Shed/Garage Roof Truss

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mwkenna

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Hi Guys

I am constructing a single skin block built garage/workshop/shed to replace an existing structure. The size is 8x4m to the outside edges. My question is on the construction of the roof truss and how to actually fix them to the top of the blocks. How thick should the wall plate be, what kind of angles do I need for run-off, how many trusses would I need etc...

If someone could please clarify some of this I would be eternally grateful!

Thanks,
Mark.
 
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Thanks guys, is it important then to bed the wall plate on cement? Would it not just sit on the blocks just as well?

Thanks,
Mark.
 
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mwkenna said:
Thanks guys, is it important then to bed the wall plate on cement? Would it not just sit on the blocks just as well?

Thanks,
Mark.
Click to expand...

As long as it's secured correctly, you'll sleep well. :)
 
Presumably when you say 'roof truss', you are referring to manufactured truss rafters?

Or are you referring to a cut roof?

Both sit on a 100mm x 75mm wall plate. This can be as little as 100 x 50mm if height is a problem.

The wall plate is bedded on mortar to the top of the masonry and is ultimately held down with bent straps.

Manufactured roof trusses are usually supplied with ancillary metalwork i.e. holding down straps, restraint straps, truss clips, twisted nails, truss shoes, hangers etc. They also supply the miles of 100 x 25mm node bracing as well as basic layout guide and calc's. You would normally inform the truss company of any nuances i.e. gable ladder sets, loose fill elements, soffit overhang, roof pitch etc, when the truss guy comes to measure once the plates are bedded.
 
Apologies, my terminology is often incorrect!

I am going to create the truss rafters myself, if it were for a house or something more important than a workshop then I obviously wouldn't but I am confident in my carpentry so long as I have a plan to follow. What I am unsure of is how to calculate the angles based from runoff requirements, thickness of the rafters, whether to cut a birdmouth or sit the rafter on the supporting beam, etc...

I could take a best guess based on common sense but I would rather hear it from the professionals.

Thanks,
Mark.
 
mwkenna said:
What I am unsure of is how to calculate the angles based from runoff requirements, thickness of the rafters, whether to cut a birdmouth or sit the rafter on the supporting beam, etc....
Click to expand...
First identify how you intend finishing the roof and if there are any height restrictions. Also decide if you intend using the roof space as storage. All these pointers will dictate the pitch of the roof.

For example, a roof pitch of 35 degrees or greater will just about allow any roof tile to be used upon it. As the roof pitch gets shallower, the choice of tiles etc, becomes more limited.

Rafter size will be dictated by span, pitch and whether or not you decide to use a purlin or not. Rafters should be b/m'd onto the wall plate and fixed to a ridge board at the peak.

We use trig' and pythag' to work out the rafter lengths, b/m cuts (plumb and seat) and all other cuts and measurements to do with the roof. We use architects and SE's to provide us with timber section sizes.
 
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