DIY Si
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| posted on 28/11/12 at 04:35 PM |
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Garage roof beam help please!
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.
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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mookaloid
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| posted on 28/11/12 at 04:49 PM |
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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]
"That thing you're thinking - it wont be that."
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loggyboy
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| posted on 28/11/12 at 05:12 PM |
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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.
Mistral Motorsport
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DIY Si
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| posted on 28/11/12 at 05:27 PM |
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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?
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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Canada EH!
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| posted on 28/11/12 at 05:46 PM |
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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.
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DIY Si
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| posted on 28/11/12 at 06:29 PM |
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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. 
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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rachaeljf
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| posted on 28/11/12 at 08:08 PM |
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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]
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DIY Si
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| posted on 28/11/12 at 08:41 PM |
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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.
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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JoelP
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| posted on 28/11/12 at 09:13 PM |
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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.
The fact that the floor joists are also preventing the roof from spreading will not have much affect - supporting the junk you put up there causes
them to bend, whereas the weight of the roof stretches them, which they can resist far easier. Myself, id just treat it as a floor and forget the roof
element.
Beware! Bourettes is binfectious.
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rachaeljf
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| posted on 28/11/12 at 09:19 PM |
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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]
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SteveWalker
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| posted on 28/11/12 at 09:41 PM |
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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.
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