Roger Moore takes a look at the possible problems caused by damp subfloors and how to
avoid these when specifying or laying flooring.
WHEN told a floor has failed, our technical team try to find out why, and how we can help sort the problem. On occasions, when we take a close look at these installations, the culprit is a problem with the subfloor or substrate. We’re often told there isn’t one magic solution to a problem; well in this case there are several. Whether specifying or fitting floors, the following could help avoid problems right at the start.
Subfloor types generally fall into three categories: concrete, wood and specials.
First: identify the subfloor you are dealing with. The following guidance is for concrete as this is the most common. Concrete subfloors could be within a newbuild or a refurbishment, so we’ll cover both. If you’re dealing with calcium sulphate, please speak with the flooring manufacturer to make sure you get the right guidance.
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.
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.
As a rough rule of thumb, new power-floated subfloors will dry at approximately 1mm per day, but can take far longer, so always test for moisture, ensuring 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 that 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 meters use a traffic light system to give an indication of the presence of moisture, but on cementitious subfloors, an electronic or air hygrometer will be needed to record a truly accurate assessment of the level of relative humidity present.
If you’re regularly testing both floors and walls, there are meters that can do both available. 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 a number of options available to you.
If you’re dealing with a refurb rather than a new build, 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.
Also consider the age of the building. Before 1970, damp proof 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 that 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?
- Adhesive-free flooring – but check first!
Altro adhesive-free floors can be laid up to 97% RH, and with no adhesive and no DPM required it can also save you time and money. They’re designed to lay flat and hold just as effectively as an Altro floor installed using adhesive. However, although all Altro adhesive-free floors can be installed without a DPM, that’s not the case for all manufacturers’ adhesive-free flooring options, so make sure to check! - Apply a damp-proof membrane
If your damp subfloor is in an area where a specialist floor is needed, such as a wet room or bathroom, you can apply a DPM, such as Altro Proof. 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 of the DPM that the surface is still too uneven, a subsequent application of smoothing compound should be applied.
Once the Altro Proof has been applied and allowed to cure and you are happy with the result, the new flooring can be installed direct, using a two-part adhesive for commercial kitchens, wet or heavy-duty installations. A pressure sensitive adhesive can be used for dry general circulation areas.
Make sure the adhesive you use can cope with moisture and off you go: 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. - Use a fibreglass underlay sheet
Altro Everlay A or B 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 Altro flooring can then be adhered to the surface of the Altro Everlay, using AltroFix 19 Plus two-part adhesive.
At 4mm thick, Altro Everlay B offers extra underfoot comfort for staff and additional sound absorption, should this be a requirement. If you choose this option, make sure you use AltroFix 19 Plus adhesive for this type of installation, as this has been designed specifically for use with Altro Everlay.
And finally, what to do if it’s all gone wrong? We’ve all seen the signs: split joints, blisters in the floor and a musty smell. In extreme cases the floor can also 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.
If this means disconnecting equipment, moving people out, causing a business loss of earnings, your time to strip it all out, treat the subfloor and re-lay, plus material costs, it could be very expensive. But I hope I’ve shown you a damp subfloor doesn’t have to cause problems as long as you know it’s there, and finding out is simple.
01462 489516
www.altro.com
Roger Moore is Altro’s European technical services manager
