Need help understanding how to fit bifold doors on garden room

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27 Sep 2022
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Hi all, I've been building a garden room over the summer, using a timber frame on concrete piers. This was because of size of the room (6.5 x 5m) and also access (a long garden) making it difficult to pour a solid concrete base.

Anyway, all going pretty well but I want to fit some bifolds, which is the first time for me, and it appears they all need a recess / rebate below the main floor level, which I can't quite work out how to do!

I have a timber frame, but it appears I need to cut out a 60-70mm rebate from the perimeter floor to avoid having the door 75mm above the subfloor? Am I misunderstanding this? Any help working this out would be much appreciated.

Here's an example from one supplier I'm looking at:
It does depend if you're having open in or out, but normally if you want a flush threshold open out then the sill with a guide groove will have to be set to finish flush with the finish floor level inside. Pvc and aluminium door sets will come complete so the size of step down will be down to the manufacturer, with timber sets the guide channel is about 20mm so you can build that into a threshold size to suit you. Bottom rolling sets will require less height than top hung if that works better for you. If you do need to alter what you have already built it will be best to cut out the section of perimeter timber and replace with smaller section rather than trying to cut down insitu.
I think you need to cut the ring beam. But you will need to double up behind to maintain strength.

Putting a step of CLS in behind the beam and then up to support the floor might work??
Thanks for the replies guys, sounds like I do need to cut out a section from my timber frame then. I've created the base from 6x2, with 18mm ply as the subfloor on top. On the basis that I need to cut out some of my perimeter frame to allow the doors to sit lower, I've bolted on a second 6x2 which then sits on the concrete piers too. Just wanted to check this is correct which it sounds like it is.

Basically all the doors I've seen have a difference of around 60-70mm between the internal threshold and the outer cill / sill, and the doors themselves sit at the lowest point of this as I understand it.

Below is a plan view of my perimeter frame, then an angled view to hopefully better communicate what I think I need to do next. Wish I'd planned the doors earlier in the process but this hadn't come up in my research for somer reason....

The doors I'm looking at with Express Bifolds (and most others to be fair) have a 75mm cill to threshold height, so if I don't cut out the base it'll be a massive internal step down!

So I think I need to cut out the depth of the cill in my external 6x2, and about another 15mm from the internal (second) 6x2, to give the depth of the door frame, and this allows the internal threshold to be just above my ply level, to align with laminate / wooden flooring as per the bottom sketch?

Sorry for the long post but this part is new to me and I've found it almost impossible to find info. Any YouTube videos I've seen just show people sitting the doors on the floor level of the room, which doesn't seem to be possible??

floor structure.jpeg
I think you will need to cut out the section and move it back then nail on a 3x2 (or a pair of them) at the right height to sit the threshold on. If the joists are running parallel to the opening this won't be too hard if they're joined to the perimeter beam you'll make it easier if you lift the floor.
I think you will need to cut out the section and move it back then nail on a 3x2 (or a pair of them) at the right height to sit the threshold on. If the joists are running parallel to the opening this won't be too hard if they're joined to the perimeter beam you'll make it easier if you lift the floor.View attachment 144388
Your wood there looks a bit pulpy too few growth rings!

Looks about right just remember the edge of ply floor needs supporting.
Is it a rebated or a low threshold bifold and is it too late to change it?
I went through all this on my build and understand your problem. It's easily solved, it's just venturing into the unknown that's the issue....
Thanks for the further replies - sorry, didn't see a notification for the responses!

It is a rebated threshold @Molynoox, which doesn't look like too big a problem if I take the approach suggested by @Jones above. The perimeter frame on the front - with the wall now in place above - is currently two 6x2s, but I can cut out a section of one of the 6x2's across the width of the door opening and replace it with a 3x2 instead.

Also the perimeter frame is supported on concrete piers approximately every metre, so even a 3x2 on the front would be supported across the whole length of the opening.
I went for low threshold, it means you don't have to do all that faffing, it's only 20mm deep which is the same as the floorboards, the sill is 20mm which is about the same as the underlay and laminate. So all-in you have about 40mm of door to lose in the floorboard and flooring and then you have a seamless threshold to the outside.
Basically it means you can sit the door directly on the frame and just build the floor up behind it.

I use Brio bi fold hardware from Marches Ironmongery, they have quite detailed drawings on their website. Isn't a small step over threshold a small price to pay for weatherproofing? do you realize the channel for the door hardware will need somewhere to drain to and open out doors will need a planted stop on the inside?