In the latest article in his subfloor assessment series, Richard explains how surface regularity is measured, why flatness matters, and the practical techniques inspectors use to identify unevenness
THE next basic requirement for a subfloor is that it should be ‘flat’. Most people assume a floor is flat unless there’s something bad enough to trip them up, but with British standards we find a measurable definition.
What’s important for flooring is that the floor is free from undulations. It doesn’t have to be level unless there’s a specific requirement for this, such as in operating theatres or where highly sensitive equipment gets moved around.
BS 8204 – which has various parts to cover different types of subfloor – provides three ‘grades’ of surface regularity. When measured using a two-metre long straight edge, SR1 is achieved if there are no gaps underneath the straight edge bigger than 3mm, SR2 is 5mm and SR3 is 10mm.
However, the standard – and even the individual standards for different types of floor coverings gives these to enable the floor to be assessed, it doesn’t state a specific surface regularity that must be achieved for flooring. We usually find that specified by the flooring manufacturer, and if nothing is specified, or if the manufacturer just states that their flooring ‘must be fitted according to national standards’ this can cause considerable argument.
Surface regularity measurement also doesn’t take into account issues such as residual marks from trowelling out a smoothing compound which may be well within SR1 but will, nevertheless, show through a vinyl flooring. These are usually a sign of poor application of the compound and a failure to double check and smooth out any ‘wrinkles’ before proceeding to install flooring.
When a flooring has failed, an inspector needs to be able to assess the subfloor to see if this has contributed to the failure, so here are some methods I use.
The British standard reference mentions a straight edge, so I carry a two-metre long spirit level and a small bag of old pound coins. An old pound coin is 3.15mm thick and made to a level of precision that makes it a reliable way of testing a floor. If you lay a two-metre straight edge down on a floor you should not be able to slide an old pound coin under it if it is to SR1.
The extra thickness above 3mm means this isn’t quite as severe a test, but in practice the difference between a good floor and a poor one is going to be more than that extra 0.15mm. New pound coins, introduced in 2016, are only 2.8mm thick and this means they create a more severe test.
You can buy precision made spacers (and I don’t mean plastic window fitting spacers) and also tapered slip gauges which don’t just give you a ‘pass’ or ‘fail’ but actually measure the size of any gaps there may be, and these are very useful when I need to be very specific, such as when reporting for a court claim.
Flatness tests can be valid even if done on top of a sheet vinyl as the flooring will have an even thickness and so will follow the subfloor very closely. However, care should taken with more rigid flooring products, such as wood planks, laminates and SPCs, as these may alter the way the straight edge lies. However, the surface of any flooring will not be more uneven than the subfloor below it, so if the flooring surface is uneven, it is reasonable to assume that the subfloor is the same or worse.
A second method I use is to measure it with a laser. I have a laser floor scanner that projects an X-shaped beam and which makes assessing surface regularity very easy, but unfortunately this is no longer made. When training inspectors I use a standard cross-line laser and a marked ‘dipstick’ which has a line marked at the height of the horizontal laser beam and then lines 3mm, 5mm and 10mm above and below this datum.
I set the laser in one corner of the area to be assessed and then put the dipstick down every mere or so and note how the floor height varies from point to point. Even if the flooring is not level, this will reveal undulations and has the advantage of being easy to photograph for reference.
Another method which I find very helpful, especially to highlight uneven joints, bubbles or lifting edges in flooring is to use a 300mm rule stood on edge. This will show where, for example, adjacent tiles are not horizontal or have steps (‘lipping’) and it will also reveal any crowning or cupping of wood and wood-based flooring. A torch behind the rule makes any unevenness easy to see.
In the real world, even if the subfloor doesn’t meet SR1, if it reasonably flat it’s unlikely to cause issues for the flooring or the users unless the flooring is a click-together product with delicate joints. Unevenness can overextend them and flexing can cause the joint profiles to break.
There’s more but we’ll make that the subject of the next article in this series. If you’ve missed any parts, take a look back at the CFJ website.
www.richard-renouf.com
Richard Renouf is an independent flooring consultant
