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Is the stability claim a myth?

Everybody knows engineered wood floors are more stable than solid ones… don’t they? Richard Aylen does some digging.

MANY people who have been involved in choosing hardwood flooring products will have faced the question; ‘Should I choose a solid hardwood or engineered wood floor?’ There is a lot of useful, qualified advice available from manufacturers and other specialists in the supply chain, but one piece of advice that is offered time and time again is that engineered wood floors are ‘more stable’ than solid ones.

I have worked in the wood floor industry for many years and have experience of both solid and engineered products and I have to say this claim raises more questions than answers and it is certainly a claim that needs to be picked apart to really understand why it has come about and if it is indeed true.

This is probably a good time to include some definitions. When I refer to a ‘solid’ floor I mean one where the full thickness of the board is made from a single piece of wood. An engineered wood floor is built up through its thickness with different layers of wood glued together. You could say it is multi-layered or laminated perhaps. Individual layers may vary in terms of their thickness the types of timber used.

I must declare my hand straight away; I believe the stability claim is a myth. For many years I have searched for proof and have found none. Despite the fact that the two floor types behave slightly differently when it comes to natural moisture-movement, I have found no evidence to support the idea that one type is inherently more stable than the other. Nor have I found that one is more prone to problems than the other in this respect, although squash court floors are an exception, which I will talk about later.

The belief that engineered wood floors are more stable than solid ones is widely held and is found in all parts of the supply chain. I want to start by giving a few examples that have come my way.
I’ve been told engineered floors will ‘keep you out of trouble’ if there is dampness onsite, but a solid floor will be less forgiving. But as far as I know no-one has produced evidence of both types being laid in the same space in identical conditions and one failing, the other not.

The bottom line is that no manufacturer will offer a guarantee on any kind of wood floor that is laid in damp conditions and both types; engineered and solid wood will fail.

Let’s look at how manufacturers define ‘damp’ or ‘dry’ conditions. If you believe an engineered floor is more moisture-tolerant you would expect the manufacturer’s instructions to recommend a humidity range that is wider than for a solid wood floor, wouldn’t you?

For my own company’s solid wood floors, we recommend humidity levels between 35%RH and 65%RH; which is a range of 30% from low to high. 35%RH to 65%RH is in fact the normal range that you will find in a typical occupied building in the UK, and this is broadly in line with design parameters used by CIBSE (the Chartered Institute of Building Services Engineers) and mechanical services designers.

To find out what wood floor manufacturers had to say I did some quick on-line research on engineered floor manufacturers’ installation instructions. I found instructions from four companies; two of which also recommended a 30% humidity range, but the other two recommended only a range of 20%RH.

Admittedly this was only a small sample, but it implies that engineered products are less stable. A range of only 20% is in fact difficult to maintain unless you have fully climate-controlled building. I will be interested to know if any engineered wood floor manufacturers will recommend and guarantee a humidity range higher than 30% – which they would have to do if they could in any way be considered ‘more stable’ than solid floors.

In squash courts most floors in the UK will be solid beech or maple. About 95% of these floors will be used with a bare, unsealed surface. Engineered floors are used to some extent but I know of one manufacturer who will only guarantee their floors if they are finished with lacquer.

The floorboard is a 23mm thick engineered board with a top layer of beech between 3mm and 4mm thick. If the floor is used unsealed the manufacturer will not guarantee it because of the risk of the top layer de laminating (ie becoming unglued from the backing board) and splitting.

They’re very open about the fact their floor in an unsealed state will not be stable enough to survive the normal humidity levels found in a typical squash court (and squash courts don’t have extreme conditions). Their concerns are justified because I have seen one of their floors, used unsealed, failing exactly as described.

Compared with an unsealed solid hardwood floorboard the engineered squash court floor, used unsealed is less stable, and the manufacturer confirms this.

Solid and engineered hardwood floors certainly move differently when it comes to the normal expansion and shrinkage resulting from seasonal humidity changes. Different; yes, but is there any element of ‘right’ or ‘wrong’ here? Engineered floors expand and shrink both length and widthways, and proportionally the amount of movement in each direction is about the same.

Solid hardwood expands more across the width (across the grain) than the length. Lengthways expansion is very small and for small to average-sized floors movement in this direction may not be noticed at all. As all floor manufacturers recommend expansion gaps around their floors it’s hard to see why an unbiased person would say one or other floor type is better or worse.

It also raises a few questions for the engineered floor manufacturer who claims their floors are ‘70% more stable’. This could be a result of some virtuosic number-crunching, but I think the claim warrants an explanation from the manufacturer. Given an explanation we can decide if it is actually something that translates into a tangible benefit to the customer.

I was given a copy of an advice sheet from an organisation in the underfloor heating industry. It was a guide to selecting floor coverings where they said that compared to solid wood floors engineered wood floors are; ‘a more stable option than solid timber, due to being cross laminated, its moisture content is less critical’.

This relates directly to the idea that ‘engineered wood floors will get you out of trouble’ that I mentioned earlier. It transpired that they were about to review their technical advice and the person I spoke with already understood that the statement was misleading and intended to remove it.
As a further example of how deep-rooted this myth has become, I have a copy of a consultant’s report on a failed solid wood floor that had been affected by high moisture levels onsite and the author, a flooring consultant, expresses his opinion that an engineered wood floor wouldn’t have been affected in the same way.

This was irrelevant to the case because he wasn’t reporting on engineered floor at all, and there was no clue as to why he mentioned it.

When I asked the consultant what evidence he had based his opinion on it transpired he had none, he was simply relying instead on the principle of ‘everyone knows’. He was probably fortunate his expertise was never cross-examined in court.

Engineered wood floor manufacturers are generally happy to sit by and allow the myth to thrive but a small number of less scrupulous companies just take it too far. Two well-known engineered floor suppliers in the sports floor market published web content intended to discourage customers from using solid wood floors.

Among their claims was the idea that solid wood floors ‘cup’ every summer because of higher seasonal humidity and because of this they have to be sanded every year to make the boards flat again. They concluded that solid wood floors have a shorter life than engineered floors. This is of course wildly untrue, and a typical solid sports floor will last for 60 years plus with proper maintenance – several times longer than an engineered one in fact.

When we hear of engineered floors being promoted on their superior stability you might gain the impression that engineered floors were developed because the world was waiting for a solution to some deep-rooted problem with solid wood floors.

I don’t buy this I’m afraid. We have used solid hardwood floors in the UK for centuries and there are many sold floors still going strong after 100 years or more. I think the truth lies in matters of cost (who knew?).

When the first engineered floors were designed, I am quite sure the conversation at the product development meeting was not; ‘how can we provide customers with a more reliable floor?’, but; ‘how can we make a cheaper floor that looks the same as a traditional solid one?’

What better solution than to cut out 75% of the hardwood and replace it with cheaper softwood?… although you have to use considerably greater quantities of glue to bond the layers together. You also use more energy because you have to saw the timber into many smaller pieces, but they probably didn’t worry about that so much all those years ago, and they probably don’t want you to think about it too much either.

So here is the crux of the discussion; why do people believe an engineered wood floor has superior stability? I think this originates from the similarity between engineered floors and plywood.
Plywood is accepted as a relatively stable sheet material, and it achieves this because it is made from layers of wood that are bonded together with the direction of the wood grain alternating between layers.

So that’s fine then, an engineered floorboard is just as stable as plywood because it too is made from layers of wood bonded together, right? Well, not quite.

Let’s look more closely. First let’s consider plywood. Ply sheets are made from layers of wood that are all the same thickness, using similar species of wood. This ensures the movement factor of each layer will be similar, with the grain direction alternating between layers, and when it is properly manufactured the moisture content of every layer will be the same.

Sometimes ply will be designed with a thicker core layer in the centre and there will be equal numbers of thinner layers above and below this core layer. Either way the ply sheet is symmetrical through its thickness and no one layer will be substantially ‘stronger’ or ‘weaker’ than the others. This is the way to achieve high stability. See above photo of plywood cross section.

Let’s compare this with an engineered floorboard. See above photo of engineered board cross section.

An engineered board is made from different types of wood, in layers of different thicknesses. You will find a hardwood decorative layer on top, this can be between 3mm and 6mm thick and will be made from oak, walnut, beech etc. Beneath this will be a layer of a different type of wood- which might be pine, either in thin layers or in ‘fingers’ called lamellas. Sometimes the middle layer may be hardwood such as hevea (rubberwood). On the bottom of the board will often be a thinner layer of softwood say 1mm to 2mm thick.

This form of floorboard construction is very different from plywood. The layers are different thicknesses and made from different types of timber and these will almost certainly have different movement factors. For the engineered board to achieve stability the amount of movement and ‘stress’ in one layer has to closely match the others. As the engineered board is made from different types of timber and in different thicknesses it is difficult to see how these can be factored to make a stable board. You also need to ensure the moisture content of each layer of the board is closely matched otherwise warping and delaminating will occur.

This train of thought is shared with timber scientists. At various times I have talked with timber scientists at TRADA (Timber Research and Development Association) and I have a copy of a TRADA report on an engineered floor that had failed on underfloor heating where the top layer on the board had split. The top layer was 6mm thick oak and this had been bonded to a plywood backing board. The TRADA consultant’s diagnosis of the cause, which is unfortunately too long to show in full, was that it was no surprise that the oak had split because its movement factor was approximately ten times that of the plywood.

When the two layers expanded and contracted under humidity changes the stresses between them were so great that the oak layer split. Ironically many people in our industry firmly believe that engineered floors are the go-to option for use with underfloor heating!

Looking at the wider world of engineered floorboards, and taking into account the enormous differences in board design, timbers used, thicknesses and construction methods, how confident can we be that the manufacturers have got their product design right every time?

It would be easy to understand how manufacturers arrive at some creative marketing strategies to sell engineered floors. If you want to maintain your profit margins you are better off not telling people that you are selling them a low-cost floor, and instead you put across arguments to convince customers that they are getting something that improves upon the original solid wood floors.

If you can also convince them that solid floors have inherent problems that you alone can solve then all the better. Even the name ‘engineered’ is a clever choice because it implies the products is created using cutting edge technology and sound design principles; something I have serious doubts about.

I have looked hard for evidence to support the idea that engineered floors are more stable than solid ones. I certainly believe the two floor types move differently and this is something that a less scrupulous marketing manager may try to polarise into a ‘good vs bad’ selling point. If real proof existed that engineered wood floors were more stable than solid ones there would have been regular instances of solid floors failing and engineered ones not, while laid in normal, dry conditions.

If such evidence exists no one has ever presented it to me. I have however found plenty of evidence to cast doubt upon the idea. There is a difference between what we truly know and simply what people say. We used to believe the earth was flat and that the sun revolved around the earth, but we have moved on…
Richard Aylen is technical manager, Junckers

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