Width of wall required to support extension roof

[Kaymo]

Hi,
So, I know that cutting out the brick wall directly below a window, to convert it to a door is fairly standard practice, but my question is how narrow would you accept a wall column before starting to consider bringing in a structural engineer, or going with a different solution to support the roof.

The pictures below might explain this a bit better showing the cutouts being proposed by the builder. This would leave the corner section consisting of the existing timber corner post which is behind the white trim, supported by the remains of the wall at the corner, which would be about 300mm width on either side of corner. I believe the wall construction is 100mm brick either skin with ~100mm cavity within. The roof itself is not particularly heavy, its timber construction flat roof behind the slate in the pic.

Now obviously the existing timber lintels and corner post have been supporting this for the last 30 years so the load on those shouldn't be an issue, but my concern is more with whether the runt wall that would be left at the corner would be structurally sound. If the remaining wall was a metre each side of the corner I probably wouldn't be asking this question, but is there any rough guide to how wide, or how many brick widths a brick wall/pillar has to be for it to be considered stable?


Thanks

 

[Kaymo]

Just wanted to clarify in relation to my original post, that I am not looking for anyone to make a judgement on whether this would be stable or acceptable, more just whether anyone is aware of a rule of thumb or regulation that might guide how narrow a wall section can be?

 

[noseall]

3 bricks in each direction all the way to the roof and tied over (with purlins or a lintel) at the top.

Otherwise, you would need a post concreted in at the corner.

 

[noseall]

A bit like the right hand corner of this building below....

Standard3

• noseall
• 23 May 2019

 

[Notch7]

Your dwarf wall will have all its return cut off.

And a post right at the end.

I think a SE might be best.

675mm is generally the min for a corner to act as a buttress (as per Nosealls pic).

You dont nec need that for a dwarf wall.

 

[Kaymo]

Thanks folks, not sure I am completely following this though.

3 bricks in each direction all the way to the roof

Why would I go up to the roof if there is already a wooden post there above the wall. Are you saying that I would need to remove the post and build it up in brick?

Your dwarf wall will have all its return cut off.

Sorry for my ignorance but I don't know the terminology. Presume the dwarf wall is the wall below the window and post, but by return do you mean the rest of the wall?

 

[Notch7]

Thanks folks, not sure I am completely following this though.


Why would I go up to the roof if there is already a wooden post there above the wall. Are you saying that I would need to remove the post and build it up in brick?


Sorry for my ignorance but I don't know the terminology. Presume the dwarf wall is the wall below the window and post, but by return do you mean the rest of the wall?

The section that will be removed for the new door. Once thats cut out the wall will be pretty much just the front -the bit left below the post will only be the thickness of the front wall.

 

[noseall]

Why would I go up to the roof if there is already a wooden post there above the wall. Are you saying that I would need to remove the post and build it up in brick?

You either have a full height brick pier as I have shown or you have a full height column/post that is concreted into the ground and usually onto a big deep cube of concrete.

By the time you have chopped at that wall, what you will have left won't be worth a waz.

 

[^woody^]

My house and thousands/hundreds of thousands of houses have no corner returns, no posts, no calcs, and no problems.

Edit. Nah, use a steel post for this.

 

[noseall]

My house and thousands/hundreds of thousands of houses have no corner returns, no posts, no calcs, and no problems.

Well observed.

However, how many have you seen where there is a dwarf wall with a post perched on top and said wall has been reduced to a ridiculous sized column? I wouldn't put my name to it.

 

[^woody^]

Well observed.

However, how many have you seen where there is a dwarf wall with a post perched on top and said wall has been reduced to a ridiculous sized column? I wouldn't put my name to it.

Yes well observed yourself sir.

I shall not put my name on it either. I'll use Notch's.

 

[Kaymo]

The section that will be removed for the new door. Once thats cut out the wall will be pretty much just the front -the bit left below the post will only be the thickness of the front wall.

Thanks. The bit that would be left would be the corner section, just over 300mm in each direction, with the concrete sill on top that the post sits on. There will be some wall left underneath where the new door would go because this part of the extension is raised, but only 1, maybe 2 courses of bricks.

You either have a full height brick pier as I have shown or you have a full height column/post that is concreted into the ground and usually onto a big deep cube of concrete.

Thanks. Ok, so what I have at the moment is neither of these things, it is a half wall, a concrete sill, and a wooden post resting on top. Are you saying that this was a poor design to begin with, or what is there currently is ok because the half wall is connected to a long section of wall under the window?

By the time you have chopped at that wall, what you will have left won't be worth a waz.

To be honest this was my own gut feeling, but the builder that I got out to give a quote was planning to do it this way. I actually queried this part of the plan specifically with him, and asked him if he was sure that the runt wall would be structurally sound, and he appeared very confident that it would be. Obviously I am not confident that it would be, or I wouldn't be on here seeking advice!

 

[Kaymo]

So, taking on board the advice above that this would need to be a new post to ground level, I took off the trim tonight to see how it was put together. The post appears to be one 150x50mm piece on the right, nailed to three 100x50mm pieces on the left, to make one 150mm² post. Resting on top of this are 200x50mm lintels going in each direction, and my impression is that these are doubled up, i.e. there is another 200x50mm piece behind each of these. The main joists for the roof are resting on top of the one going off the the left.

It wasn't really what I was expecting - is this a normal arrangement?

If I was to get the wall taken away completely, and replace with a post top to bottom, would it just be a case of taking the windows out, supporting both lintels, and replacing the the timber that forms the post with the same but top to bottom, or would a single 150mm² post would be preferable?

 

[Notch7]

I wouldve thought a SHS with plates welded on top and bottom.

Time for a structural engineer.

The existing timbers look rot at the bottom

 

[Kaymo]

I wouldve thought a SHS with plates welded on top and bottom.

Time for a structural engineer.

Was hoping for wood as I know how to do that! Was hoping to get someone in to do this but the only person who has so far quoted is the person who wanted to cut out and leave the narrow brick pillar, none of the other joiners or builders I've contacted seem interested. I've emailed a structural engineer today and contacted building control to try and get a sense of what would get approved.

The existing timbers look rot at the bottom

Yes, they are. When we moved in the plywood trim at the bottom was rotten and I patched that up quick, but it had obviously been in that state for some time before that as it is rotten all around the outside edge.