Garage roof structure advice please

[Anonymous]

I'm building a modest garage with a pitched roof, covered in pantiles. The garage will have a floor area of less than 30 m2 so won't need building regs consent.
However, I need to make sure that my roof structure is correctly specified to support the weight of tiles. I propose rafters of a simple inverted V in softwood with tie beams fixed across the wall plate - possibly with a hanger from the ridge board down to the centre of each tie beam.
Can anyone please recommend the correct section and distance apart of softwood timbers for me to use for the rafters, tie beams and hangers? The tiles will be clay pantiles, 41kg/m2 and each face of the roof will be approx 20m2 (5m wide x 4m high).
I hope that's enough information, and thank you in advance for any help.

 

[Anonymous]

David, what is the span from wall plate to wall plate and do you know either the pitch of the roof or the height of the ridge?

 

[Anonymous]

My 1985 building regs book says rafters of the sc3 grade 47mm by 100mm on a 22.5 to 30 degree pitch would be a maximum 2.32metres long ( Sc4 2.92metres )with a loading betwee .5 and .75 Kn/m2 and rafter centres at 400mm . I think the grading classification has changed now, normally use oak so out of touch. This sounds as if good old 4x 2 should cover you ok.
If not give me a shout and I will look at the charts again.

 

[Anonymous]

Personally I would have no hesitation in constructing this type of roof with PT graded 150x50 joists and collars.

Roof at 45 deg
Collars bolted and nailed at roof plate level.
angle plates nailed at all joints ( left&right)
All at 400 centre
Wind braces ( 100x25) 4 no, nailed on inside.

If storage needed above collars, strap from ridge plate to collars.

Ridge plate 175x25

All in a nutshell , I would be very confident.

BUT:-
If you want a lower ridge,( down to 35deg with pantiles) you are in different stress country.

I am sure someone posting on this forum, will get their slide rule out an will put you (and me) on the right track.

 

[Anonymous]

Thanks for your response, John. Span plate to plate will be 6m and roof pitch 41 degrees.

 

[Anonymous]

Many thanks for that, Middi. I had intended a roof pitch of 41 degrees - does that make a big difference, d'you think? Plate to plate span of trusses will be 6m.

 

[Anonymous]

By the way, please could you tell me what PT graded means? I'm from a wood furniture background, so construction gradings are alien to me. If I just tell the timber supplier "PT grade" will he automatically know what that means?
Thanks again.

 

[Anonymous]

Many thanks for that Jon. Actually I'm planning considerably more pitch than that - pitch will be 41 degrees, so rafters about 4m long. Trusses will be 6m long.

 

[Anonymous]

Sory, Middi - one other question: when you say "collars bolted and nailed at roof plate level" does that mean bolted to the rafters and nailed to the plates... or the other way round?

 

[Anonymous]

PT=(pressure treated)
Graded ( lowest grade in your case, usually C31)
they used to be SC1,SC2,SC3 etc..

Nowdays graded timber is not what it used to be, they grow it at 200MPH, but if it is pressure treated and well build it should last a very long time.

the ungraded stuff at times is a pile of shite.

Rafters and collars:-

Presumably the rafters will be " bird mouthed" on the eaves plate with an overhang.
Once in position spike them ( 4in nail) on either side and one from the top onto the plate.
Position your collar on to the plate , using a clamp hold onto rafter, drill and fit M10 bolt(one only) with washers.
You can also spike the collar from the side onto the plate.
For total security, if exposed to winds, fit thin zinc plated "L" plates to either side of this connection .
This is cooking it a bit for a garage roof but they are cheap.
Coupled with bracing it will be a very stong roof.

By the way , why 41 deg?

 

[Anonymous]

I make the rafters about 3950 long - which is on the long side for a single length - i.e. with no purlin support.

If you were following building regulation standards, Table A13 of ADA for a rafter 50x150 SC3 timber at 400centres gives a max rafter length of 4M.

In fact that is slightly overkill in your situation and I would either use the same but go to 600 centres or I would use a 50x125 instead. Both these will be plenty.

Have you costed truss rafters? I recently did a double garage for a client and the truss rafters cost just over £300 for the whole set delivered. Nice and quick to put up.

 

[Anonymous]

David, sorry didnt read your post thoroughly, yes 6 x 2 is what you want for a cut roof. Of course if you go for trusses they will be more expensive than a pile of wood but shoulkd take only an hour or two to erect.
Middi has suymmed it up pretty well.

 

[Anonymous]

Many thanks for the spec John. I had briefly considered trussed rafters, but (in my ignorance) assumed that they would be vastly more expensive than constructing the roof from simple sawn timber. On the basis of what you say, I will certainly look into them. Definitely a much quicker and easier way of doing the job.
If you use trussed rafters, presumably you don't have a ridge board? If this is the case, does one rely simply on the battens and the diagonal wind braces to brace the whole roof structure?

 

[Anonymous]

Many thanks for all that advice, Middi.

Why 41 degrees? Well, my house is listed, and in Suffolk, so I reckon a good steep pitch is necessary for the structure to be sympathetic with the house, and given that the garage will be deeper than it is wide, that's the way the pitch worked out on a purely aesthetically pleasing basis.

That's my attempt to make a somewhat random decision seem logical and technical!

 

[Anonymous]

David,
Trusses rafters could well be cheaper as they use less timber e.g your truss as a queen post (one vertical central web plus 2 diagonal would be 35mm x 72mm rafter and 35mm x 97 ceiling joist or if you prefer it in 47mm timber a King post ( 1 vertical centre web) 47mm x 122mm Rafter and 47mm x 97mm ceiling tie both of these are at 600mm centres using standard tile loads.
Bracing would be 97mm x 22mm at each node point i.e. 2 for the King post 4 for the Queen post plus 4 diagonals under the rafters for both.
It's quick & cheap the chioce is yours.

T'other David.

 

[Anonymous]

David

Many thanks for those ideas. I think I have pretty well decided that I will go for trussed rafters. I favour the kingpost design because it leaves me a clearer roof space for storing my timber! I understand everything that you have so clearly described, except "node" points. Whereabouts are these on the kingpost design please?

Thanks again,
Namesake

 

[Anonymous]

David,
The node points are where members join one another, for your King post that will be at each heel (rafter to ceiling tie) peak (rafter to rafter and top of the internal web) and ceiling tie to the bottom of the web. The only longitudinal braces you will need are at the last two as the wall plate will act as the brace for the heels. You will also need diagonal braces on each rafter (braces are 22mm x 97mm softwood)
If you e-mail your address to ddyeratbtconnect.com I'll post you a guide to trusses.

David