roof truss design for living roof

[catfordbags]

i've built the walls for my new 3.6 x 7.6m outbuilding and now i am in deep thought about the roof ---proper diy - zero progress outside - 100% head scratching inside.

here is the basic idea for my roof truss from a book i've been following (building green)

however i have changed the shape somewhat and this is what i'm thinking of building....

1. please pick the bones out of my design and crucify it if necessary.

2. the bloke in the book doubles up his rafters and then increased his centres from two foot to four foot. He then decks the roof out with 6 x 2 t & g planking (to support the heavy living roof) - this looks really nice and i am keen to follow suit but i'm not sure if i can afford it - would thick marine ply do the same job or are there any other alternatives for decking four foot centres ?

3. in the book he uses 8 x 2 for his trusses - i was hoping to get away with 6 x 2 - any thoughts ?

 

[wms]

you could always take your ideas to a roof truss manufacturer explain to them what you intend to cover it with they have programs that will calculate the loading etc so you will know what timbers will be used also easier to satisfy mr building control if involved.

 

[^woody^]

Can a sedum roof be on such a pitch? Wont the soil, plants and moisture all gravitate towards the bottom of the pitch leaving the top parts bare?

I cant see 1200mm centres being satistactory.

That guy is way over spec'ing the truss timber sections

A truss relies on the inherent shape, and not on the thickness of the timber. What is the point on having thick timbers at 1200 centres? Why not thinner trusses and closer together?

And don't forget that trusses need horizontal and diagonal bracing too

 

[catfordbags]

sedum roofs - can generally go up to 30 degrees -you sometimes need to add a timber grid onto the deck to prevent slippage..

the bloke doubles his trusses and goes for 4 foot centres for asthetic reasons since his roof is exposed from beneath.

likewise - i'm now thinking of doubling my 6 x 2 truss and going for 813mm centres to fit in with 8ft x 4ft plywood sheets......? - effectively four trusses holding up the 8ft of plywood....?

1. anyone know how strong 18mm wpb is ? is marine ply stronger ?

2. ..and another question - ridge beams - should my ridge beam be a single length of timber - 7.6m - or can i join 2 or 3 pieces together ?

 

[^woody^]

A trussed roof does not have a ridge beam - ridge beams are for cut rafter roofs They can be joined - normally with a scarf joint

There is no way 18mm or even 25mm ply will span 800mm without deflecting under load. Marine ply is not stronger, just more water resistant

Don't forget you have to allow for about 250mm of snow too

 

[noseall]

i don't think the o.p. is listening woodster.

 

[catfordbags]

woody - sorry for the endless questions but here is the next....

would a deck made up of 6 x 1 be stronger than 25mm ply ?