Steel Beam Above Bifold Doors

8 Oct 2006
Reaction score
United Kingdom
Hi all

About to get a builder to crack on with a single storey extension on the back of house that we desperately need and wanted for last ten years!

Theres a bit of back and forth at the moment regarding the structural calcs and beam design above the width of the bifolds on the back of house

We are have a vaulted ceiling so have an I beam in the ridge beam and theres no issue with that however for the the beam above the bi-folds the structural guy has designed that with a square profile steel cross section.

While the calcs show no bending and all that - is there any benefit with a square profile - any idea why he used this cross section rather than an I beam

Also ther is (I hope anyway) a lintel above a door opening we have already and the roof ridge I beam seems to just come in line with that lintel edge - the structural guy says no effect on it but I spoke to another one and he said put a steel lintel intel in there instead - I want to avoid that as it will screw up the internal room I would guess but just seeing if anyone has any thoughts on that too

See drawing and his comments in yellow below and the rough location of the lintel too - in blue

Thanks for any thoughts all

Last edited:
Sponsored Links
You're paying the structural engineer to worry about stuff like that. My only concern with a box profile would be the possibility of corrosion (if any water did get into it it wouldn't drain naturally).
I don't see what the problems are or why a box section should be vulnerable to water corrosion. What does the architectural detail show?
Thanks for the responses all - yeh I know its not really for me to worry about all this but when you have one structural guy saying the lintel above the current door needs to be replaced and the you have another structural guy saying it doesn't doesn't fill you with confidence!

Im not that worried about the box profile really just wondered any reason why they would use that section

My main concern in the existing lintel above the current patio doors (highlighted in yellow) its very close to the new ridge beam - but I suppose if the structural engineer says no issues I suppose thats fine but I like to check things where I can

Sponsored Links
1. The new ridge will not affect the existing lintel.

2. When a beam has a welded plate below it supporting an outer skin of brickwork, the total load acts downwards but away from the
geometric centre of the beam. This offset (eccentric) load induces torsion (twist) in the beam, which adds to the total bending moment
in the beam, and has to be allowed for in the figures.
I-beams are not all that good at resisting torsion, especially on long spans and under higher offset loads, whereas closed, box-section
beams are more efficient in this respect.
Having said that, the span (3600?) and off-set load are not great and there are a number of I-beam sections that would have been OK
in your situation - a 152 x 152 x 30 with a welded plate would have been able to cope with the torsional effects, and probably cheaper.
It also makes fixing plasterboard on timber noggins easier than trying to fix to a flush metal surface.
I suspect your SE is being a bit pedantic and too concerned about theory rather than practice.

3. All beams bend (deflect); the trick is to make sure the deflection is kept to a point where it is not noticeable to the eye, or doesn't impact on
the working of the bifolds.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local