Subfloor for engineered wood

[Albie22]

Hi, I am planning to lay 20mm engineered oak with a 6mm wear layer in my Victorian house. It has suspended timber floors with a void underneath.
I’ve been advised to lay caber floor, then a ply layer and the boards on top.
Does that seem an excessive amount of subfloor? I had assumed ply would be enough. I’m concerned about door thresholds and the difference in heights as I can only afford to do the living room just now.
Thank you!

 

[Jolita]

we removed all boards and insulated between floors and straight on joints we lay 2.2cm engineered oak flooring and absolutely enough support.

 

[Cubist]

I would be worried about using a 'caber' type sub-floor for ground level floors. Most manufactures claim the material to be water resistant but I am not confident that it could handle being exposed to the damp conditions that can often be found beneath suspended wooden floors for decades. Frankly, in your shoes I would not bother with this as the ply-wood base beneath the 6mm veneer would be much more capable of handling those conditions than the caber material. You could also consider treating the underside of the your engineered boards before installation or even laying them over a membrane as used over concrete floors.

 

[Albie22]

Thanks for the reply. I don’t know the benefit of a caber floor under ply. I’ve been quoted 1.6k to lay the caber floor which was an additional expense I wasn’t expecting

 

[Cubist]

How big is this room? Does the £1600 include the new engineered oak boards?

 

[jamest]

If that's for the caber floor only, it's outrageous. You can buy 20 square metres of 18 mm moisture resistant caber floor panels for £200 (including VAT). But why would you want to? 20 mm engineered oak boards are structural strength, so like Jolita, you could lay them directly over the joists and forget the caber and the ply, if you were so minded.

 

[Albie22]

36sqm. That price is just for caberfloor. Then a separate price for ply, then again another cost to lay the actual floor.
This includes materials except the engineered floor. That’s been paid for separately

 

[jamest]

OK, so they are basically charging you about £1000 to lay the caber floor.

My guess would be that they intend to remove the existing floorboards (assuming that's what you've got) and replace them with the caber panels, that would explain the high cost. Why would they do this? Presumably because they want to get the subfloor as flat as possible. People tend to complain if a new floor isn't quite level or creaks or if some gaps between boards are larger than others. To guarantee that none of this happens, the want to get the subfloor as flat as they can. I can see no other reason for doing it. They are proposing to fit a floor that will be 'perfect' in the sense that it will be flat, level, creak and movement free - that way, they ensure that they won't have to come back to correct any problems (which could cost them, not only in time but in materials too)

 

[Albie22]

Yes, the floorboards will be lifted back to the joists. Is ply not sufficient for this? I had assumed ply with T&G floorboards or caber if it was click. I hadn’t anticipated laying both plus 20mm engineered floorboards

 

[a twig]

Yes just ply probably is but as @jamest says there are some conscientious installers who go belt and braces to avoid callbacks. Frankly based on my experience you’ve done well to find one

 

[a twig]

Either that or they are concerned about bounce from old joists that are usually thinner than modern building regs specifies

 

[Cubist]

Were these guys wearing masks when they called to assess the job? My guess is that they'll leave the Transits engine running when they come to ask for payment.

You need a reputable supplier and installer ASAP.

 

[jamest]

If the joists are a bit thin/weak as a twig suggests, then strengthening the subfloor makes sense - a tongue and groove modern engineered floor can't take much flex or there is a risk of the tongues splitting, especially if the subfloor isn't quite flat. Late Victorian houses usually have pretty strong joists on the ground floor though. I would have thought that 20 or 18 mm ply would be more than enough.

An alternative approach is to sister up any weak joists and add noggins between them to create a firm base, then insulate and then just lay the new boards over the joists, screw or nail them and not bother with any ply or caber. You might not be able to find a contractor who'll offer that service.

 

[Cubist]

There's little here that makes any sense to me and I'm convinced that @Albie22 's supplier is trying to rip him/her off. The only 'good' reason for a subfloor to be installed over extant joists is that the layout of the joists does not easily lend itself for use with engineered boards of the standard lengths supplied without inducing very high levels of wastage - the boards must always meet directly above a joist just as a traditional floorboard does as to do otherwise risks partial failure of the floor.

The original suggestion to use a layer of caber plus a layer of ply plus the 20mm floorboards - is utterly bonkers! The quoted price quoted for the caber alone is extortionate and no consideration seems to have been given to the overall weight question - all of which, in this configuration, could add up to circa 2 tonnes of dead weight which would have to be carried by the joist ends whatever condition they may be in! Also, the additional layer of plywood is absolutely superfluous as the engineered boards will have their own 14mm plywood backing below the 6mm oak veneer!

In terms of advice to the OP @Albie22 - like it or not you will probably need a plywood subfloor of 18-20mm thickness and nothing less. Your final engineered floorboards would undoubtedly be fixed using the 'secret nailing/screwing' approach and sufficient depth of subfloor material will be needed to hold the nails/screws. As a consequence you can expect to have to adjust the heights of skirting boards, any doors opening into/over the new floor and the door frames themselves. Consideration also needs to be given to where this new floor may meet others such as hallways as you may need to handle some height discrepancy there.

 

[Albie22]

Thanks everyone for your input. It’s much appreciated. I don’t think(hope) our joiners are taking us for a ride. I suspect the person that said they were being very cautious is correct.
However my thinking is in line with @Cubist that it’s massively complicating a simple install and the cost is ££££.
Will get further quotes in and see where we are. Skirting is all getting replaced but the doors will definitely need trimmed too with that amount of flooring going down. Also I hadn’t considered the load on our very old house so thank you for pointing that out too

 

[Flyfisher]

Given that this is turning out to be an expensive project, is it worth considering just laying new oak floorboards instead?

I had to replace a floor above a cellar (including the failing joists) and considered an engineered oak floor but in the end it seemed much simpler to just install new oak boards. I didn't have any issues with undersized joists because they had to be replaced anyway and the 6x2 joists I used are almost certainly over-spec'd for the span involved. I then laid 28mm oak planks and the floor is rock solid. Mindful of the oak 'moving' because it was over a cellar (which was one of the reasons for considering an engineered floor in the first place) I screwed down the boards (slotted screws 'natch) and also biscuit-jointed them (they are just butted up against each other, not T&G or lapped). Two years on, there's no sign of any movement, except for the cellar hatch that has slightly bowed. Anyway, just a thought.

 

[Cubist]

biscuit-jointed them (they are just butted up against each other, not T&G or lapped

28mm oak planks, biscuit jointed to boot.... Damn @Flyfisher you're either a perfectionist or a masochist - but I seem to recall you pushed back on the latter epithet so maybe a body-builder as well?

 

[Flyfisher]

Ha! Probably a bit of both, with a tendency towards over-engineering things, though definitely not a body-builder
The floor in question was discussed in this thread, with a photo showing the end result:
https://www.periodproperty.co.uk/forum/threads/new-oak-floor-treatment.19078/#post-249121
So far, so good.