Roof rafter question

[amsie]

Hi all,

Im looking to build a roof this weekend on my single storey extension. Timbers are 200 x 50, and the wall plates are 100 x 50. My question is do I need to do a birdsmouth cut at the top as well as on the wall plate that sits on top of the brickwork (bottom of the rafter), or can I do a plumbcut at the top and a birds mouth at the wall plate on the brickwork?

Also when I fix the wallplate to the brickwork on the existing house what size Rawl bolts/anchors would I need?

Thanks in advance

Paul

 

[catlad]

Plumb cut is ok skew nailed to a timber bearer, I use 14mm thunder bolts.

 

[noseall]

A lean-to roof would typically have the (vertical) pole plate sleeve anchor bolted to the wall and the lower (horizontal) wall plate bedded in mortar with bent straps fitted down onto the walls.

Some b.c's are asking for either a b/m on the top plumb cut or in leu of this fix a clip or bracket to the junction.

The above is only relevant when you locate the pole plate under the rafter tops as opposed to level with the rafter tops as you would when fitting a duo-pitch ridge board.

When locating the pole plate under the rafters, it is better to fit all the rafters (or at least the two end felleas) so that they butt against the wall then offer the pole plate up to the rafters after.

 

[noseall]

TOP TIP:
Check that the wall you are butting against has not got any excessive bows, twists or bumps in it as this can seriously effect the rafter length cut, particularly when setting up your first pattern rafter.

It is best to cut a pattern rafter then try it in different locations along the run, checking that the top of the plumb cut is running level with a horizontal line along the masonry. Don't rely on the brick joints being level, i.e. strike your own horizontal line.

 

[amsie]

Thanks for the reply Noseall and Catlad.

So regarding the birdsmouth cut to the top is it worth calling BC to check before I start? I was hoping to get going this weekend.

Cheers

Paul

 

[noseall]

Thanks for the reply Noseall and Catlad.

So regarding the birdsmouth cut to the top is it worth calling BC to check before I start? I was hoping to get going this weekend.

Cheers

Paul

I think something akin to a truss clip will be ok, but make sure you buy the right width clip to suit your timber thickness, as trusses are typically 38mm.

 

[^woody^]

From a structural, and good practice POV, a birdsmouth will stop the rafter sliding and put the load down vertically

Just skew nailing wont hold a rafter trying to move, and a truss clip will just bend as its not designed for this purpose

 

[noseall]

a truss clip will just bend as its not designed for this purpose

Ok, use a glide shoe then.

 

[^woody^]

That will need a notch . LOL

 

[RonnyRaygun]

From a structural, and good practice POV, a birdsmouth will stop the rafter sliding and put the load down vertically

Just skew nailing wont hold a rafter trying to move, and a truss clip will just bend as its not designed for this purpose

A rafter can't slide on a lean to as long as it's well connected to the pole plate...agree that if butting up to the pole plate I'd want to do something more substantial than skew nailing. Notched onto joist hangers maybe...or angled nail plate both sides perhaps...

 

[noseall]

A lean-to rafter can't slide anywhere if it is butting against the (vertical) masonry unless the wall plate pushes out - relevant perhaps if the roof is vaulted and there is modest 'give'.

 

[amsie]

thanks for the replies all.

Think were gonna go with birdsmouth cuts to top and bottom.

Another question regarding the pitch of the roof. We can only get 10degrees max due to windows above. The architect who did the calcs and design has screwed up (thats another story) and has gone quiet on us. In the calcs and drawings he has specified a 100 x 50 wall plate, but with a low pitch would 100x50 be sufficient for 200x50 C24 timbers plus the roof tiles? (50Kg/m2)

Thanks in advance

Paul

 

[^woody^]

Wall plates don't do anything structural, so use what you like

 

[amsie]

Thanks Woody appreciate your reply.

 

[amsie]

sorry guys one more question.

For fixing the wallplate can I use M12 stud anchor bolts?

Cheers

Paul