Birds mouth seat cut overhang

[an203888]

Hi all,

I want to drop my rafter by about 20mm where it meets the wall plate. I wouldn't be cutting more than 1/3 from the rafter but the bottom of the rafter won't all be on the wallplate. I know it's not ideal but would it be OK?

Screenshot_20200507_174824_com.google.android.keep

• an203888
• 7 May 2020

 

[23vc]

Yeah it’s the depth of the cut that’s important, you’ll always get that on shallower pitches

 

[an203888]

Yeah it’s the depth of the cut that’s important, you’ll always get that on shallower pitches

So building control won't mind then!?

 

[bobasd]

You correctly have the 2/3 HAP height above plate. BCO wont mind.
The ceiling plasterboard will cover what you see as an issue.

If you are a DIY'er then well done for taking on a bit of roof cutting.

 

[noseall]

I want to drop my rafter by about 20mm where it meets the wall plate.[/GALLERY]

What size plate is on there now? I personally don't like to see any seat cut unsupported.

 

[an203888]

What size plate is on there now? I personally don't like to see any seat cut unsupported.

Is a 4x2 wall plate. I don't particularly like it but the space I have for the pitch below first floor window means that anything I can get is needed. Can't really make the wall lower as I have a 2.1m high bifold with a lintol above which is a block high and then the wall plate on top of that.
I'm thinking of adding some small hangers to the plate just to support the bottom of that unsupported but of rafter but I will take pics when I start.

 

[an203888]

Just offered a rafter up... Would this be OK? The pencil mark in picture is 1/3.

IMG_20200510_103337

• an203888
• 10 May 2020

 

[^woody^]

Why didn't you put the plate lower?

Or use smaller rafters?

 

[an203888]

Why didn't you put the plate lower?

Or use smaller rafters?

Couldn't put the plate lower as it sits on top of the large lintel which supports a standard height door of 2100. Architect recons on drawings that I should be able to get a 16 degree pitch but now I'm actually doing it - no chance. I have about 30mm above the tiles to under the window to put lead in.

 

[^woody^]

The thing is, whatever you're notching out of those rafters is reducing the effective rafter depth, so you may as well use smaller section sizes, and may well need to space them closer together too.

If you had used/designer specified a different lintel style then the plate could have gone lower. But sounds like he was crap anyway if his drawings don't work out properly.

What's so important about the 16° pitch?

 

[an203888]

The thing is, whatever you're notching out of those rafters is reducing the effective rafter depth, so you may as well use smaller section sizes, and may well need to space them closer together too.

If you had used/designer specified a different lintel style then the plate could have gone lower. But sounds like he was crap anyway if his drawings don't work out properly.

What's so important about the 16° pitch?

I hear exactly what your saying although don't agree about the smaller rafters. There will be more strength in these than using smaller rafters just not a lot. I did look for a different lintel but they all seemed to be same depth. I'm thinking uf adding hangers under the notched item to support the unsupported wood

 

[^woody^]

The effective section depth of a rafter is whatever you have cut it to at the wall plate - which is the bearing where the load is. You've reduced that rafter by what looks like 50mm, so it's performing as a smaller rafter.

Is there a hanger designed for that scenario? I don't think there is. If not then there is no point.

Strictly, you would need to gusset the area to deal with the additional stress at that point.

 

[an203888]

The effective section depth of a rafter is whatever you have cut it to at the wall plate - which is the bearing where the load is. You've reduced that rafter by what looks like 50mm, so it's performing as a smaller rafter.

Is there a hanger designed for that scenario? I don't think there is. If not then there is no point.

Strictly, you would need to gusset the area to deal with the additional stress at that point.

thanks for info woody. Do you have any details of what you mean by gusset in this scenario or somthing similar? Thanks for help

 

[^woody^]

Google for gusset plate.

Basically a sheet of ply fixed either side of the rafter. Probably about 400mm long.

But that's only if the rafter is over spanned due to the reduced thickness at the bearing. Reducing the rafter spacing will have the same effect as it reduces the load on each individual rafter.

 

[garyo]

You've reduced that rafter by what looks like 50mm, so it's performing as a smaller rafter.

Is it really that simple? Isn't it about shear strength at the end, versus overall deflection, and the notch is only going to affect the shear?

What's the span on the rafters, ann20?