DIY Triumph February 2009
Why Wooden Flooring: homes with wooden floors sell for more

More pointers in installing wooden flooring on underfloor heating

Today we received a interesting story about a wood flooring project that involves underfloor heating. The story and questions can be found here, we've copied and pasted the most relevant ones in this new article.

Woodchip asks:

I am in the process of procuring wood for our new house.

New, as in long term project (4yrs now).

We have piped in under floor heating upstairs and downstairs

The downstairs floor is 4" concrete and not perfectly level, so before we lay any wood, we would require to level it with a self leveling compound of some sort. Something I have no working knowledge of. Can someone suggest a good self leveling compound/screed mix that we could use for a fairly large area.  

I did see something in B&Q, but have no idea how good it is, or the brand name.

Acrylic level compounds are always much better - stronger - than latex self-levelling compounds. Especially when underfloor heating in concerned. See here for more information on preparing your underfloor and using levelling compounds.

 

Our downstairs rooms are (approx.)7.2m x 5m, 5m x4m, 5m x 3.5m & 7.2m x 4m.

The upstairs floor has 2 different types of floors. One room with concrete layed on Lewis plates 7.2m x 4m.

The rest is a biscuit system, with 22m moisture resistant chip board, with 2" x 1" strips nailed to the joists through the chip board, with underfloor heating pipes layed between the 2" x 1" strips & covered with a sand cement mix. These room sizes are (approx.) 6m x 5m, 5m x 3m + a landing 7.2m x 1.2m.

The floors as they are, have all been down for well over a year now, and finally we are at the stage where we want to lay solid wood on all these floors.

I'd rather not put down chipboard upstairs and then lay the wood on top, as I think there is enough weight on the joists already. Obviously we can nail the floor that uses the biscuit system, if need be.

As long as every room is treated as a separate area you don't really have to worry about creating one type of underfloor.
However, we are not in favour of installing solid wood flooring on Underfloor Heating Systems, there is a larger risk in shrinkage and cupping than with wood-engineered flooring. Our Duoplank Oak range for instance has wide boards AND is guaranteed on underfloor heating systems.

We have looked at OAK some at 120mm x 18mm, and 140mm x 22mm. We have been told by some to avoid a wide plank, with underfloor heating but this minimum width seems to have grown wider over time.

The only thing that doesn't seem to have changed is the minimum moisture level of 8% in the wood.

If your heart is set on installing solid Oak floors your best bet is indeed narrower width, we wouldn't recommend wider boards than 120mm. And it is good to read that the minimum moist content for Oak solid flooring is mentioned everywhere correctly. Do note this should be the absolute minimum moist content when your wood arrives in your home. If your home climate is even dryer there is a risk the wood will loose even more moist.
We also hope they also mention the maximum moist content solid Oak wood flooring should have: 11%.

Advice on the floors has varied some saying glue it, some saying float it.

With regard to putting down a 2mm underlay, we have been advised against it "as it tends to stop heat coming through, more than if the floor was first covered with 18mm chipboard and then had the wood layed on top of that". Wood being a poor insulator

I did notice a comment on this site, not to lay a solid floor on underlay if the room was longer than 5m.

With all that I've been told so far, the method I would prefer, is just glueing the floor straight on to the leveled floor.

Most manufacturers recommend to fully bond a wooden floor to the level subfloor where underfloor heating is concerned., specially solid wood flooring. Always use flexible adhesive and a correct notched trowel. The subfloor needs to be as level as possible to avoid air-gaps - adhesive isn't a 'filler', as some may think.
As said above, if you treat every room separate we don't see a need for installing chipboard first (chipboard wouldn't be our first choice for creating one type of subfloor: plywood is much better suited).

You are correct in stating solid wood floors in rooms wider than 5m shouldn't be installed floating on underlayment, much better is to fully bind them.

Do note the rule of thumb in regards of expansion gaps with Solid Oak flooring: for every meter width of the room add 3 - 4 mm gap with a minimum of 10mm. Based on your details this means a gap - everywhere! - of around 18 - 20mm.

We would like to direct you to two of our guides:
Wooden flooring and Underfloor Heating
and our Wooden Floor Installation Manual

 

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Comments

Woodchip

Thanks Karen

Bob Allott

I have just read this article on installing solid oak floorin over a under floor heating system. I was concerned regarding the comments refering to the moisture content of the flooring.
BS8201: 1987 in 6.5.3 Recommended moisture content at laying. Under floor heating 6% to 8%.

Wood You Like Ltd

Hi Bob

Thanks for your comment. Every wood species has its on best moisture content for installing, be it on or not on UFH.
Solid Oak for instance should be between 8% - 11%, Pine can have a higher moisture content (up to 15%) before it would give you problems once installed.
The 1987 guideline was written before wood-engineered floors "took over" and before better UFH systems and other materials were invented.

Best course of action: follow the manufacturers guide lines, that'll keep your warranty in place. Our Duoplank manufacturer (wood-engineered boards with 6mm solid top layer) recommend max 11% moisture content, which falls in the normal specs no matter if there is UFH involved.

Time and inventions evolve, BS guidelines are known to have problems to keep up I'm afraid.

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