Oak flooring... again (sub floor question)

[Therms]

Morning all,

I come here following extensive, but fruitless research and I’d like to call on your collective knowledge.

I’m laying a solid oak floor. The oak is 15mm tongue-and-groove boarding and I’m laying over floorboards, which sit on joists. I’m removing the skirting boards. I’ve loads of questions, but for the sake of not taking liberties with your time and making sure I understand the answers I’ll not ask them all at once. I hope no one minds.

I have spent some time looking for answers and not just arrived here to take up your time, but the flooring is costing me a lot of money and I don’t want to mess it up.

Subfloor

My floor is even’ish. I’m going to have to lay a sub floor and I’m thinking of 15mm plywood. I’d like to have used thinner because that’ll end up lifting the floor height by 30mm. I guess my first question is how uneven can my floor be and how thin a subfloor can I get away with.

My other question is a little daft – do I need to leave an expansion gap for the plywood subfloor?

Finally for now – I know I need to drive a lot of screws through the subfloor and into the joists, avoiding the pipes, but what screws are recommended?

Actually I’ve another quickie – do I need an underlay like they suggest you use with laminate flooring?

 

[WoodYouLike]

The thickest plywood we use then the floor is really uneven is 12mm.
Screw the plywood every 25cm (without hitting pipes!) use counter sunk wood screws. Leave a 'credit card' wide gap between the plywood boards.

And yes, on your plywood subfloor when installing floating you need foam underlayment (without DPM!)

 

[Therms]

Great reply – thanks a lot.

I was going to nail (secret nail through tongue) the new oak floor. Is this still called a floating floor and will it still need an underlay? How think is the underlay, just a roll of foam under the oak or under the subfloor – sorry, that’s a really stupid question.

Do I need an expansion gap round the edge of the room for the subfloor?

 

[WoodYouLike]

When you secret nail your floor doesn't 'float' and you don't need underlayment (no use).
Yes you do need to leave expansion gaps around the perimeter of the sublfoor.

Read this too about expansion gaps!

 

[Therms]

Again, thanks.

Can I ask what the underlay is for?

 

[WoodYouLike]

For floating methods it is for sound-insulation. When secret nailing the top floor is fixed tightly to the subfloor, creating one 'solid' floor where foam underlayment in between has no use at all.

 

[Therms]

While I’m on a roll of asking stupid questions I might as well ask one more.

If each board is nailed how does it expand over it’s whole length? Surely every board is fixed tight.

 

[WoodYouLike]

Your whole floor - when the humidity changes - will move (shrink or expand) - even when every individual board is nailed (every 30cm!). Expansion gaps are essential to cater for the expansion to keep the floor - as a whole - from buckling. The boards can react individually (lift up from their nails a bit, moving sideways and pressing against other boards).

We cannot changes the laws pf physics.

 

[Therms]

You are truly a font of knowledge and I thank you for your time.

 

[WoodYouLike]

Thanks! I should be really. (It's my job )

 

[CrazyHorseB]

Can I tag a few stupid questions of my own to this thread as it seems at about my level....

If the original flooring is a bit uneven to you need the plywood base for both Floating and nailing methods... or can you get away without using it all together (my new floor is 22mm)?

Given that the floor is slightly uneven does Floating or Nailing have an advantage?

What method do you use WoodYouLike?

 

[WoodYouLike]

What happens when you nail (every 30 cm) a floorboard onto an uneven sublfoor? The new boards will follow the 'dip, then try to 'flex' back into their original state and might lift the nail up a bit: Japanese alarm system - creaking floorboards when walked upon

We hardly use the nailing method when there is a subfloor/underfloor - only when we need to install directly onto joists.
When you new floorboards are of a good and thick quality (like 20mm solid or 20 mm wood-engineered) these will be more able to tackle some unevennesses than thinner boards when installing floating - they're more rigid of their own.

 

[CrazyHorseB]

For some reason I just want to nail them (in my mind nailing them makes them more 'part of the house') but after reading many posts on here you've finally convinced me to float them... thanks!

Do I definitly need a plywood base when floating then? The original floor boards aren't that bad but they are 80 years old so won't be consistent everywhere. Id just do it but seen as i'll be adding a 3mm underlay, another 3mm plywood takes the height of the floor up nearly 3cm in total which seems a lot .... but if you say it needs it then your the boss!

 

[Therms]

Interesting – I also was going to lay a plywood base and nail the oak (secretly through the tongue) because it seemed more proper.

Maybe, just maybe, I need to rethink.

 

[WoodYouLike]

Crazyhorse, another option would be to use Barrier - a more decent than normal underlayment (without DPM!) than can tackle some unevennesses better than the standard 3mm.
Is around 3.6 mm thick (and weighs a ton!)