Chimney Stack Support in Loft: Bearing steels on 100mm internal spine wall?

Hi everyone,
I’m in the early planning stages of thinking about removing the chimney breasts from the ground and first floor of a semi-detached house. I’m looking for a sanity check on the structural approach before I engage a Structural Engineer and start the Party Wall notices.


The Situation:
House: Standard UK semi-detached 1930s.
Chimney: "Gathered" flue. Two separate breasts (front and back rooms) merge into a single stack in the loft.
Constraint: From my reading, building control generally does not permit gallows brackets for this setup (especially on a party wall), so I am planning for a steel beam solution.
Party Wall: I am aware I need a Party Wall Agreement and a Structural Engineer’s calcs. Just trying to understand the likely steel layout first.

The "Proposed" Plan:
I want to avoid spanning a single huge beam the full depth of the house (front to back) due to it's height in interfering with the roof.
Instead, I’m hoping to use the central internal wall as a mid-span support?
Beam 1: Spans from Front Wall -> Central Spine Wall.
Beam 2: Spans from Rear Wall -> Central Spine Wall.

The Question:
My central spine wall is a single skin brick wall (approx 100mm).
Is it standard practice to bear two steel beams (supporting a shared stack) onto a 100mm internal wall using spreader padstones? e.g. 440x100x215
What if these 2 beams were to have end brackets and bolted together to make it a "single" UB
Or is a 100mm wall typically considered too slender for this load, forcing me to span the full depth of the house (external to external) or install brick piers/steel posts downstairs (which I want to avoid)?
Has anyone done this successfully on a half-brick spine wall without needing extra vertical supports in the bedrooms?
I feel like this is a common problem but I can't seem to find a solution.
Thanks in advance for any insights!

I’m not an SE but very sure it’s doable with the right bearing detail. The fact that the wall is single skin doesn’t mean it can’t take the chimney load.

I would expect to simply use 2 beams with a simple splice on the wall. Structurally they remain 2 beams. the splice just to locate them but up to your SE.

I take it the wall does goes down to ground?

It depends if the partition wall is load bearing or not, and on a suitable foundation.

stevie888 said:

I would expect to simply use 2 beams with a simple splice on the wall. Structurally they remain 2 beams. the splice just to locate them but up to your SE.

I take it the wall does goes down to ground?

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^woody^ said:

It depends if the partition wall is load bearing or not, and on a suitable foundation.

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Thanks for the reply, that makes sense regarding the splice plate.

Regarding the central spine wall: Yes, it appears to be a load-bearing brick wall that continues vertically through the whole house. I believe the 1st floor joist go into this wall. It runs from the loft, down through 1st floor, then the ground floor, and I can verify that it continues into the cellar where it goes into the earth with visible foundations.

My main concern was whether a spreader plate is usually sufficient in this scenario?

Mines exactly like that. The SE said get a square plate welded on the ends and bolt them together.

Fortunately my local steel place had two columns with plates already welded on so I grabbed them.

At the outer ends if you chop out the wall plate they might just fit without interfering with the roof to save you chamfering the ends.

It shows a full brick wall but it's a half - just recycled drawing.

OP,
Why even do what you propose - removing your chimney breasts?
Your rooms could end up looking like plain boxes with no familiar point of family reference?
And at a time when all older features are valued for giving the house a character thats prized by many house buyers

cdbe said:

Mines exactly like that. The SE said get a square plate welded on the ends and bolt them together.

Fortunately my local steel place had two columns with plates already welded on so I grabbed them.

At the outer ends if you chop out the wall plate they might just fit without interfering with the roof to save you chamfering the ends.

It shows a full brick wall but it's a half - just recycled drawing.

View attachment 402445

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That really is music to my ears! relief to know you didn't need a brick pier on the 100mm wall

You mentioned using columns rather than beams—was that to keep the steel profile low enough to clear the roof slope? I’m trying to gauge how tight the fit will be at the eaves. Do you remember roughly how deep the section was that you used? I’m hoping I can get away with 152mm to save chopping the timbers, but I suspect the SE might insist I need a taller 203mm section.

But thanks again, this is super helpful —it's the sanity check I needed—I feel much better about approaching an SE now.

ree said:

OP,
Why even do what you propose - removing your chimney breasts?
Your rooms could end up looking like plain boxes with no familiar point of family reference?
And at a time when all older features are valued for giving the house a character thats prized by many house buyers

Click to expand...

That’s a fair point, and I actually agree with you regarding the main living areas—I do like the character it gives to the room.

However, the bedrooms in this house are on the smaller side. The chimney breasts take up prime wall space that makes positioning a bed or wardrobe difficult and all annoyances and solutions point to the chimney. So removing them upstairs is purely a practical decision to gain that crucial floor space.

Also, I am actually keeping the chimney breast in the living room (just boarding up the opening for an electric heater) so we will definitely retain that focal point and character downstairs. It's just the upstairs that needs the extra breathing room

OP,
Thanks for your reply.
Single flue bedroom chimney breasts are typically narrow, much narrower than ground floor chimney breasts. Say, 600mm to 750mm with an even narrower projection.
Cabinet units can neatly be built around the B/R chimney breasts.
But, if you demolish, is it worth it for that small gain in wall space?

152x152x23kg - 4m span. Yes they're not as tall as beams. They come in "heavier" sizes so you should be able to use that section.

As cbde says, 152 x 152 x 23 would be more than adequate.

Dont forget a thin (3mm) steel plate spanning from the top of the beam into a joint in the party wall. just to stop any lose bricks in the wythes from dropping down.