Overboarding with a gap in the floorboards

[Deerhound]

I'm retiling my hallway.

The previous tiles were on 12mm ply, laid over the Victorian floorboards.

After lifting that ply, I've found a gap in the floorboards from where a doorway was moved (see link). In the gap there's the old base of the wall. The centre of it is almost but not quite level with the top of the boards but there's a few cm around that where whatever is laid in top is going to be unsupported.

I should say that the section with the gap never caused a problem with floor stability with the previous 12mm ply. (There were stability problems, just not near that section!)

My previous plan had been to replace the 12mm ply with 12mm cement backer boards.

However, I've got a feeling that cement boards will do even less well than ply with a small unsupported section. Is that right?

I don't really want to replace the floor boards with thicker ply because in the back half of the hall one of the joists you'd want to rest a new ply sheet on is hidden under the stairs.

I know 18mm ply is much better for overboarding but that's going to give me big issues with floor level Vs the surrounding rooms. The builder also wants to use a ditra mat which he says will add 3mm in height.

What should I do?

https://photos.app.goo.gl/CuksbHgQ2rXvy98WA

 

[RandomGrinch]

Block the holes and poor in self leveling compound?

 

[Deerhound]

Thanks RandomGrinch.

It's going to be super hard to make that pocket water tight for the SLC because it's so irregular and has gaps to the subfloor all around.

I had a good call just now with Hardiebacker's technical support line. The guy recommended screwing a 0.6/0.7 gauge steel sheet over the gap before laying backer board on top. Not perfect but sounds like a decent plan?

 

[RandomGrinch]

Sounds like a plan!
Though being a confirmed bodger, I would have a go blocking the holes with foam, or making a dam around the biggest holes before using compound. At least the unsupported section could be significantly reduced in size.
Good luck!

 

[Deerhound]

@RandomGrinch

So, not sure this plan is going to work.

The board is going to need a lot of fine cutting to get around a 1m curve in the hall and around the bottom of the stairs.

Am I right in thinking that's going to be much harder with Hardie boards than ply?

Also, looking much further ahead, if we ever wanted to re-expose the floorboards, is that even possible when there's a thick layer of adhesive on it like you need with cement board?

 

[RandomGrinch]

Hardiebacker is a lot harder to cut than ply. There are threads on here suggesting alternative ways to cut it, rather than the usual "score'n'snap". E.g.
https://www.diynot.com/diy/threads/hardie-backer-board-help.505742/
It is doable with a jigsaw, or angle grinder, but dust production should be minimised and good PPE worn.
It's also possible to screw down the boards (stainless steel screws are recommended to survive the alkalinity), that could help minimise adhesive use?

 

[Deerhound]

Thanks @RandomGrinch

Based on my conversation with Hardie, I think the adhesive is important in the plan to cover the gap with the steel. The thick adhesive layer acts as a kind of self levelling compound to give you a properly flat surface for the board despite having a sheet of steel over part of the floorboards.

I'll check if the builders* are ok to cut the curves in the board. Otherwise I guess it's back to 12mm ply. But that's what we had when the tiles came loose (quite a few years after installation, but still) so I don't have much confidence in it. The floorboards are solid but they aren't dead level and the previous ply left some splodges of adhesive that stick up a fraction of a mm. So maybe the new ply would still need a thick layer of adhesive like you'd use with Hardieboard to get it solidly level, as well as the screws?

All these problems go away if we cut the floorboards to get a proper thickness of ply down so maybe that's what we need to do after all. We'd need to get an extra joist down under the new ply to support one edge of it. I'm also a bit concerned that cutting the boards at the edge of the stairs removes some support from the stairs themselves. The edge of the stairs would sit about 4cm outside the joist that's supporting them. Does that sound stable enough?

Sorry, I'm totally unsure what to do now.

* I'm on a DIY forum under false pretences - sorry!

 

[foxhole]

Replace the damaged board.

 

[Captain Nemesis]

What should I do?

Find some appropriate boards via architectural salvage resources to replace the short one(s) or let a section in, clean sand and oil the boards, and hey-presto, good looking floor

 

[curry-kid]

I would just fill the hole with tile adhesive and smooth it level with the floor boards.
Use the 12mm ply as before.
Rather than the Ditra, you could use BAL Rapid-Mat. This is < 1mm thick.
I used the BAL rapid-mat in my hallway and tiled on top - no problem with the tiles and the doors open without needing planning.

Cheers