Lintel for graden room bifolding doors

Jul 11, 2026 Last reply: 1 week ago 5 Replies
K

Hi,



I'm looking for some advice on selecting a lintel/beam for a set of bifold doors in a timber-framed garden room.



The bifold door opening will be 3000 mm wide and is located in the front wall of the building.



The structure is as follows:



Roof: light flat roof constructed from 45 × 145 mm (6×2) C24 timber joists at 400 mm centres, spanning from the front wall to the back wall. The roof deck is 7200mm x 4200mm, 18mm OSB and EPDM rubber covering. There are 7 roof joists bearing directly on the lintel above the door opening.



Front wall: 45 × 120 mm (5×2) C24 treated timber studs at 400 mm centres.



Back and side walls: 45 × 95 mm (4×2) C24 treated timber studs at 400 mm centres.



I'm trying to determine the most suitable lintel/beam to support the roof over the 3 m opening.



Would you recommend:


  • a Rectangular Hollow Section (RHS),
  • a flitch beam, or
  • a Universal Beam (RSJ)?

If one of these is preferable, what size would you suggest for this application? If there are any additional details that would help determine the correct beam size, please let me know and I'll provide them.



Thanks in advance for any advice.


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T

I have seen a flitch beam used on that sort of opening, I suppose it comes down to cost and time.

Richard

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T

You cant go wrong with an RSJ or even a precast lintel - or a stressed steel one.

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R

I'd suggest working out the load it'd need to support and thinking about what the beam is going to be resting on.

I'd have thought an RSJ would do the job but that'd weigh about 50kg on its own - plus the weight of the roof/snow etc. Be a bit wobbly atop a bit of 4x2 . . .

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T

I'm no structural engineer but that seems to be a fairly lightweight construction, both the roof and the wall. I would question whether you actually need a steel for this - it's not like there's a brick wall above to support. If it's a heavy steel you might next need to reinforce the wall to take its weight.

Glulam as Andy suggested is an interesting idea, as is a flitch beam. Need to have some rigidity to stop the beam sagging, which I think either will give you.

I might be tempted to do some calculations of your wall strength: how much weight can your wall support, in its current configuration? How much weight is your roof going to be putting on it? That might give you an idea of how much headroom you have in the calculation for the additional weight of a beam.

I suppose another option might be some kind of internal timber (or other) frame to take the weight of the roof, with the wall turning into just a facade to keep the weather out. eg if you fancied some oak posts and beams as part of the interior decor, they could handle the weight and then the window just needs to support its own weight and maintain its own rigidity.

Theo

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