Cut rafter tweaking required

[Digmixfill]

I've run into a snag with my cut rafter plan. My ridge beams are level, and the new wall plates are level, but I didn't measure the span distance between the ridge and the walls all the way along. I foolishly assumed everything would be parallel - I really should have known better.

 

One side of the roof is proceeding as expected. 50x150mm rafters at just under 35 degrees with a birdsmouth seat length of 60mm. The side with the progressively diminishing span isn't going to work at the shortest end. The angle changes, and the projection of the rafter at the foot is quite a distance away from the eaves edge.

 

If I put a piece of 22mm timber under the worst offending rafter with the current birdsmouth cut, everything works, but i'd obviously need to use progressively smaller bits of timber on the other rafters.

 

If i reduce the birdsmouth seat length, I can get it to work with the existing wall plate, but i'll be heading toward 30-35mm seat length. What is the accepted minimum seat length? I can only find details for maximum birdsmouth depth.

 

An exaggerated plan view of the roof attached. Red=ridge beam Black=eaves.

 

How would you resolve this?

[Nickfromwales]

If that side of the house is "out of whack" then you can only build up the internal side and mask it, leaving the wonky stuff only visible outside; assuming the building is out of whack vs the plate having splayed outwards?

[Digmixfill]

It's definitely the building being somewhat less than square. The wall plate hasn't budged at all.  At the worst point the span is > 75mm less than the opposing side.

 

You would keep the 60mm birds-mouth seat and shim the wall plate height for each affected rafter?

[Big Jimbo]

I would shim the rafters, so that the plane of the roof remains constant. This obviously has consequences for inside the building with ceiling heights etc. If this is a new build, i feel for you. The importance of keeping walls square when building is often overlooked. People seem to concentrate on levels. Squareness and angles are just as important. If you have room on the inside, i would think about dropping a false ceiling in, to keep that level.

60mm, although it sounds quite small, is going to need some get arounds, to not end up looking shite. If a new build, was it set out correctly, or has One of the walls got a lean in, or out while being built. Regardless, bit late to do anything about it now, if that is the case.

[saveasteading]

9 minutes ago, Big Jimbo said:

The importance of keeping walls square when building 

It may still be worth checking that squareness is the cause. If the building is rectangular,  measure diagonally and they should be the same within a few mm.

[Digmixfill]

9 hours ago, Big Jimbo said:

I would shim the rafters, so that the plane of the roof remains constant. This obviously has consequences for inside the building with ceiling heights etc. If this is a new build, i feel for you. 

 

I'm converting an old brick barn. The original walls are all over the place. I have built a new inner leaf following the best line I could. Everything is a work around

By the time i've stuck insulation under the rafters I won't see any evidence of rafter adjustments.

 

Would a shim cut from the C16 wall plate offcuts be sufficient? Just cut them to the width of the rafter and glue & nail underneath the seat?

 

9 hours ago, saveasteading said:

It may still be worth checking that squareness is the cause. If the building is rectangular,  measure diagonally and they should be the same within a few mm.

 

I'll check, but the odds are against square.

[saveasteading]

1 minute ago, Digmixfill said:

the odds are against square.

In most new buildings too. It's just worth the exercise to see if the numbers feel compatible with your issue.

 

I've got a pair of 100m tapes for this purpose. Where the right numbers cross, I drop a brick on the ground. I can use hitech stuff but this works better and doesn't need calibrating.

Later in the construction the same tapes remain ideal but the brick becomes too approximate and is surplus.

[Big Jimbo]

If it's an old barn let the roof follow the wonky walls. That can look great IMO

[Digmixfill]

5 hours ago, saveasteading said:

In most new buildings too. It's just worth the exercise to see if the numbers feel compatible with your issue.

 

I've got a pair of 100m tapes for this purpose. Where the right numbers cross, I drop a brick on the ground. I can use hitech stuff but this works better and doesn't need calibrating.

Later in the construction the same tapes remain ideal but the brick becomes too approximate and is surplus.

 

Internal walls are in the way for an end to end measurement, but I measured the diagonals of the largest room. It's around 4.5m long and 4-ish-m wide. One diagonal was a good 5cm longer than the other.

 

2 hours ago, Big Jimbo said:

If it's an old barn let the roof follow the wonky walls. That can look great IMO

Argh, that's not how I roll. It would bug me for the rest of my days if I didn't try to level things out

 

Which is preferred - glueing and fixing the shim to the rafter seat or glueing and fixing the shim to the wall plate?

[Big Jimbo]

I would shim the wall plate.

[Digmixfill]

By that do you mean you would lift the wall plate itself, or fix the shim to the wall plate?

 

[Digmixfill]

I spent a while experimenting with different things and I've opted for a lump of C16 under at least 2 rafters. A handful of stainless screws and once i've finished cutting the rafters and levelling i'll finally fix with cascamite and screws. The timber adjustment pieces get gradually thinner to follow the adjustment required. This gives me the same birdsmouth cut as the unadjusted rafters, so less mucking about marking and cutting.

 

[torre]

Bit late to this but I think I'd have preferred to seat the rafter on the 30-35mm you already have and supplement that with some substantial metal connectors. That would keep the plane of the roof flat. A company like Simpsons can probably suggest a connector (I joist rafters can be attached without any birds mouth after all)

 

Won't raising the rafters at one end like this give you trouble later as there'll be a bit of a bow on your roof battens - hopefully not enough to stop individual slates sitting flat.

 

With what you have, I'd still want a better mechanical connection between rafter and wall plate and would nail not screw (unless they're structural timberlok or something) for more shear strength. You need to be a bit careful you're not fixing your rafters mostly to the thin timber and leaving your wall straps holding down the wall plate below it, but not so much holding down the roof itself.