How To Stop Sub-Floor Moisture From Damaging Your New Wood Floor

This is one of the biggest questions for people spending a small fortune on a new wood floor and rightly so! Who wants to throw their money down the drain?! Believe me, many have and many will. By finding this article, you may have just saved yourself future heart ache and misery. Read on to get the answer you want..

I want to start by letting you know exactly what the solution you are going to be given is referring to. That is to stop moisture from concrete or solid sub-floors damaging your new wood floor.

It’s said that death and taxes are the only things guaranteed in life. Well, I’d like to add ‘wood floors will fail if left in close contact with excessive moisture’ to that guarantee. My little add on isn’t quite as catchy but true all the same.

So you want to install a wood floor on top of your concrete or solid floor and have either found out it’s wet, believe it’s wet or are unsure.

Well in this game, taking a chance won’t double your money. You’ll either get away with fitting your floor without failure or you won’t!

How to test for sub-floor moisture in a solid sub-floor

The most accurate and only method currently excepted by British Standards under British Codes of Practice BS 8203, BS 5325 and BS 8201 to test for moisture in concrete sub-floors, is with the use of a hygrometer humidity box or RH probe. These instruments, used in conjunction with a relative humidity reader, measures the rate of evaporation of moisture from the concrete slab. If a reading of 75.01% relative humidity or over is recorded, a wood floor of any kind should not be installed without further preparation!! Regardless, of the fact you may be using an underlay with a moisture barrier. These types of underlays are only good up to a relative humidity of 75%..

But I came here for a solution?

Well, I will not disappoint! Concrete slabs can take months and even years to dry in certain circumstances. To say a concrete slab is dry just by looking at it or even going from the time it was installed and using rule of thumb guidelines, is just not good enough. A concrete slab can look perfectly dry, however, moisture evaporation from the core is very rarely visible! Take heed..

Concrete slabs often have a moisture barrier or insulation installed underneath. This helps prevent the concrete slab from taking on moisture when water levels increase. In houses built before 1960, ground floor concrete slabs were not required to have this additional moisture barrier. So, whether you wish to hurry up the installation process after a new concrete slab has been installed, have an older property without a foundation moisture barrier or are simply just not sure and require a cast iron guarantee, you’ll need the right products to do it!

Ardex 1 Coat Damp Proof MembraneThe solution is to use Adrex 1 C (coat) Damp Proof Membrane (DPM) in conjunction with Ardex NA latex based self levelling compound.

Ardex 1 C can be used on concrete slabs with a relative moisture content of up to 98%!! It can even be used on concrete slabs that don’t have a moisture barrier installed under the slab.

This type of liquid DPM doesn’t trap the moisture within the concrete slab (In the case of post 1960’s slabs with a sheet dpm installed) . In effect, it actually lowers the rate of evaporation so the slab can still dry but not effect your new wood floor. Ardex 1 Coat and Ardex NA are both fast drying solutions, that will enable you to install your wood floor within a week of installing the DPM and Ardex NA (Confirmed by Ardex).

Ardex logoSometimes in life, things can sound to good to be true. However, Ardex as a company have been around for over 50 years and stand by all their promises using the wealth of experience they’ve gained in the industry.

You wanted a solution and here it is. It’s now up to you to take on board all the guidance given by Ardex when installing their products and have complete peace of mind that your new wood floor will be down problem free for years to come.

Caution : Ardex DPM 1C is not recommended on slabs subject to hydrostatic pressure. In most cases, properties are built in a way that prevents the build up of pressure i.e. with adequate drainage, property situation etc. However, hydrostatic pressure may well be a concern for consideration i.e. basement excavations/renovations or local geographical changes etc. If in any doubt, contact ‘Ardex technical’.

Of course, there are other solutions available than Ardex. Take a look at other liquid Dpm options here..

© Copyright 2013 Wes, All rights Reserved. Written For: Fitmywoodfloor
About

I am a Pergo trained professional installer of 20 years. I've been up close and personal with lots of floors and have the knobbly knees to show for it...Should you have any questions or comments please feel free to add them below. Thanks for taking the time to call by and I hope the information you've found has given you some insight!........................................................................................................................................................................“When you click on links to various merchants on this site and make a purchase, this can result in the site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network, amazon.com, amazon.co.uk, amazon.fr, amazon.de, amazon.it, amazon.ca, affiliate window network.” This statement is to comply with current internet regulations regarding transparency to consumers.

Posted in Engineered floor fitting, Installation and project guides, Laminate floor fitting, Solid wood floor fitting, Solid wood flooring, Sub-Floor preparation Tagged with: , , ,
  • Kristy

    Hi Wes

    We had some Karndean laid in our new extension. However they didn’t do a moisture test and 3 weeks later there were bumps in our flooring. We now have hygrometer down to measure the moisture (we as UFH installed so the digital ones won’t work). However they applied a self smoothing compound when initially laying the flooring.
    Will the smoothing compound stop the screed drying out properly (even delay the process) and are be better laying Adrex over it? Or is digging up the smoothing compound needed before Adrex can be laid? I have little confidence in any recommendation/advice the flooring company so turning to you 😉
    Hope you can help.

    Kristy

  • Hi Kristy,

    Can I firstly confirm what the screed below the smoothing compound is please? I.e. Calcium sulphate/Anhydrite, standard concrete etc (Please be specific if possible).

    How long had the screed been down?

    What ‘smoothing compound’ was used by the Karndean installers?

    Regards,

    Wes.

  • Kristy

    Hi
    Thank you for getting back to me. The screen is a traditional sharp sand and cement. It was down (from memory) 3/4weeks before the flooring was put down. The builders confirmed it was about 60/70mm deep
    Unfortunately I’m not sure which smoothing compound was used.

    Thanks
    Kristy

  • Three to Four weeks is pushing it to say the least. They may have been relying on the screed being force dried with the UFH, but force drying isn’t recommended with a normal sand/cement screed which I’m sure your builder noted. Regardless, the slab should have been tested before hand.

    ‘If’ the original compound has blown, it would be best to remove it, let the screed dry properly and start again.

    If the compound is fine and the bumps are due to moisture effecting the adhesive, resulting in debonding, the compound should allow the moisture to pass through. It shouldn’t effect the drying time too much, although it will extend it. This is all providing the compound is moisture tolerant.

    Ultimately, you won’t know until the Karndean, or sections (for testing), has been lifted.

    Regards,

    Wes.

  • David Gibson

    Thanks Wes, that’s what I thought but fortunately we’ve managed to come to an accord

  • Adam Joseph Carolan

    Hi, I had a hard wood floor fitted in my apartment that was glued onto the concrete floor directly. I was happy with the fitting but noticed it started to lift. Upon inspection parts of the wood have turned black and I am concerned regarding damp.

    I’ve installed two dehumidifiers in the hall as this seems to be where the issue is occurring unlike the rest of the apartment. There are no windows in the hall way and it does lead onto the bathroom.

    My contractor has assured me you traditionally glue solid hard wood to the concrete directly and the rest of the flat seems fine though the other rooms are better ventilated.

    Should we be using a membrane or membrane solution on the concrete before installing the hard wood floor, even if only in the hallway? I’ve had this replaced once already and can’t afford to keep doing so every 2 months on a floor supposedly guarenteed for 20+ years.

    Thanks
    Adam

  • Hi Adam,

    Firstly, there should be a degree of checks carried out to try and ascertain what the actually problem is and the root cause. Simply applying a liquid dpm may not help if the issue is perhaps a leak or excessive humidity.

    Can you tell me the age of the property and what level you are on?

    Regards,

    Wes.

  • Hi Adam,

    With the property being so high up, bar a leak, the issue is unlikely to be moisture from the sub-floor. The black substance looks like loose bitumen residue/adhesive that has pulled up as the floor has expanded/contracted. The residue should have been ground (At least 80% to 90%) to remove any loose residue as well as any other contaminants that may react with the adhesive.

    Regards,

    Wes.

  • Hi Adam,

    Your best bet would be to purchase a resistance moisture meter. With this you could measure to moisture content of the timber (Top & bottom). This would give you an indication.

    I assume the warming isn’t heating pipes??

    If there is moisture in the slab, as it is so far up and 25 years give or take old, I’d say the only possible reason for moisture would be a leak of some kind. I just can’t think of anything else it could possible be. Leaks can be very hard to trace in apartments by the way. Check around soil/waste pipe boxing. A leak may not be coming from your apartment either. It could be from your upstairs neighbours or even the neighbours above them or the roof. Concrete acts like blotting paper and the source of moisture entry can often be a distance away from the immediate expected area.

    Is the timber out of shape, warping in any way? If there is moisture coming from below, you will often find the underside will swell whilst the top side stays dryer. This would result in what we call ‘cupping’ (Google ‘wood floor cupping’ then hit images)

    Regards,

    Wes.

  • Adam Joseph Carolan

    Hi Wes,

    Certain boards have shown signs of cupping.
    Just to confirm (as I’m looking at having the work done this week), would it be worth me buying a membrane substance for the floor to put down under the wood?

    I have the exact same flooring down in the living room and as yet haven’t had any issues.

    Thanks
    Adam

  • Hi Adam,

    I just can’t see how an 18 year old concrete floor on the 6th floor would still be wet. As you mention this is an isolated area, this strengthens my thoughts that there is water getting to the concrete somehow. I can actually see in one of your pictures the cupping, which definitely suggests moisture from the concrete.

    You could put a liquid dpm down, but this may not fully help the issue. If you would like to go down this route regardless, you’ll need to remove (as much as possible) the bitumen residue via grinding – using safety equipment i.e. Extraction, full respirator mask etc. This will remove any loose residue and any other contaminants prior to applying the dpm. Do follow the manufacturers guidelines carefully.

    Regards,

    Wes.

  • Hi Vin,

    I’d certainly not advise you to try and speed the drying process up. Doing so can cause further issues i.e. debonding, cracking, and a general loss in structural integrity of the screed.

    The best course of action would be to install a liquid dpm. Typically, and in normal circumstances, this can be carried out at around the 30 day mark (With readings below 98% RH). Some products like F-Ball F77 can be applied after 7 days of the slab being installed (According to F-Ball). However, we’d certainly want to wait.

    I take it we’re talking about a bonded screed, at that thickness? Has that been installed over an old concrete slab? How old is the property?

    Regards,

    Wes.

  • Vinny

    Hi Wes,
    Much appreciated for your guidance.
    The screed is just a levelling screed, which is circa 30mm thick and has been laid over the existing concrete slab of the property (around 150mm thick). The property age is circa 1970. I will liaise with F-Ball to see what they are able to offer (thank you for this). If we choose to apply the liquid membrane, will the moisture be trapped below the DPM and have no where to go?
    kind regards,
    Vin

  • Hi Phil,

    Always the overwhelming best course of action will be to remove the existing slab and start again using modern techniques. Although the current slab may well be in good condition, it is still around 80 years old. Certain things can obviously improve with age, however, in this case and baring in mind we’re talking about a ‘new’ extension, I’m firmly in agreement with your builder.

    I will however, go through the processes for you of both scenarios…For the purposes of this reply, I am going to assume you will be gluing the wood floor to the substrate.

    ‘Removing the original slab and replacing with new’

    When a new concrete slab is installed, there are two major factors to consider prior to the installation of a wood floor.

    Firstly, the quality of the concrete paste and installation i.e. Good mix and correct curing of the concrete. Assuming you have a good builder, this shouldn’t be a problem.

    Secondly and most common issue, the moisture content of the concrete. It can take new concrete a long time to dry out prior to it being in an adequate state for the adhering of a wood floor. Typically, six month and often upwards. Do not underestimate this comment. If such time is not convenient, a product like Ardex DPM 1C can be applied after around thirty days, subject to acceptable hygrometer readings and removal via grinding of any laitance. Therefore, you would be utilizing the same technique and expense (Plus new concrete and installation) as if you were to leave the current slab in place and remove the bitumen.

    ‘Leaving the original slab in place and using the Ardex DPM system’

    This approach would require the removal of bitumen via grinding. This would also prepare the concrete by removing any laitance, providing the concrete is in good condition. Then once prepared correctly, a system like Ardex DPM 1C or similar could be applied providing there is no hydrostatic pressure and as before hygrometer readings fall within tolerance (98% RH or less). Then an adequate smoothing compound, to encapsulate the DPM and rectify any final levelling issues, would be applied.

    Comparing the cost and upheaval between approaches is something that’s hard to estimate. The best and surely obvious way to approach this would be to calculate the cost of new concrete, preparation, supply and application of a system like Ardex against solely the preparation of your existing slab and Ardex system etc. Initially you’d think the latter to be the less costing of both approaches, but this would clearly depend on the quality, condition, and levellness of the existing slab.

    I hope that’s given you, at the least, a clearer view of the potential options available to you.

    Regards,

    Wes.

  • Phil

    Thanks for the Reply Wes,
    Yes I was planning on gluing the wooden floor down but I am open to suggestions on other methods or alternative materials to timber that may be more suitable for my scenario. I am really not in a position to remove the existing 1930’s floor construction as my family will have no where to go while it is done. I have read that it may be possible to use Ardex AP directly on-top of the bitumen followed by a Ardex DPM 1C & then a final layer of Ardex AP. What are your views on this method depending we are not above the 98% RH? This would save all the breakout & excavation works as well as the grinding process but am unsure of whether it would work. Would re applying some bitumen over the damaged areas be an option followed by some Ardex AP?

    I am now thinking it maybe better to use a alternative material to timber like ceramic tiles which will have a better tolerance to the moisture.

    In the meantime I have a damp proofing contractor to give me some advice & to undertake a survey of the issue.

    Regards
    Phil

  • Hi Phil,

    I can’t say I’ve heard of ‘Ardex AP’, so honestly don’t know what it is. I can’t find it on the Ardex UK website, perhaps it’s a US product? Could you post a link or more information?

    Your main issue regards the bitumen, is if any of it is loose/weak (Not always obvious, and very likely the majority of it will be loose/weak). If any layer in the preparation process fails, then the floor will lift/debond. When a wood floor is glued down, as it expands/contracts, shear forces will pull on the substrate, therefore, it is absolutely critical the bitumen is ground. The concrete doesn’t necessarily have to be completely free of bitumen, but as much as possible should be removed as to avoid the potential for failure.

    I’d be very interested in what the damp proofing contractor suggests. It’s always good to hear other trades current approaches.

    Regards,

    Wes.

  • Phil

    Hello again Wes,

    I’m sorry I meant to put Ardex NA rather than AP, I have no idea where I got AP from.

    The damp man gave me a few different options including removal of complete slab & removal of screed. He did however give me two options where the existing would be left in place.

    1. Asphalt with a thin layer over the bitumen which then a timber floor could be laid directly on top of. He did however say this would be expensive.

    2. Use a good quality damp proof membrane laid over the bitumen followed by a layer of 18mm chipboard followed by the timber floor.

    What are your thoughts on those two methods?

    Thanks

    Phil

  • Hi Phil,

    The use of Ardex NA is a slightly grey area in your particular circumstances. Many in the trade feel it is to soft to have a wood floor adhered to (at 16 N/mm compressive strength). Many prefer a compound with a higher compressive strength at around the 30 N/mm range. However, after raising this with Ardex themselves, each of the technical representatives I’ve raised this with has suggested that NA is fine to adhere a wood floor too. It may well be worth calling ‘Ardex Technical’ yourself to clarify this. As a result of the murmurings, I tend to veer in the direction of the higher compressive strength. Without wanting to sound contradicting, I am yet to see a wood floor fail due to the use of correctly applied Ardex NA, both first hand or online.

    I can fully see why NA would be recommended to you in a DPM sandwich scenario (NA > DPM > NA), noting that NA can both be installed beneath a DPM as well as go over bitumen residue. However, with the latter point, do consider the shear forces I mentioned in my last post. Any loose bitumen (simply test this with a knife – pick at the bitumen), will potentially lead to failure if not removed.

    In reply to the numbered statements :-

    1. Ashpalt, although a great material, should not be used when adhering a wood floor. Again, the shear forces involved when a wood floor expands/contracts can – and have on many occasions I must STRESS – pull the Ashpalt up, resulting in a complete mess and failure. Baring in mind Ashpalt is not a bonded product.

    2. I personally would not use this option. Chipboard is a terrible material, prone to creaking and due to the treatments/oils in many chipboard products, is not normally advised to adhere too. You may also find issues with the floor sounding like a floating floor i.e. clacky/hollow etc as the chipboard would essentially be ‘floating’.

    Apologies for being such a pessimist, it’s just we tend to have very firm opinions in this field, as the monetary cost of failures is often high not to mention the upheaval.

    I’m not saying the methods above wouldn’t work, I’ve seen diy’ers and pro installers get away with all sorts. To add balance to that, I’ve seen some dramatic failures as well. All I can do, is try and limit the potential for your floor failing by giving the best advice I can. I believe both numbered statements above to be a high value gamble.

    Personally, I would prefer to see the bitumen ground > DPM 1C > P82 Primer (Belt and braces) > Something like Ardex K39 or Ardex K15 (As a side note, these two products are water based and far easier to use than a latex such as NA for a diy’er in my opinion).

    Regards,

    Wes.

  • Phil

    Hi Wes,

    Do not apologise for being pessimistic as I’m really appreciating you’re time & advice on this issue of mine.

    I’m starting to think I may be better going for an alternative material to timber/engineered timber as this preparation will undoubtedly eat into my budget that I had for the floor. Do I assume correctly that a laminate or vinyl floor covering will not have the shear forces that will be present in the timber floor? This would then allow me to put a damp proof membrane down before adding some marine ply or similar followed by the vinyl/laminate flooring.

    This is not really what I wanted to do but unless I spend a lot of money & go through a lot of upheaval for myself & my family I am sort of stuck.

    Regards

    Phil

  • Hi Phil,

    I do understand this putting you off somewhat.

    You are correct in relation to the shear forces being minimal with a vinyl product, although, you would no doubt still be advised by the manufacturers to remove most of the bitumen. Of course, if we are talking about a bonded system and not a vinyl click/loc.

    As with the laminate, this would be a floating floor, therefore, shear forces would not apply.

    I do have a suggestion for you. There is a relatively new product/supplier (well, new to me) in the UK called ‘softlay’ (http://www.softlay.co.uk/). This company provides underlays that can go directly over concrete substrates with RH readings of 98% or less. In essence, an underlay with real DPM properties as opposed to the standard foil backed underlays on the market that only protect up to 75% RH or less. They also do a vinyl bonded underlay system. It may well be worth contacting ‘David Jenkins Tel 01245 929122’ to discuss the options available to you.

    I just can’t see the need in a ply or chipboard layer. I do understand that if you put the flooring directly over a 1000 to 1200 gauge DPM sheet you may well get issues with noise/rustling due to the thickness of the DPM sheet, but if you’re going to veer towards the floating route, I’d personally get rid of the ply/chipboard layer. Unless, of course, you need it to raise the height of the sub-floor. Otherwise, it’s just an unnecessary expense.

    Glad my murmurings are helping in some way Phil.

    Regards,

    Wes.

  • Stephen Lyon

    Hi
    We have quarry tiles in our kitchen and unsure wether just to plywood over them or to sceen over? No doubt that there no DPM underneath however moisture level is between 5% – 18% on the low side but if we cover them this could cause the levels to triple hence the reason over asking.
    Thanks in advance

  • Hi Stephen,

    Applying a moisture tolerant smoothing compound over them would by far be the best method in my opinion.

    What floor covering are you installing?

    Regards,

    Wes.

  • Yourport.com

    Hi, Can I ask a question regarding a new engineered wood floor I am having installed on top of a new concrete floor. The construction of the concrete is (from bottom up) Hardcore, Insulation slabs, Damp proof sheet, then 75mm concrete. My question is this. Can I simply glue/mechanically fix the subfloor (osb3 18mm) onto the concrete was there should be no moisture in the concrete as it already has a damp proof membrane below it? The concrete was poured about 3 months ago. Also can you recommend a suitable glue and screws that is suitable for the job? Regards,

    Dean

  • Hi Dean,

    Can I ask why you wish to fix OSB to the concrete? I fear if you start drilling a huge amount of holes in a three inch thick screed, you may create fault lines/cracks (from hole to hole – especially as you insert the screws and the raw plugs expand/tighten) and potentially loose the integrity of the screed. Just a thought..

    Providing the concrete is dry (with respect ‘checked’, rather than assumed), you could adhere directly to the concrete.

    Regards,

    Wes.

  • David Seaton

    Hi Wes,
    My 100yr old ground floor parquet in the hallway had numerous loose blocks in a 2m square patch at
    the base of the stairs, so I lifted them, hacked out the loose concrete down to
    where it became firm enough, PVAd it, poured water-based self levelling compound
    from Wickes which varies from 2mm to 2cm thick, cleaned the bitumen off the
    blocks, stuck the blocks down with Lecol adhesive onto the new compound, sanded the entire floor,
    painted with water based varnish. Now 18months later the blocks have all popped
    up as the compound has domed up bang in the middle of the patch. It has literally created a cavity under the compound. From what ive
    read elsewhere my feeling is that the self levelling compound has taken on
    moisture and expanded and domed. It is nicely solid and joined at the edge
    still where it abuts the original concrete. Shouldn’t the moisture have been pushed
    aside underneath the new SL compound patch and out via the rest of the floor
    that is original and obviously able to breath sufficiently?. Does my guess sound
    likely? I’ve only read one article online about water-based SL compound not
    liking high humidity at all but I’m assuming that is what has happened to mine.
    I want to fix the patch again but don’t know what to put down this time, should
    latex based compound by fine or is there a breathable alternative. Any advice …
    Unfortunately in my front room ive also laid SL compound to fix half the knackered sub-base floor and was about to start sticking down blocks, then I noticed the hallway blocks all lifting so stopped before I started sticking.

  • Hi David,

    There are some water based SLC’s that are moisture tolerant, but most aren’t. As is likely the case with yours. Any moisture will wick through the compound and blow it. PVA is not an adequate primer either.

    As your property is 100 years old, it’s likely it doesn’t have a DPM beneath the concrete. Ideally, you’d want to use a moisture tolerant compound (Typically latex based), then apply a liquid DPM, then adhere the wood blocks. Following all manufacturers installation guidelines, often found via product data sheets online. To point you in the right direction, look at Ardex or F-ball products.

    I hope that’s of some help, and apologies for the delay in getting back to you.

    Regards,

    Wes.