Afternoon all. I am, once again, in need of some help/advice.

Next Friday I will finally be moving house. The new house has a single sectional concrete garage round the back that I need to pull down and replace with a bigger version, but with a gable roof for extra storage space. The garage will be 3.5M x 7M, with the roof pitched across the 3.5M width. Due to digging back into the garden, I am allowed a ridge height of roughly 3.75M, so giving 1.5M within the roof. This gives a roof space that will look something like this:


Garage roof outline

The roof will be a traditional cut roof rather than trussed, so there will be no additional supports for the roof other than the walls. The rafters will be made from 2" x 6" beams, which I know are fine for the roof weight. The problem I'm having is spec'ing the joists. I know how to calculate their size for the storage loading (as if they were flooring joists) but I am unsure how to factor in the strength needed to maintain the roof shape as well. Does anyone either know how to do this, or can point me to somewhere I can figure it out for myself?

Before anyone says, I could ask the guy who's designed the garage, but that's me! The garage is sized to be built without needing planning or building regs, although I want to build it to at least equal the regs.

I have just done this but my garage is also 6m wide and to avoid having ceiling joists (I am putting in a 2 poster ramp) I used steel RSJs for the purlins and ridge.

Edit: Just re read your post and you do want a ceiling. I would say if you are happy that the joists are ok for storing loads of stuff in the loft then they will be fine to hold the roof together. If you are not using trusses then you should consider some purlins depending on the roof covering material.

[Edited on 28/11/12 by mookaloid]

I assume by your posting on being confident on strenghts then you must have dome some calcs, if so there is plenty of info on loadings in the old version of Part A of the building regs:
http://www.planningportal.gov.uk/uploads/br/BR_PDF_AD_A_1992.pdf
towards the end are all the truss/rafter sizes etc.

MMmm, looks like 2" x 8" at 400mm centres should be plenty, which is what I was aiming for.

Anyone see any issues with using that size?

Can you use a gambled roof, barn type, my garage is 20'x 30' with a barn style roof with an area of'8'above the main floor ceiling.

This gives a storage area of 12'x30'x6.5" which allows for storage of 3 snowmobiles a garden tractor and other unused toys.

I could send a picture, but I am far from computer literate.

The roof is supported by what are referred to as Barn Trusses, here in Canada. The main floor walls are of 2"x6" wood studs at 16"centres.

I know the sort of roof you mean, I know it as a mansard roof. And whilst it gives more usable space, it's also quite a bit harder to make. And since I'll be doing all the roofing myself, ease of construction is important. As is speed, as I'll be doing most of this in Jan/Feb and I expect it'll be proper brass monkeys outside.

Si,

Use 50x200 at 600 centres for the floor joists.
Use 50x125 at 600 centres for the roof joists.
For simplicity use a 50x200 ridge beam and 50x125 diagonal wind braces fixed to form a 7m x 3.5m "X" in plan view, fixed to the underside of each intersecting rafter.

That all complies with BS 8103-3.

Cheers R

[Edited on 28/11/12 by rachaeljf]

Rachael,

Thanks for that! Those beam sizes are what my existing garage uses for the rafters, so that's good! They'll be cheaper too.

Do you happen to know the weighting limit of the floor joists? I'd rather double up in certain areas for the heavy weight bits like engines and so on rather than use OTT beams for the whole floor.

quote:

---

Originally posted by DIY Si
The problem I'm having is spec'ing the joists. I know how to calculate their size for the storage loading (as if they were flooring joists) but I am unsure how to factor in the strength needed to maintain the roof shape as well.

---

1.5 kN/m2 is the usual domestic floor load, which is roughly 150 kg/m2, OR a 200 kg concentrated load (not both together!), which is a fair bit to get up into a small roof space like yours. The typical solution for really heavy loads is to double up the joists where you want to put the load.

[Edited on 28/11/12 by rachaeljf]

Not forgetting that the load tables are based on acceptable deflection in a domestic floor, not on failure loadings. If you do overload the ceiling a bit, it'll just deflect a little further, which is most likely perfectly acceptable in this situation.