Shed roof: joists or no?

JMSE

Hello one and all,
Ive built the 4 walls of my big garden shed 8m long and 5.25m wide. The roof I am building will be hipped with a 30 degree pitch up from both sides. Being over the minimum size I got planning permission the drawings of which show a roof with joists going from wallplate to wallplate. Are the joists necessary as there will never be a floor up there under a 30degree pitch? The tiles will be normal Roadstone Spanish roll tiles so they are a factor. Could I not put a purlin from rafter to opposing rafter higher up ... say halfway up. Anyone know the limitations? 5.25m is 17ft 3 inches.

Tails142

The joists are designed so that they act in tension - to give a triangular resolution of forces between the rafters and the walls

if you dont have the joists, the roof could theoretically spread out wider with the load of the tiles applied and if the deflection is large enough - it could cause the walls to deform and ultimately collapse

JMSE

It could, I suppose I might get away with no joists with a narrower span but maybe 17' is just too much. Maybe an option is to put in a joist every second rafter or so. It might seem a little negative to be trying to cutback on the amount of joists but theres also the greater access to the overhead space on the positive side, theres hardly a need for a joist every rafter when theres no floor or ceiling slabs relying on it.

Mellor

Use a collar istead of joist. A joist spans from rafter to opposite rafter about halfway up, could even be close to 1/3 from the top. This will save timber and increase head room.

JMSE

Is there a ready reckoner type guide anywhere to dictate when one can go for a collar. Also what size timbers 4x2? Anyone out there use a collar and what was the span and type of tile on top?

*edit* a collar would probably only suit a tin roof, cant see the weight on a heavy roof been supported properly

Mellor

What you can go for depends on span and loading. I don't know what you are basing tht a collar is only suitable for a tin roof on, but it is incorrect, for short spans a collar is fine. We used one in our extension. The size of members will be dictate by the span and load. This is set out in IS 444. Remember roof members in a simple roof will be larger than a complex prefab truss.


EDIT: assumming that the 5.25m is the internal measurement between walls, and 30 degree pitch
IS444 calls for rafters to be :
44x115, C18, @ 400c/c,
44x125, C16, @ 400c/c
44x150, C16, @ 600c/c

For anyone that isn't sure what these numbers mean, it's ;
Dimensions, Strenght class, Centre to Centre distance

JMSE

Mellor thanks a million for that data, its exactly what I was looking for. I wasnt basing my collar ideas on anything, I was only reckoning.

The 5.25m is actually the outer measurement. I think I will go for 600mm centres and the corresponding 44x150s, with collars between rafters but being the overkill individual that I am I will probably put a joist in too. I was even thinking of adding hangers down from each collar end down onto joists.

Mellor

If thats the external size then the rafters can be reduced. Collars are great for head room, thinking about it unless you really need the head room, then having joists to hang awkward tools and bits off is handy.
Get your hands on IS444 (its included in homebond if you have it, but available online)
and have a look at different options foe sizes and centres and use this to work out cost.