Cutting a birds mouth under a rafter?

[Frankychops]

Basically I want to make this cut in a roof rafter, I can screw or bolt another timber to it if needed.

220x47mm 4.7m clear span length, 400mm apart, slate roof.

It feels wrong to do it, however the roof is massively strong…. It’s about 45cm from the bottom of the rafter…



https://imgur.com/a/hIyL6Iz

Edited by Frankychops on Tuesday 6th May 20:24

[Frankychops]

ETA, I know its not the done thing in an area of tension, however with the rafters being "over" spec, Could I remove an inch or 2!?

[smokey mow]

What makes you say the rafter are “over-spec” ? Based on the very limited detail you have given us, the rafter appear to be very close to their limit of span before you even start heavily notching them.

[Frankychops]

smokey mow said:

What makes you say the rafter are “over-spec” ? Based on the very limited detail you have given us, the rafter appear to be very close to their limit of span before you even start heavily notching them.

c24 at 5.0m is showing 175mm in some tables, the 40mm would help. although I am looking at a different option to the problem i'm trying to work around.

[smokey mow]

Frankychops said:

smokey mow said:

What makes you say the rafter are “over-spec” ? Based on the very limited detail you have given us, the rafter appear to be very close to their limit of span before you even start heavily notching them.

c24 at 5.0m is showing 175mm in some tables, the 40mm would help. although I am looking at a different option to the problem i'm trying to work around.

What pitch and dead load though and are they C24 or only C16? This info is important to ensure you’re looking at the right table rather than just the one which gives the most favourable answer.

Without all the details we’re just guessing.

[LooneyTunes]

What is it you’re actually trying to achieve by making the cut? There might be another way to achieve whatever it is you’re trying to do?

[wolfracesonic]

A birdsmouth should be no deeper than a 1/3 of the depth of the plumb cut but that’s not a birdsmouth, more of a notch, which for floor joists as a guide, shouldn’t be greater than an1/8 of the depth of the timber.

[Frankychops]

smokey mow said:

What pitch and dead load though and are they C24 or only C16? This info is important to ensure you’re looking at the right table rather than just the one which gives the most favourable answer.

Without all the details we’re just guessing.

C24, 38deg

[smokey mow]

Frankychops said:

smokey mow said:

What pitch and dead load though and are they C24 or only C16? This info is important to ensure you’re looking at the right table rather than just the one which gives the most favourable answer.

Without all the details we’re just guessing.

C24, 38deg

Dead load?

If you don’t know, ask your engineer as you’ll probably be asked to give this detail to building control when they come to check it. I certainly would do if I saw a notch that deep.

[TA14]

LooneyTunes said:

What is it you’re actually trying to achieve by making the cut? There might be another way to achieve whatever it is you’re trying to do?

This is the critical question.

Cutting 40mm will lose about a third of the bending strength so not desirable. You could probably design splice plates on either side to restore the strength but an alternative solution which doesn't involve cutting into a C24 would be best. Notching a rafter in the way proposed is always going to be an 'ugly' solution.

[Frankychops]

smokey mow said:

Frankychops said:

smokey mow said:

What pitch and dead load though and are they C24 or only C16? This info is important to ensure you’re looking at the right table rather than just the one which gives the most favourable answer.

Without all the details we’re just guessing.

C24, 38deg

Dead load?

If you don’t know, ask your engineer as you’ll probably be asked to give this detail to building control when they come to check it. I certainly would do if I saw a notch that deep.

Deadload is under 0.5 kN/m²(slate roof). Not worried about BC sign off as it’s an internal mod, bc will not be around to check.

[Frankychops]

LooneyTunes said:

What is it you’re actually trying to achieve by making the cut? There might be another way to achieve whatever it is you’re trying to do?

As petty as it sounds, making a bit of space for a hidden projector screen. I’ve a few options to look at.

[dxg]

wolfracesonic said:

A birdsmouth should be no deeper than a 1/3 of the depth of the plumb cut but that’s not a birdsmouth, more of a notch, which for floor joists as a guide, shouldn’t be greater than an1/8 of the depth of the timber.

Exactly this. The horizontal cut of a proper birdsmouth rests the rafter onto the wallplate, leaving the projection to do nothing other than support the last foot or so of the eaves. Accordingly, tension doesn't become a factor, but knowing the pitch of the roof most definitely does...

[dxg]

Frankychops said:

LooneyTunes said:

What is it you’re actually trying to achieve by making the cut? There might be another way to achieve whatever it is you’re trying to do?

As petty as it sounds, making a bit of space for a hidden projector screen. I’ve a few options to look at.

Bring the project screen further into the room / further away from the wall. No one will notice when it's down. And if the retraction slot is neat, no one will question whether it's 6 inches or a foot from the wall. Only downside would be a *slightly* smaller image from the reduced projector throw.

[Frankychops]

dxg said:

Frankychops said:

LooneyTunes said:

What is it you’re actually trying to achieve by making the cut? There might be another way to achieve whatever it is you’re trying to do?

As petty as it sounds, making a bit of space for a hidden projector screen. I’ve a few options to look at.

Bring the project screen further into the room / further away from the wall. No one will notice when it's down. And if the retraction slot is neat, no one will question whether it's 6 inches or a foot from the wall. Only downside would be a *slightly* smaller image from the reduced projector throw.

I can't do that, the screen needs to be where it is. Also need to work in a limited projector to screen distance.

[TA14]

Frankychops said:

dxg said:

Frankychops said:

LooneyTunes said:

What is it you’re actually trying to achieve by making the cut? There might be another way to achieve whatever it is you’re trying to do?

As petty as it sounds, making a bit of space for a hidden projector screen. I’ve a few options to look at.

Bring the project screen further into the room / further away from the wall. No one will notice when it's down. And if the retraction slot is neat, no one will question whether it's 6 inches or a foot from the wall. Only downside would be a *slightly* smaller image from the reduced projector throw.

I can't do that, the screen needs to be where it is. Also need to work in a limited projector to screen distance.

Paint the truss black and put the screen behind the truss. Losing 40mm from the top corner of a very big screen in the loft won't be noticed. Or is the screen at 90 degrees to the rafter and that's a cut out for the back of the screen? Where's the projector and why can't that be moved back 40mm?

Edited by TA14 on Wednesday 7th May 10:25

[Frankychops]

TA14 said:

Frankychops said:

dxg said:

Frankychops said:

LooneyTunes said:

What is it you’re actually trying to achieve by making the cut? There might be another way to achieve whatever it is you’re trying to do?

As petty as it sounds, making a bit of space for a hidden projector screen. I’ve a few options to look at.

Bring the project screen further into the room / further away from the wall. No one will notice when it's down. And if the retraction slot is neat, no one will question whether it's 6 inches or a foot from the wall. Only downside would be a *slightly* smaller image from the reduced projector throw.

I can't do that, the screen needs to be where it is. Also need to work in a limited projector to screen distance.

Paint the truss black and put the screen behind the truss. Losing 40mm from the top corner of a very big screen in the loft won't be noticed. Or is the screen at 90 degrees to the rafter and that's a cut out for the back of the screen? Where's the projector and why can't that be moved back 40mm?

Edited by TA14 on Wednesday 7th May 10:25

Yes, the edge of the screen hits the rafter, I can't move it forward otherwise it hits an old staircase, same reason I can't have it lower. the case currently intersect with the rafter, i'm working on some other ways of making it all work!

[smokey mow]

Frankychops said:

Deadload is under 0.5 kN/m²(slate roof). Not worried about BC sign off as it’s an internal mod, bc will not be around to check.

And I’m out. If you’re doing it right, you’d have nothing to hide.

Professionally I can’t get involved if you’re not doing everything legally.

[Chumley.mouse]

Cut the notch out and bolt some additional support to it , metal work or timber. Saw a whole roof purlin removed once , there was a bit of sag in it but otherwise roof was solid .

[Frankychops]

smokey mow said:

Frankychops said:

Deadload is under 0.5 kN/m²(slate roof). Not worried about BC sign off as it’s an internal mod, bc will not be around to check.

And I’m out. If you’re doing it right, you’d have nothing to hide.

Professionally I can’t get involved if you’re not doing everything legally.

I'm not sure what i'm not doing legally, its an existing roof that I was looking to modify(within regs). I'm pretty sure you do not need BC signoff for a notch in a rafter?