woodworking question - scarf joint.

[bigegg]

I want to make 3, 20ft long, sloping, beams to repair my (20ftx10ft) garage roof.

I've managed to get hold of a quantity of 13ft scaffolding boards for a very reasonable price, but obviously I have to scarf them...

I plan to also have the beams cut on a slope:
if my maths is correct, by cutting a board from 9in wide at one end to 4" at the other, that will give me a 30" overlap, and after I cut 40" off the "offcut" it will give me a 2" lower end.

^^^ a 20ft sloped beam, 9" at one end, 2" at the other.
in the middle (ish) there will be a 30" overlapped section, 5" sloping to 4".

I need some help designing the joint:
I'm planning on using gorilla glue and coach screws.
a vertical 1inch "nib"(?) on each end then the slope between them.

am I better with a 1/2 nib, and a steeper slope?
or am I overthinking it?

the outer beams will rest on the outer wall of the garage. the centre beam will be supported at the back on the wall, and at the front by a 9" cross beam (using joist hangers)
there will be some noggins between the long beams, and some shims between the existing (very bowed) plywood roof and the new beams

 

[addjunkie]

Ive been looking at manufactured wooden I beams, and span tables etc for my next project. And even with no joints your board doesnt sound man enough for the task.

 

[TechnicAl]

I tried to joint 2 boards for the centre beam in my shed....just kept breaking....I tried to do finger joints...I think I cut 4 if my memory serves me well....glued em, screwed the outside ones, clamped em....broke every time..........made a post to rest them on...worked no problem

 

[bigegg]

Ive been looking at manufactured wooden I beams, and span tables etc for my next project. And even with no joints your board doesnt sound man enough for the task.

I don't think it will be an issue:
it will be effectively fully supported by the (very bowed) roof which is already on, which is
6x2 joists at 2' centres, 36mm of ply on top.

this will just be a frame to increase the slope, and remove the bow.

I think that even if I built two separate 10ft sq frames it would still work.

 

[bigegg]

I tried to joint 2 boards for the centre beam in my shed....just kept breaking....I tried to do finger joints...I think I cut 4 if my memory serves me well....glued em, screwed the outside ones, clamped em....broke every time..........made a post to rest them on...worked no problem

yeah, finger joints are no good for this.
it's a leverage thing.
but scarf joints have been used for centuries for this: apparently, with modern adhesives, the joint is stronger than the surrounding timber.

as I say, maybe I'm overthinking it.

 

[tflnemesis]

Didn't know what a scarf joint was but google came up with this page...

https://timberframehq.com/tag/scarf-joint/

 

[bigegg]

Didn't know what a scarf joint was but google came up with this page...

https://timberframehq.com/tag/scarf-joint/

that's exactly the info I was after.
thanks!

 

[eLuSiVeMiTe]

Why not span the 10ft and do away with the scarfs.

Any joists ive had to joint mid span have been doubled up with joints staggered and bolted at 600c with spiked washers.

 

[Screwdriver]

When I needed to make some superlong joists, I double them up and through bolt. Like this:

https://www.mig-welding.co.uk/forum/threads/do-i-need-a-surveyor.76163/page-3#post-1084533

They have been holding up well. Thats a lot of wood mind...

 

[bigegg]

Why not span the 10ft and do away with the scarfs.

Any joists ive had to joint mid span have been doubled up with joints staggered and bolted at 600c with spiked washers.

mainly because it means ripping a load of 10ft boards to an exact width to make the slope, then leveling each one in turn, and shimming.
I have 13ft boards so I'd be discarding 33ft of offcuts if I cut them to 10ft.
by using the long beams, one of them effectively becomes the lintel over the window - which saves me another problem - because running them crossways means I'd have to support 3 of them above the window anyway. the 20ft beams would effectively become they're own straight edge

 

[addjunkie]

If its all sitting on a stable supported roof, why joint them at all, fix down, and the new roof sheets will hold it all together, if you fix through.

 

[eLuSiVeMiTe]

What way do the original joists run out of curiosity.

Offcuts go to noggins.

 

[Burdekin]

Span table for flat roof for you.

 

[addjunkie]

Thread hi jack....im rebuilding my barns, with a view to conversion once the buildings are fixed with new roofs. So looking at the span tables for C16 and C24 first floor joists, Im a few mill short between the random stone walls. Going for an open layout, and dont want to have a steel, mid way as itll make plumbing etc a nightmare.

Would it be acceptable ( building regs etc) to run a timber ledger along opposite walls and fixing them like in the picture, then hanging the joists from them. Ledger and joists would be all the same cross section.

 

[Burdekin]

View attachment 154752 Thread hi jack....im rebuilding my barns, with a view to conversion once the buildings are fixed with new roofs. So looking at the span tables for C16 and C24 first floor joists, Im a few mill short between the random stone walls. Going for an open layout, and dont want to have a steel, mid way as itll make plumbing etc a nightmare.

Would it be acceptable ( building regs etc) to run a timber ledger along opposite walls and fixing them like in the picture, then hanging the joists from them. Ledger and joists would be all the same cross section.

From Eurocode 5 span tables.

 

[addjunkie]

Im confident in the span etc, its the fixings to the walls, Im more interested in, as the thickness of the ledger brings me with in the span table, mid joist with a hanger or cut into the stone.

 

[Burdekin]

Im confident in the span etc, its the fixings to the walls, Im more interested in, as the thickness of the ledger brings me with in the span table, mid joist with a hanger or cut into the stone.

I did previously did find the wall plate bolting and size requirements. It’s totally fine to do it like you are planning, bolting to the wall and joist hangers. Just not sure of the size of the wall plate and bolt sizes. Will see if they are referenced in a book I have.

 

[addjunkie]

I did previously did find the wall plate bolting and size requirements. It’s totally fine to do it like you are planning, bolting to the wall and joist hangers. Just not sure of the size of the wall plate and bolt sizes. Will see if they are referenced in a book I have.

Thats Ok dont worry about it, I shall tell my drawing, building regulations man to put it on the drawing, hes very good at drawing and stuff, but not the best at thinking out the box. So I find it best to tell him exactly what I want, he just adds the detail then. He would have either spilt the open plan living area in half or had some big joist through he middle.

 

[mpats]

Leo does a vid somewhere of doing scarf joints, he made a jig so he could use a router to trim things to the right angle.
https://www.youtube.com/channel/UCg-_lYeV8hBnDSay7nmphUA

 

[bigegg]

What way do the original joists run out of curiosity.

Offcuts go to noggins.

original joists run 10ft