Wood flooring and underfloor heating
Solid wood flooring, engineered wood flooring, and underfloor heating: what to consider, what to expect, and how to choose the right product.
First things first, it may seem obvious, but wood is a natural product. It was a living, photosynthesising plant before becoming your floor. It will take maintenance, and it will reward you with warmth, patination and character.
Please do not expect perfection, or for your wood floor to remain in it’s first-laid, pristine condition. It will age with you, and show the hallmarks of your way of life. This is to my mind a very positive thing but it must be embraced before you decide to purchase. Denial of this will lead to much frustration.
Engineered vs solid wood flooring:
Engineered flooring has a thin layer of wood, and several layers of ply underneath which are often laid across from each other. This provides a stable base, and there is less potential for the wood to swell with ambient moisture, and move with expansion and contraction. There will still be movement, but leaving expansion gaps at the edge of the room (to be hidden by the skirting) means you will not notice this.
There are three basic structures of engineered floors:
- Cross ply birch plywood back
- Sandwich Board where the whole structure is made up of the same species of tree
- Poplar back where you normally get just two layer of poplar this is the least expensive engineered board
Top quality plywood boards are the foundation for any quality engineered flooring, as good foundations are for a solid house. The plywood is made up of several layers of birch; each layer has the grain running in opposite directions which will ensure that the top layer has a very hard stable surface on which to adhere. You should always look for a 100% birch plywood back to give extra stability. A mixture of woods is not good for the long term stability of any engineered wooden floor.
One downside of this method of construction is that as the top layer of wood is thin it will not be able to be re-sanded or repaired to the same degree as a solid wood (think porcelain tile vs a natural stone tile, the “finish” stops at the surface). An average refinish removes between 0.75 and 1mm off the surface
Here are the general guidelines to keep in mind:
- .5 to 1mm wear layer: cannot be sanded and refinished; only recoated with a layer of urethane.
- 2mm wear layer: can be sanded and refinished 1 to 2 times.
- 3mm wear layer: can be sanded and refinished 2 to 3 times.
- 4 to 6 mm wear layer: can be sanded and refinished 3 to 6 times.
Engineered wood is durable, but not in comparison to solid wood. While an engineered wood flooring is less like to cup* than solid wood flooring, if it does there is less remedial work that can be done (due to the thin veneer).
Solid wood is (as the name suggests) solid wood. There is no base layer. This wood can be refinished or re-sanded many times as the wood is thick (thicknesses vary from 20mm to 38mm but if you are thinking of having it over UFH, you will be looking at the thinner side, to ensure a low TOG**)
Solid planks of wood will be attached directly to your concrete/screeded floor or floating floor.
Both engineered and solid wood flooring will have movement as the product expands and contracts with seasonal variations (heat), and with the presence of ambient humidity (moisture) – with, or without UFH.
Solid wood will have more movement than engineered wood because it fully reacts, whereas engineered is “trapped” to a substructure that restricts it somewhat.
Moisture expansion and contraction:
If you were to put a dry solid wood plank in a wet room, it would absorb the moisture of that room and swell, and the reverse if your timber was wet, and you put it in a dry room. While fabulously helpful in the live tree, this causes trembling in the face of purchasing thousands of pounds worth of flooring.
To reduce the chance of moisture creating cracking, bowing or gapping in both engineered, and solid wood floors there are two important steps you should take:
- the flooring that the wood is going to be laid onto must be at or under 8% stable moisture levels before you lay the floor (see specification of your provider for the figure they require – failure to follow this will null and void the guarantee on your product). This prevents immediate after-laying movement of the wood.
This is usually more relevant if the space is new-build, or the UFH is newly laid.
- your wood must be allowed to acclimatise to the space in which it is going to be laid BEFORE it is laid. Some providers state 2 weeks, others less. This prevents gapping and warping soon after laying, and ensures the expansions gaps left can deal with the true movement of the floor.
Neither engineered nor solid wood are recommended for bathrooms as the ambient moisture level varies frequently, as does the temperature of the room (if you want the look of wood, think about Karndean flooring in these spaces). Some producers do not recommend laying in the kitchen, however I have had no problem with this, after ensuring the family aren’t too splash happy with the dish-washing!
Any leaks around your wood flooring, engineered or solid, will swell and warp the wood if not dealt with quickly. Some engineered wood offers a surface level of protection against this with lacquers which makes moisture bubble on the surface rather than sink in, but I find these products (sadly) tend to look like laminate because of the plastic-y finish this requires. Water which penetrates this often has no easy way of drying out, creating more of a problem elsewhere…
Heat expansion and contraction:
This is a large part of the concern when people consider UFH, but it is also relevant where a space has standard central heating, or where areas of floor are exposed to direct sunlight.
To counter the heat expansion of your wood flooring, expansion gaps must be left between the floor and the walls. The width of these gaps differ depending on how the floor is fixed: if it is nailed or screwed down (NOT RECOMMENDED WITH UFH!!) it needs 5-8mm, and if glued or is a floating floor, it will have 12-15mm. Your flooring professional should have the correct guidelines for your choice of floor.
If you have decided on solid wood, you should be aware that the wider the planking you go for, the more movement you can expect. So a superwide 400mm plank or board could move as much as 5mm annually, where a 120mm plank or board would be expected to move 0.5mm annually. So if having little or no visible gaps between boards is important to your aesthetic, choose a narrower board.
This is also true of engineered flooring but the movement is generally less as the backing boards hold the top veneer more tightly, not allowing the same level of movement.
Personally I prefer a really wide board, or a variable mix of wide, mid wide and mid, and feel that small cracks and gaps annually appearing (worst case scenario) add character and originality.
UFH – Underfloor Heating:
There seems to be common “knowledge” that you can’t lay a solid wood floor over underfloor heating. The thought appears to be that a solid wood laid over UFH will leave the floor unusable within weeks, due to movement, cracking and swelling.
This is not true.
If the substructure is dried to the guideline requirement (speak to the flooring provider for their protocol), the wood doesn’t reach a surface temp of 27 degrees (which would be quite uncomfortable to walk on), and the flooring has a tog** value of under 2.5, both engineered and solid wood can be laid over UFH.
Note be very careful putting large rugs or furniture over the wood flooring if there is UFH, as this can increase the tog value thereby trapping heat and causing hot spots which will damage either engineered or solid wood flooring.
I recommend a water based UFH as opposed to an electric when going under wood, as the electric heats and cools more rapidly so can put any solid, or engineered wood flooring under expansion-contraction stresses in a short period which may ultimately lead to the material itself cracking.
Floor stress also depends on how consistently you use your UFH. If you leave your heating on annually, allowing your system to maintain a minimum temperature (ie it won’t come on if the ambient temperature does not require it), or seasonally put it on, and maintain a constant temperature during the season with slow warm-up and cool-down periods, the wood has time to adjust.
However if you arrive to your freezing cold holiday home in the dead of winter and turn the heating on full, expect the floor to be upset! I refer you to my opening paragraph. Wood is a natural material with character and charm. It should be prized and respected amidst the daily hammering. Treat it with a little thought and you will have a welcome worth returning to, for decades.
To have a view on the sustainability of the product you are purchasing, it is important to know the provenience and the species of the wood. Investigate if the manufacturer replants to replace, and how they/their supplier manage their woodlands.
If you are choosing a slow growing hardwoods (such as Oak), engineered wood flooring uses about 75% less surface wood than a solid plank. This is due to the veneer of “real” wood on the surface being around 5mm thick, versus the solid wood planking being around 20mm thick. So you may (theoretically ) get up to 4 engineered boards out of every cut from a solid plank.
Of course there is also the cross-layer wood backing of an engineered board to consider: these plywoods tend to be made of faster growing hardwoods (birch, poplar). They make up another 12-20mm depth of the board.
Part of the sustainability model is lifetime of use. Solid wood floors tend to be kept for longer than engineered, often being restored by future owners of a home to suit their preferred style, or if taken up they are sold as reclaimed.
It is possible to get long and wide solid wood planks from fast growing species (such as the Douglas Fir). Companies such as Dinesen have been using this species for their flooring for over fifty years, with floors laid by the company’s founders still in place (see the Danish Castle “Sønderborg Slot”, laid in 1965).
This largely depends on the finish of either floor, and on your desired look.
If the finish is a varnish or lacquer it is the equivalent of putting nail polish over he wood. If it wears or chips, the wood will be unprotected and the look will be uneven, so the floor will need refinished.
Some flooring (usually engineered) will be protected by the manufacturer using a coating that can make it look a little “plastic”. This is the most resilient (think of a commercial restaurant floor) and the manufacturer will be able to inform of the amount of footfall if should withstand until needing repair.
If you opt for a stain or oil to get the flooring to your colour of choice, this soaks in and protects the wood, but needs monthly or weekly topping up with a special wood floor cleaning product.
If you have a lye, or soap finish, these floors will need a monthly application of soaping product to keep it looking as good as it can.
Your manufacturer will be able to provide detailed instructions for care and you should read these to make sure you are happy with what you will need to do, before you make a final decision.
You will find that main routes of traffic (eg doorways) will patinate more quickly than less used areas, but be warned if you want to resurface you will need to do the whole room, not just a select areas.
There are some rules of thumb for the day-to-day living with a wood floor:
- Dry or damp mop (to requirement)
- Do not leave the surface wet
- Do not use bleach
- Clear up spills as soon as they happen
Well done on reading this through! As a prize (haha) I have made this article into a PDF which you are welcome to download using the button below.
I hope you that when you do get a wood floor, your reading of this guide will have helped your understanding of what to consider, what to expect, and ultimately how to choose the right product for you so your enjoyment of this natural product is enhanced.
* When a plank of wood “cups”, it bends itself as if it were wrapping around the heart of the tree – you can look at a plank of wood and see the grain of the plank, to see which direction this would be in. The edges of the board raise higher than the centre.
**TOG value is the insulation factor for the flooring. The word comes from the informal “togs” to mean clothing, which in turn comes from the Roman “Toga”.