Building mono-pitch roof

[Jack07]

Hello,

I was hoping someone could give me a bit of advice as I'm about to start building a mono-pitch roof over my new single storey side extension to my existing house.

The building inspector has asked for a birdsmouth cut on the top wall plate as well as the bottom. I'm more than happy to do this as I'm building it myself I would prefer a more belt and braces approach anyway.

The issue with that, is fixing the wall plate and working out how much lower it will now need to be to allow for the timber above the plate. They will be 6" rafters and I know that I can't cut more than 2" into the timber.

I was going to string a line from where the rafter will finish on the existing wall to the lower wall plate allowing for a minimum of 4" of timber on both.

From there I could work out the location of the upper wall plate.

As I've never done this before I would appreciate it if someone could give me some rough guidance on if there is an easier way to go about this?

Regards
Jack

 

[foxhole]

You could do a mock up on a sheet of ply laid on the floor, allows adjustment then you can transfer dimensions when your happy with layout .

 

[Jack07]

You could do a mock up on a sheet of ply laid on the floor, allows adjustment then you can transfer dimensions when your happy with layout .

Thats a good idea, I have loads of ply here and am still waiting for timber (weeks by the looks of it!) So I've got plenty of time to work it out.

Thanks for the reply!

Cheers

 

[noseall]

I would appreciate it if someone could give me some rough guidance on if there is an easier way to go about this?

Regards
Jack

You cut a pair of rafters first and let them dictate the position of the wall plate. You need to be aware that with monos (lean-to) roofs, that the wall they are abutting isn't always perfect.

We cut say three rafters and use one pair as a guide at each end of the roof. We use the third to test the viability across the whole roof, dropping it in several locations along the plates. This tests whether the wall is running true (in or out) and whether one-size-fits-all is ok.

 

[23vc]

You cut a pair of rafters first and let them dictate the position of the wall plate. You need to be aware that with monos (lean-to) roofs, that the wall they are abutting isn't always perfect.

We cut say three rafters and use one pair as a guide at each end of the roof. We use the third to test the viability across the whole roof, dropping it in several locations along the plates. This tests whether the wall is running true (in or out) and whether one-size-fits-all is ok.

that’s what I should have done on my last one. I set the top plate dead level then ended up cutting several slightly different rafter patterns across a 10m run.

 

[mrgobby]

Roof pitch determines birdsmouth angles, need only be cut wide enough to sit flat on plate. 4" wide may not necessarily mean 4" deep, shallower the pitch = less height is cut out. Don't over cut depth more than necessary, just enough to sit level & flat on the plate. Your eaves detail determines rafter overhang length. Whether you need/want a soffit or are fitting it tight to wall with no soffit. Usually determined by aesthetics, if rest of the house has wide soffit you might want to mirror that (or not!). If built reasonably square then as Noseall says you can check that along the length with the template to see if one fits all solution is possible which is easier. You have some leeway adjusting the birdsmouth position but you will want your overhang lengths to match so your fascia runs parallel and doesn't look like it's doing a Mexican wave. - I'm not a roofer or carpenter, google lots of images and watch youtube vids will all help give you a better idea of to and what not to do.

 

[Jack07]

You cut a pair of rafters first and let them dictate the position of the wall plate. You need to be aware that with monos (lean-to) roofs, that the wall they are abutting isn't always perfect.

We cut say three rafters and use one pair as a guide at each end of the roof. We use the third to test the viability across the whole roof, dropping it in several locations along the plates. This tests whether the wall is running true (in or out) and whether one-size-fits-all is ok.

Many thanks for reply.

I was going to make sure I had a few rafters to check the levels throughout and 3 seems a good idea.

Unfortunately I don't have any rafters here due to shut down! I have the wall plates but until I can get hold of some timber i can't really proceed. That why I wanted wall plates up first so I can get on with that!
I currently have scaffolding at correct height so I would rather fit wall plates off scaffolding than ladders.
Looks like I'll have to let scaffold go and manage with timber afterwards.

Thanks for advice

 

[mrgobby]

You could screw together pieces of your ply to replicate a rafter length and use that to create a template ready for when timber is available, save you trying to do that bit off ladders. You might find a timber merchant doing a delivery only service, it's supposed to be essential building supplies only but businesses are still operating out there under the operational rules in place.