read up on them in the past, but if your calculations are correct, it would appear that you have a dead load of 0.85KN/m2.
We list our calcs and possibly someone can confirm them.
Square foot = 18lbs x 10.763 = 193lbs metre square = 87.3kg ms = 0.85KN/m2.
With a half span of 1.2 and pitch of 30degrees and a pitch line of 1.35, according to Trada tables, C16 47x100 at 400 centres will fit the bill. Imposed snow load is included in tables.
Would welcome any comments.
As stated we are crazy, but would go the route Tony suggested.. Torch on green mineral felt cap.
Regards oldun

 

[noseall]

With a half span of 1.2 and pitch of 30degrees and a pitch line of 1.35, according to Trada tables, C16 47x100 at 400 centres will fit the bill.

Surely if he is going to have a 'garden' on the roof a duo-pitched fella is out of the question?

My reasoning is he will have a gentle mono pitch i.e. flat roof with furrings etc hence 150mm x 47mm.

 

[tony1851]

Don't mean to be unduly pessimistic but after a year or so, he will have a roofload of mud and weeds.

 

[noseall]

Don't mean to be unduly pessimistic but after a year or so, he will have a roofload of mud and weeds.

A bit like the back of my chimney then!

 

[xdave]

Don't mean to be unduly pessimistic but after a year or so, he will have a roofload of mud and weeds.

It doesn't quite work like that. Assuming the OP is not going to be growing grass up there then the plants like sedum, etc, will outcompete the grass and weeds and completely colonise the space. It would only need light weeding a couple of times a year until it becomes established. There should be no mud on a green roof because you use a free-draining aggregate for the growing medium above either a reservoir or an absorbant mat. It is not as simple as unrolling turf over a felt roof.

The reason for having green roofs is not to replace felt roofs for functional reasons (a warm roof with extra insulation will achieve the same result). They are installed to improve the amenity of the area they are used in, to increase the biodiversity (bees, butterflys and ground-feeding birds love sedum roofs) and to aid with water runoff. In areas with few green spaces they also make a contribution to air quality.

If the OP really means an intensive green roof, and not extensive (as I've described) then he will struggle to get that on a shed. Intensive green roofs are specialist things to install that have at least eight inches of soil over rigid reservoirs and will add a significant amount of weight. On a small free-standing timber building it would be so top heavy that it would collpase in the first strong winds. Intensive roofs can have a weight in excess of 400kg/m2. Extensive are about 100kg/m2.

 

[KevGillan]

Hi shaundiy

Don't be put off by the pessimists - I was in Norway last summer and they've got green roofs on everything and they look great, especially with a 6' spruce growing out the top

I've been inspired to try this myself. Haven't done it yet so no experience to go on, but found the following joist size table.

Untitled

• KevGillan
• 14 Mar 2013
• 1

Its from a DIY Guide to Green Roofs available here:
http://livingroofs.org/about-livingroofs.org-living-roofs/greenroof-diy-guide.html

Good luck!

UPDATE: whoops - just realised how old this post is - please excuse the newbie..