Steel Beam for Extension advice

[diyant123]

Hi there,

I'm going to have an extension on the rear of my bungalow.
I want to demolish most of the existing rear external wall and have steel beam inserted which will span the full width of the extension (about 6 metres).
This beam will rest on the remaining edges of the external wall.
However this external wall is a cavity wall with 100mm brick, 100mm cavity then 100mm block.
I want the inner walls to run flush into the new extension without any nibs or lips.
The steel beam will only be holding up the existing roof, I imagine positioned directly under the existing wall plate.
My question is, will this be possible?
Is it ok for the steel beam to just sit on the inner skin of blockwork (100mm), with a padstone beneath to spread the weight?
I beleive it could not rest on both the inner and outer skins due to cold bridging issues, is this right?
What kind of depth would the beam be approximately?
I am aware that I will need to get a structural engineer to do calculations, but i just want to know if its all fine in principle before approaching them.

All help much appreciated

 

[^woody^]

The beam will be as low as, or lower than the head of any existing widows or doors in that elevation.

But the good news is that it can sit on the blockwork.

 

[Doggit]

We can't give you any advice on the size of the steel needed, so you'll need a structural engineer to determine the size required. You may be asked for engineering bricks instead of a padstone, and as you've already guessed, it'll rest on the inner wall, and not bridge across to the outer one, but essentially, it's all do-able. In not wanting any nibs in the wall, what about the ceiling, are you intending to try and set it in the ceiling, or are you okay with a beam inside the room.

 

[diyant123]

Hi thanks for those replies, sounds like I just need to get the SE in now to do the calcs.
I'm surprised 100mm bearing is enough on the inner skin to pass building regs, I thought it was a minimum of 150mm.

I am hoping for a flush ceiling too without any nibs. Ideally i will be able to set the beam in the ceiling as much as possible (as you mention). I don't think the full depth of the beam will be disguised though so I'd just have to bring the rest of the ceiling down a bit to the base of the beam. I will have a look in the loft in a moment and measure the depths.

 

[^woody^]

You won't get a beam in at the eaves and have a flush ceiling.

If that is what you want then the beam needs to go in the roof space and the roof and ceiling timbers adapted.

 

[freddiemercurystwin]

It really depends on the existing section but typically for bungalows it won't be possible.

 

[Nozzle]

Or you could use a glulam beam which doesn't need to be boxed in, and you can make a feature of it

Nozzle

 

[^woody^]

^^ And mind your head

 

[Nozzle]

^^ And mind your head

It wouldn't hang any lower than an I beam - and doesn't need fire protection like a steel does

 

[freddiemercurystwin]

It wouldn't hang any lower than an I beam - and doesn't need fire protection like a steel does

I don't see the advantage other than saving 15mm of FB?

 

[Nozzle]

It wouldn't hang any lower than an I beam - and doesn't need fire protection like a steel does

I don't see the advantage other than saving 15mm of FB?

Perhaps I missed the point of Woody's post, but I think he was suggesting a glulam would some-how take up head room in a way that a steel would not.

 

[tony1851]

A 203 x 203 UC would do it easily. The existing wall plate would sit on the bottom flange - giving approx 100 downstand seen from within the existing room.
New roof joists could be flush with the underside of the steel.