Bifold door support

[saveasteading]

We will have this opening for a bifold door. Big and heavy, and moving so it must not jam.

 

Looking for any experience of issues from such a  door deflecting.

Its not the Engineering in question but any idiosyncrasies of such doors.

 

We have two choices.

 

1. a steel macross the opening. \there is room for 178mm RSJ and then timber around it. It would sit on timber so we would add 2 more uprights at each end.

 

2. Simply retain the timbers as currently designed and plate both sides with high quality 12mm plywood, and screws every 150mm. The ply could be whole sheets so say effectively 1200mm high, but 600mm will surely suffice.... the outside will be faced in osb anyway. This make it an immensely deep timber beam of the James Jones / Pasquill type.

we would probably add suds between the horizontals to ensure a precise level beam before plating. And perhaps noggins/ dwangs at the top of the 600 sheet.

the timbers are 6 x 2 tanalised.

 

 

 

You may get the idea that I like the latter, because it is a neat composite design, and we dont have any issues how to fix the door. But my only doubt is in the  reality in use: do bifolds  

 

[MikeGrahamT21]

I can honestly say I like mine, but my opening is only 2m, I would imagine with the amount of leaves you are planning as above it could start looking quite cluttered when they’re all stacked up fully open, and may well look cleaner with a sliding door, but if you are set on bifold, then yes they are good, as with most things, if you get a good one. Don’t get top hung as they aren’t as robust as ground bearing. If PVC, Liniar modlock are very good, what I’ve got, 10 years in and they’re as good as new, just don’t get chrome hardware outside!!

[Andehh]

We opted for big sliders, as 85%of the time it's all closed up, so maximum glass views. Lot less to go wrong as well! 

[Super_Paulie]

not sure how much use this info is but i have some 4m bifolds, but under a steel beam and there has been no deflection. All work fine the last 10 years, it was the glass with integrated blinds that kept packing in. The beam was probably massively overspec.

[Spinny]

Aluminium Bifolds 3.5m wide and 2.4m tall, top hung, stack can be positioned to either side of opening when open.

Fixed to timber and steel beam above. (The timber was added because we changed the choice of doors and the height of the opening had to be reduced to suit.)

Best to have professionally fitted by careful, conscientious and experienced fitters.

 

Getting the threshold position correct is critical if you want something close to a level threshold, so knowing where the finished floor level will be.

PROCESS: Prepare opening, survey measurement of opening and marking of reference level markings around the area, detailed drawings with tight opening size, bifold manufacturing size etc, carefully check drawings and amend as necessary then sign off drawings, send to manufacture, professional delivery and installation, test and acceptance.

 

Beware the limited accuracy of laser levels. The company claimed floor level had been changed between survey and fit but this was nonsense. Their lasered levels differed by 11mm over 9m from survey to fit because laser levels clearly not reliable/calibrated. My water levels showed they were wrong. So buy a water level and mark careful reference levels.

 

Make sure the fixing methods and positions are defined and there is going to be solid material in the right positions and alignment for the frame to be fixed to. Insist on careful preparation, inspection and checking of the opening - builders lie and cover up - thus in our case the first fitting visit was abortive.

 

 

Edited Thursday at 18:24 by Spinny

[G and J]

Echo buy good quality. We've just left a house with 23 year bi-folds, loved them BUT they faced the west (lovely views) but as they aged, when the wind blew across the plains of East Anglia they were both drafty and deflected and they weren't quite 2m. They were however top hung

[saveasteading]

On 18/12/2025 at 14:49, MikeGrahamT21 said:

Don’t get top hung

Thanks, I will look into this. So the tops are simply guides and taking wind load, not the weight? I've only seem them top hung, with the advantage of the bottom being less critical of grit etc.

 

On 18/12/2025 at 15:24, Super_Paulie said:

probably massively overspec.

Thats where Im coming from . My career is largely based on removing overspec as a designer precaution at client expense.

 

23 hours ago, Spinny said:

Beware the limited accuracy of laser levels.

I'm all too well aware. Even an inaccurate level can be used for precision if you know how. Too many builders assume they are accurate and don't really understand.

 

 

We subsequently realised there is an option 3 which is to use  thin steel plate instead of ply. It isn't cheap to buy though and detailing might be messy. Also 4: we have some recovered and some new angle steels which could maybe be  screwed to the timber.

 

 

[Spinny]

Don't forget that when the doors are stacked you have the full weight of all doors in one place. We gave the door weights to our SE to confirm all would be ok.

[Russell griffiths]

You won’t be spanning four metres with timber alone. 
with that wall above it will definitely deflect too much. 
steel flitch beams or some sort of steel beam with insulation and timber all sandwiched together. 

[saveasteading]

10 minutes ago, Russell griffiths said:

it will definitely deflect too much. 

That's my point though. Definitely? It is definitely very easy to use a steel without further thought.

I'm looking at it as one of these but with ply screwed to both faces to avoid twisting.

These will span up to 8m as house load joists, at 450 deep (I have done that on a commercial project where depth was not an obstacle)  . I'm looking at at least that depth but double the flange timber so very strong. Plus the ply can continue past the supports for extra stiffness.

AND a sliding or bifold door slides away from the centre where stress is most significant and towards the supports where it is much less.

 

It might save me £500, and then the rest of the world can follow suit.

 

Once upon a time I used to work these out from first principles but it would take me 3 days to get my head round the principles again.

I'm not expecting one of the SEs on here to do this sum for nothing... I was seeking, and have received useful experience that nobody's has deflected and got stuck, and that everyone has a steel.. Overdesigned or just right, is another matter.

 

Thanks for listening.

 

 

[Russell griffiths]

I’ve just re read your original post, I thought you were proposing to build a beam 178mm high, which I based my answer on, but looking again your saying plate both sides in ply 600mm high. Now that would work. 
I would want to extend the plywood onto both side walls, not finish the ends level with the sides of the opening. 

[Adrian Walker]

3 hours ago, Russell griffiths said:

You won’t be spanning four metres with timber alone. 
with that wall above it will definitely deflect too much. 
steel flitch beams or some sort of steel beam with insulation and timber all sandwiched together. 

Flitch beams are generally only supportive in one direction. Bifolds will have a torsion load.