What is Granwood? How would you recognise it? And what do you need to do to deal with it? Richard has the answers.
WHEN training estimators, one of the shorter subtopics is about Granwood. The first slide on this topic is usually met with blank stares, even by estimators who’ve been working in the trade for decades. What is that? How do I recognise it? What do I need to do to deal with it?
A surprisingly large proportion of halls and sports venues have a floor which looks like wood blocks or strips but which doesn’t behave like wood at all. I came across a typical example recently when I went to look at a community hall where the new sheet vinyl flooring was bubbling in lots of areas.
The bubbles were hard because the plywood was lifting below the vinyl. The installer couldn’t understand why, so I was asked to take a look by the Community Association.
The hall was built soon after the second world war. Any property built before about 1970 is unlikely to have a dampproof membrane between the ground and the concrete slab, so I suspected there would be a high moisture content. When I took readings using an RF meter they were very high, so I investigated a little further.
In the entrance hall and corridors around the affected room was a very old linoleum floor. The bright red colour was now faded. Underneath the mats, which had clearly been in place for a very long time, the original colour could be seen in stark contrast to the exposed areas.
The moisture meter gave very high readings in these areas, too, so there was definitely not a structural DPM and any moisture readings taken before the installation would have shown this.
The lady in charge was able to tell me the new flooring had been laid on top of some old mustard and brown mineral tiles. These hadn’t been removed and the plywood had simply been nailed through into the floor below. Mineral tiles are, in themselves, a clue to the likelihood of an older building without a structural DPM (and so are parquet floors, just in case you weren’t aware).
These products were created in the era before plastics, so before double glazing, central heating and other modern amenities which we now take for granted. The flooring had to be moisture tolerant, so beware when you are asked to fit a plastic floor over such products as it is very likely to cause issues if you don’t prepare the subfloor properly.
But there was more information to be found. The room had two matwells by fire escapes and these had been fitted with fibrous entrance mats. I peeled them back to look, I hoped, at the tiles, but found instead what looked like a strip wood floor.
I knew immediately it was not wood: the clues were the lack of wood graining (it looked closer to MDF in visual texture) and the uniformity of colour – it looked bland and uninteresting when you looked closely.
I then scraped the surface and found a lacquer peeled away and left a crumbly substrate that scraped off as a fine greyish powder. It was Granwood.
Granwood is a composite material which does have a small amount of wood dust in it. This is mixed with cement, linseed oils and other materials and then turned into smooth-faced blocks that can be laid onto a slab even if there is no structural DPM.
If, as I suspect, the majority of floor layers have little if any knowledge of this material, would the installer be liable for the problems it had caused in this case? The simple answer is ‘Yes’ and for a very good reasons: the issue in this case was not the Granwood. This had been down for many years, probably since the building was constructed. It had given excellent service before it was overlaid with mineral tiles. The issue was the moisture, and that this was now being sealed up by plastic flooring. Any competent estimator and/or installer would have found the high moisture readings without even being aware of the Granwood below. The problems were owing to a failure to take moisture readings.
Had the high moisture levels been detected, the next step would have been to find out why. At the very least one or more of the mineral tiles would have been lifted to check the floor below, and if at that point the installer was unsure s/he could have sought advice from any of the subfloor preparation materials manufacturers and the right specification could have been drawn up to prevent the problems arising, or the current suppliers of Granwood could have been consulted directly. That might have resulted in them doing the work instead of the flooring contractor, but in this case I suspect that would, with hindsight, have been welcomed.
The Granwood would probably have been taken up and the slab below prepared as for any unprotected screed by way of smoothing compounds and a surface-applied liquid DPM.
Instead, the flooring contractors chose to nail plywood to the Granwood/tile sandwich, and not at the 150 mm centres required by BS 8203 (and 100 mm around the edge of each panel). Then the waterproof flooring had been bonded to the ply, creating a moisture barrier which allowed the rising ground moisture to permeate the ply. the moisture had swollen the ply causing it to lift and burst the fixings.
There are other floor types that installers may not have come across, but in each case the simple steps set out in the British Standards for wood-based, resilient and textile flooring would reveal any problems and indicate the need for further investigation and advice.
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Richard Renouf is an independent flooring consultant