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Bird's Mouth inner face cut?

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pike

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Is it ok for a bird's mouth joist cut to over hang the inside face of a header plate? I want to use stronger wider joists but have limited height to play with.

Thanks,
Carl.
 

Rob Platt

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no if i`m picturing what you are saying right, because you are reducing the size of your rafter at the load point.
all the best
rob
 

Dibs-h

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To have the cut overhanging the inside of the face, would mean that the vertical cut exceeds the 1/3 (max) depth of the rafter. This would leave far less depth of rafter at the bearing surface. Load bearing is a function of depth - you run the risk of the rafter end snapping off and the problems that come with it.

Dibs
 

pike

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I understand that you can't cut more than a third out of a joist otherwise you're making it too weak.

However, here's a pic of what I mean. It's a 100mmx50mm plate and a 125mmx50mm joist. It's a very very low pitch existing lean to. I'd be building on the existing back wall and ideally the height needs to stay the same as I'm continuing roofing on from attached garage.



I am starting to agree it is reducing the strength at the load point, but then don't most bird's mouth cuts do that?
 

pike

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Sorry for posting on my own post but looking at this and then a regular bird's mouth I'm starting to see that this one would be prone to snap at the main stress points. Cheers.
 

Dibs-h

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For a normal birdsmouth - on the top surface of the wall plate (the bearing surface), the depth of the rafter is 100% where contact begins. With what you are suggesting - the effective depth (i.e. actual load bearing) is less and that element below the horizontal cut is doing nothing from a loading perspective.

A better way would involve "cogging" the rafter into the wallplate - the following gives you an idea,



Structurally better, but time consuming.

Dibs
 

pike

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Thanks Dibs. I see it now.

I might have to just accept a higher roof and work out how to continue my onduline/coraline up a bit from the lower adjoining garage height.

Ironically, said garage is using underspecced joists and no wall plate.... but still standing.
 

Dibs-h

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pike":36mcwjgv said:
Ironically, said garage is using underspecced joists and no wall plate.... but still standing.
I've had the same conversation with my SE about my new garage. The existing one is 3mx6m and single skin brick with a rosemary roof (been up for 20/30 yrs). The new one is 4m x 6m and he wants double skin with steel windposts, etc.

Only the other day when I was counting the number of rows of rosemary tiles on the workshop (same width and roof pitch) did I realise that the roof area on the new garage (on one elevation) is almost equal to the total roof area of the existing garage and hence the wind loads will be far higher. So kinda realised why the beefed-up construction.

Dibs
 

pike

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Details schmetails ;-) Point taken.

Now I'm wondering if I should go around fixing strong tie fasteners onto the garage joists. Seems daft when it's been there that long, but I can't help feeling it's going to collapse. Ignorance might have been bliss!

I suppose if an old existing structure is only just under spec then it's not necessarily dangerous, just not ideal. It's when you're way under spec that it's a health hazard. Also no sense at all cutting corners when repairing or building new.
 

pike

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Graham, hanging off what? Right now I've got a brick wall which is 100mm short of where I want the roof top to be.

Are you saying use, say a 100mmx100mm timber plate (two 50mm) then joists hangers off that? Or d'you mean build the wall another course, then fix hangers to that?

I do like the idea of using say a double wall plate on the brick and then joist hangers. I guess the hangers will be part in the plate and part in brick though.

Cheers, sounds like half a plan. Will think on.
 

GPB

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I would keep the rafters and plate as in the sketch then bend the hanger legs back over and nail into both the face and top of the plate.

Screwfix do various 50mm wide joist/truss hangers that I'm sure could easily be made to fit.

Graham
 

pike

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Oh I think I see what you mean, bending a jiffy hanger. So that the joist is supported before the cut. Cheers Graham, that sounds doable.
 

bosshogg

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Jiffy or speedy joist hangers are what you want. The stress point of the joists, in your instance, is at the intersection of the cut over the header piece, by wrapping and nailing the tails of these hangers
speedy hanger.jpg
Transfer most of the load to the header, if the joists spacing matches the stud spacing and are aligned, then part of the load transfers to them making the frame load bearing.
In truth your sketch and description describe more a half straddle joint rather than a birdsmouth, witch are shaped < exactly like an open beak of a bird...bosshogg :)
If you are out to describe the truth, leave elegance to the tailor.
Albert Einstein (hammer)
 

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pike

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Thanks bosshogg. This join is the back and lower side of a lean to, and is brick. The higher other side which is timber will just have regular bird's mouths.

Cheers all, appreciate your thoughts. I'll probably do the cut I suggested but support if with a bent jiffy/speedy joist.
 

pike

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I've had another idea. As the pitch is very low, just sit the joists on the brick and add another course inbetween them. Holding the joists in place with twisted straps.





The only thing is that seems a little weaker with less nailing involved? Any thoughts?
 

Dibs-h

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pike":3a0x9k3x said:
I've had another idea. As the pitch is very low, just sit the joists on the brick and add another course inbetween them.
I was going to suggest that earlier on - but was a little unsure as to the angle of the roof, as you'd mentioned it was a lean-to.

Dibs
 

pike

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I've been thinking I had a 2 or 3 in 12 but it turns out on closer inspection to not even be 1 in 12.. So I think I'll just sit the joists on the brick.

Thanks for all your help.
 
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