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More questions on underlayment (and gaps)

Underlayment is one of those items where we receive many questions on (and that's no wonder, there are so many different products around - all with their own benefits, instructions and promotion slogans).
Last week we had the following (on-line) conversation:

Freddy asked:

Sorry in advance if this is going over old ground. Have recently bought bamboo flooring and will use the "floating method" to install, the underfloor is all old concrete.
Could you give me some guidance on underlay etc. Is it better to use foam and hardboard or the all in one "feltboard type"? Also what kind of expansion gap should I leave as the suppliers told me it is minimal 5/6mm as bamboo is virtually shrinkproof.
Thanks very much for any advice you can give me.
Cheers Freddy

We answered:

Dear Freddy

If your concrete floor is level (may have a gentle sloop of 1 - 2 mm per meter, but no sudden drops or 'hills') it's bes to use a combi-underlayment. We always recommend to leave 10mm gaps all around, no natural wooden flooring is 'shrink' or 'expansion' proof.

Hope this helps
Wood You Like Ltd

Which resulted in the next question from Freddy:

Thanks for the guidance,(the underlay I mean comes in blocks/slabs and you just cut to fit.The floor is not too bad a little slope running down the hall about 3/4mm over about 2 metres.
I do have to be careful with not gluing the boards to the underlay don't I.
Also do I need to use cork expansion strips?
Thanks again for the help, it's much appreciated.
Cheers Freddy

Our answer:

Hi Freddy

On concrete underfloors it is best to use an underlayment that contains a DPM. The underlayment you mean I don't think will create a continues barrier. Another option for you would be the ticker Timbermate Excell (5mm versus the standard 3mm of the combi)

You're right about having to be careful when glueing the T&G's, any spills on the underlayment can 'strop' the floor when it 'moves' during the seasons.

Cork expansion strips just fill up your expansion gaps! You don't need them.

Hope this helps
Wood You Like Ltd

Freddy replied as follows:

Thanks very much for the advice, it's very much appreciated.


Feel free to ask your own question, either by leaving a comment underneath this post - or any other, or in our category 6 - in the relevant FAQ post your query is about:
Benefits, Preparations, Installations or Maintenance and After Care

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Stuart Campbell

I intend laying t+g oak boards onto a dining room floor (floating method). However, the dining room leads into the kitchen. Would 20mm x 120mm oak boards be suitable in the kitchen if properly finished and sealed?
If yes, the concrete floor they'd be laid onto sits above a dry cellar. Would you still recommend an underlay with DPM?
Many thanks

Karin H.

Hi Stuart

With kitchens we normally recommend wood-engineered boards - they are more stable in areas with more moist, but keeping the solid boards rather narrow like you have 9 times out of 10 it shouldn't really cause a problem.

We do recommend to install a combi-underlayment in the kitchen to prevent any residue moist from the concrete getting into the wood. The dry cellar can cause a colder climate underneath with possible - on occasions - condensation. The DPM will help prevent problems caused by this.

Wood You Like Ltd

Stuart Campbell

Thanks, that's great. The boards are planed and unfinished, which of your two finishing products do you recommend. I'd like to keep the Oak as white as possible and am aiming for a satin finish. I notice they both contain oil, but which has the least colouring effect. Many thanks.

Karin H.

Hi Stuart

Any 'natural' neutral hardwaxoil or varnish will over time give the Oak its natural characteristic 'honey coloured' appearance - or 'golden yellow'.

To keep the Oak as white as possible you have to apply a colour oil - white or white/grey. Either two coats or one coat colour and second coat natural.
No matter what colour you choose, it's always best to try it out on a cut-off plank and then still every plank will have its own character and reaction to the colour oil.

Wood You Like Ltd

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