roof truss questions

[pippo]

Hi all,

I am currently having a garage built, it will be single leaf wall construction and 6.3m long x 3.3m wide. It will be ready for the roof trusses soon as the blockwork is getting started this week so I have phoned a couple of joiners for quotes. They have asked me what size/type of timber and what spacing between the trusses however I dont really know what to ask for so I was hoping for some help here. The trusses will obviously span the 3.3m and will be around 1m high to the apex to keep the overall height of the garage around 3.5m. I phoned building regs but they were not interested in helping as the size and position of the garage does not require building regs. I am unsure at this stage if I will be tiling or pvc/metal cladding the roof so I would like it to be strong enough to take tiles incase I go down that road. So I need to know:

1) what size of timber for the trusses should i ask for?

2) what type of timber? (will treated stuff be required if it is left for a couple of weeks before the roof is boarded? )

3) what centres should the trusses be spaced at?

Thanks in advance for any help.

Pippo

 

[noseall]

If you are using a truss rafter roof, this implies you are buying them from a manufacturer.

The manufacturers works out all the sizing and node joins and provide calculations and a brief installation guide regarding strapping and bracing. They are machined up from a pre-set timber section.

Also, the manufacturers can provide you with the straps, bracing material, fixings, truss clips etc, and will also give you the option to have them treated.

Most truss roof designs are specced up for 600mm (max) centres.

I put mine at 450mm centres because i do not like 600mm centres of anything.

However, if you decide to build a traditional cut roof......

 

[pippo]

Thank for the reply noseall,

Just looked into what you said and I should have wrote it was to be a standard cut roof with ridge board, joists, rafters, purlins and birdmouths etc. I will ask them for a 450 spacing. Would 6"x2" be good enough to hold the weight of tiles at that spacing/span?

Cheers

Pippo

 

[noseall]

I am not in possession of span tables but what i can tell you is that at 1m tall your rafters will have a clear span (ridge to wall plate based upon 3.3m wide garage) of around 1.9m and will be set at 31 degree pitch.

You may get away with 125mm x 50 rafters and 150mm x 50mm joists at 450mm centres but i think the joists may be a bit lean.

Use a 175mm x 50mm ridge though 38mm thick would suffice but it will probably be machined from 50mm thick timber anyhow.

No need for purlins.

Use 75mm x 100mm tanalised wall plate.

Use galvanized m/s bent holding down straps at 1200mm centres.

 

[joe-90]

Did I read somewhere that garden buildings now have to be a maximum height of 2.4 metres? Maybe I had a nightmare?

 

[pippo]

Joe-90, Up here in Scotland its 4m with pitched roof and 3m with flat roof. Well it is with my local council anyway.