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My one golden tip: test for damp!

Time and again, we’ve seen a damp subfloor causes floors to fail. Regardless of the type of subfloor and how old the building is, the golden rule that can head off a host of problems and potentially save you money is: test for damp, says Roger Moore.

YOU’D be amazed at how often no reading is taken, or wires are crossed and an installer gets the green light to install the floor, yet no one has actually tested for damp.

And yet we’ve all seen the signs: split joints, blisters in the floor and a musty smell. In extreme cases flooring can stain from damp being drawn up into it and unfortunately there is no quick fix. The only solution is to take it up and start again, and that can be very expensive. So best head it off from the start!

Subfloor types generally fall into three categories: concrete, wood, and specials.

First, identify the subfloor you’re dealing with. The following guidance is for concrete as this is the most common.

As a rough rule-of-thumb, new power-floated subfloors will dry at about 1mm per day, but can take far longer, so always test for moisture, ensuring that the reading is 75% relative humidity (RH), or below, before installing the new floor.

Don’t let anyone push you into installing any sooner, unless the reading is right – I find it’s best to do the reading yourself so you can be absolutely sure.

A reliable hygrometer or protimeter doesn’t have to break the bank but an investment here could save you a lot of money long term. Some metres use a traffic light system to give an indication of the presence of moisture, but on cementitious subfloors, an electronic or hair hygrometer will be needed to record a truly accurate assessment of the level of relative humidity present.

When you take an RH reading for the floor and it’s 75% or less, go ahead. If the reading is over 75%, there are several options available to you, and we’d be happy to give guidance.

If you’re dealing with a refurb rather than a newbuild, inspect the old floor before you remove it. If it shows signs of damp issues then pull up the floor and take a good look at the subfloor.

Before 1970, dampproof membranes (DPMs) were not mandatory, making the damp reading even more important. The relative humidity (RH) reading will indicate how you should proceed and, if above 75%, a surface DPM should always be applied.

Whatever the reading you may have to cover old adhesives, so it’s important you choose a levelling compound that’s suitable to cover them. Your chosen levelling compound supplier will have guidance on what’s best to use in each case.

So, the subfloor is damp. What are your options?
1 Adhesive-free flooring: this can be laid up to 97% RH, and with no adhesive and no DPM required it can also save you time and money. It’s designed to lay flat and hold just as effectively as floor installed using adhesive.

2 Apply a damp-proof membrane: if the damp subfloor is in an area where a specialist floor is needed, such as a kitchen or a wet environment, you can apply a DPM. If the subfloor is uneven use a moisture tolerant smoothing compound at sufficient depth to provide a smooth flat surface to receive the surface DPM.

If you feel that following the application the surface is still too uneven, a subsequent application of smoothing compound should be applied. Once this has been applied and allowed to cure, the new flooring can be installed direct, using a two-part adhesive for commercial kitchens, wet or heavy-duty installations.

Make sure the adhesive you use can cope with moisture: if you don’t use a moisture-tolerant levelling compound on a damp subfloor, it’s a bit like when you take a plaster off your finger and it goes wrinkly – the moisture has been drawn out of your skin, the same thing will happen to the smoothing compound and it could fail.

3 Use a fibreglass underlay sheet: these can also be used where RH levels are above 75% and an adhesive-free floor is not an option. Once loose-laid to a smooth subfloor, another more suitable flooring can then be adhered to the surface.

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