Sub-Floor Preparation

A floor contractor installing a levelling compound

In this page we will look at different combinations between sub-floors and wood based floor coverings and how the sub-floor should be prepared to accommodate the floor covering. This is general advice. You should always consult the manufacturers installation and floor preparation literature prior to installing a wood floor. Incorporated are links within the information that will help familiarise you with the different products available for best long lasting results and good practises. There are also links to our glossary page to describe particular words and terms that are not often heard in day to day use. Please contact us should you have any questions regarding the information in this page.

Click on the required topic that suites your individual project and you will be taken to the relevant section. We do advice all visitors to read the general sub-floor preparation section as this compliments all the other sections.

General sub-floor preparation :

When fitting any wood based floor there are always several main aspects to assess and consider in relation to the sub-floor.

  1. Even. If a sub-floor is slopping, this isn’t necessarily a problem to the life of a wood floor. Although, to the eye this may be unappealing. If a sub-floor is uneven, this can have a very bad effect to the life and feel of any wood based floor. The way we commonly refer to the evenness of a sub-floor is the difference of height over a short distance i.e. If there is a difference of more than 2 mm over a 1.5 metre span, we would class this as uneven. A simple test with a straight edge (preferably a minimum 1.5 metre length of wood or spirit level), is to place it on the sub-floor. If the straight edge see-saws/rocks you’ve found a high point. If the straight edge stays still but a measurable gap is visible in the middle of the straight edge, you’ve found a low point. With a solid floor or wood sub-floor, a self levelling compound can be used to rectify such problems. You should always follow the advice of the self levelling manufacture prior to installing a compound. The overall evenness of the sub-floor is a key consideration and should be taken seriously.
  2. Warped moisture damaged floor from rising moisture damageMoisture. The moisture content of the sub-floor is a huge consideration. The relative moisture content should not exceed 75%. A direct moisture content (MC) should be ideally no greater than 4%. Please do not mistake these two measurements as they are very different. Any undue moisture will cause failure to your floor. There are several ways to test for moisture. On a solid sub-floor several 0.5 square meter polythene sheets can be taped to the sub-floor in different areas and left for a minimum of 72 hours. If any condensation or dark patches appear under the sheets in that time, we would consider that a clear indication the sub-floor is retaining moisture. Alternatively a digital moisture tester can be used. Be careful that the readings you get are not salt. A moisture meter generally tests the electrical conductivity of the surface. Salt on the surface can sometimes show the same readings as moisture. If you have any signs of moisture, these would need rectifying prior to installing a wood based floor covering. The only real way to know if a sub-floor doesn’t contain excess moisture, is by using the British standards surface British standards compliant for testing relative humidity be expelled from a concrete sub-floor.relative humidity box method. The humidity box would be placed on to the relevant surface and left for 72 hours in preparation for meter reading to be taken. Sub-floors that are of timber construction (wood floorboards, ply board) can also have moisture problems. These should also be tested. A digital moisture meter is the only way to gain an accurate assessment. Again, any signs of moisture should be investigated further. Blocked air bricks, poor circulation under the floorboards or failure of the damp protection course (DPC) can all be factors to look at.
  3. Sound, firm, dust, dirt, grease and grime. When fitting a solid or engineered type wood floor directly to a solid surface with adhesive, the surface should be structurally sound. This means not flaking, cracking or giving (movement). A close inspection and assessment should be made of the condition. The floor should be cleaned thoroughly prior to fitting any adhesive to it. Adhesive will only stick to it’s immediate surface. If the surface is dusty, dirty, greasy or grimey, there will be no solid bond. This is a huge factor in a long lasting wood floor installation.

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Laminate floor floated over a solid sub-floor (inc concrete, ashpalt/bitumen, tiles)

  1. Installer fitting a laminate floorRun through all the checks in the general sub-floor preparation section.
  2. An underlay should always be used. These can differ from 2 mm thick up to 7 mm thick. An underlay will help absorb sound and take out any slight unevenness.
  3. On ground floors, a moisture barrier should always be used. Often these can be purchased already built into the underlay supplied by retailer. This is an important factor as over time undetected moisture from a solid sub-floor can works it’s way to the surface. A moisture barrier will prevent this effecting your laminate floor and give it longevity as well as validate your flooring guarantee.

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Laminate floor floated over wood sub-floor (inc floorboards, ply board, hardboard and chipboard)

  1. Run through all the checks in the general sub-floor preparation section.
  2. Cupping. When it comes to fitting a wood floor on a wood sub-floor you should consider the condition of the floor boards. They can tell you a lot about the surrounding environment. If the floorboards are cupping, as illustrated, this would suggest the atmosphere above the floorboards is dry or has been dry. In other words the air humidity is or has been low and the floorboards have dried out. Think of a dried out leaf, and the way it curls up at the edges. Cupping can also suggest that the underside of the floorboards have become wet, resulting in the boards swelling and then drying out quicker on the surface. If the conditions that caused this are still there, this same effect could happen again, only this time on your nice new floor. If the conditions that caused this have been rectified then all you need to address is the cupping itself. If the laminate is to be laid at a 90 degree angle to the direction of the lengths of the floorboards, providing the cupping isn’t too severe, the laminate can be laid. If the laminate is to be laid in the same direction as the lengths of the floorboards, this can have drastic effects on the feel and longevity of the new floor covering. As the uneven surface will allow the floor to rock when walked on resulting in locking system abrasion. Cupping can be rectified by punching the floorboard nails a few millimetres below the surface of the boards, then simply by running across the boards with an electric plane or course papered sanding machine, the high points can be taken down. Where the edges of the floorboards my be difficult to get to with a plane, a smaller corner sander could be used. An alternative to this would be to either remove the floorboards and attach new boards or 18 mm WBP ply board to the joists. Alternatively a layer of 12 or 18 mm WBP ply board (we recommend 18 mm ply in most situations) could also be laid directly on to the cupped floor boards but considerations for the rise in floor height should be made. Again, to stress, if the original conditions that caused the cupping have not been resolved, all of the above tasks could be a waste of time and money.
  3. Crowning. If the opposite of the above has occurred we refer to this as crowning. A result of humid air on the surface and dryer air on the underside of the wood. This should be treated in the exact same manner as above. In both cases a relative humidity tester should be purchased to monitor the air humidity over a period of time to give a solid idea of the room climate and to gain a better understanding as to the cause for the cupping or crowning. Please read the general sub-floor guidelines for clarification.
  4. Should the sub-floor consist of ply board, hardboard or chipboard, a simple levels, moisture and firmness check should be carried out. A latex based fibre re-enforced self levelling compound (SLC) can be applied. Always follow manufacturers guidelines on how to apply a SLC. High points can be plained or sanded. Generally these kind a surfaces are ideal for laying an floating floor.

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Engineered floor floated over a solid sub-floor (inc concrete, ashpalt/bitumen, tiles)

  1. Engineered wood floor installed on a underlayRun through all the checks in the general sub-floor preparation section.
  2. An underlay should always be used. These can differ from 2 mm thick up to 7 mm thick. An underlay will help absorb sound and take out any slight unevenness. Please note, that underlay will not remove or correct large undulations in any sub-floor. These undulations should be rectified prior to installation.
  3. On ground floors, a moisture barrier should always be used. Often these can be purchased already built into the underlay supplied by the retailer of your floor covering. This is an important factor as over time undetected moisture from a solid sub-floor can works it’s way to the surface. A moisture barrier will prevent this effecting your engineered floor and give it longevity as well as validate your flooring guarantee.

Top ^           Glossary

Engineered floor floated over wood sub-floor (inc floorboards, ply board, hardboard and chipboard)

  1. Run through all the checks in the general sub-floor preparation section.
  2. Please refer to ‘Laminate floor floated over wood sub-floor‘. The same values apply.

Engineered floor directly glued to a solid sub-floor (inc concrete, ashpalt/bitumen, tiles)

  1. Installing an engineered glue methodRun through all the checks in the general sub-floor preparation section.
  2. The process of gluing any wood floor to a solid sub-floor should be considered carefully. Engineered wood is an asset to any home. Once glued to the sub-floor, it can be a sound and gorgeous looking floor for life. However, moisture from a solid sub-floor can be extremely damaging.
  3. As well as the general sub-floor preparation guide, it is often advised to use a liquid DPM and then a self levelling compound over the top. Once dry, this gives an excellent surface for wood floor adhesive to bond. This is not always strictly required but should you have any doubt, we would strongly suggest you to utilise these precautions to give you a long lasting installation. Where possible, it is always recommended to use a complete system of liquid DPM, levelling compound and adhesive from the same manufacturer (for instance Ardex). This is to ensure that no unwanted chemical reactions occur at any point between the applied layers. If a full system is not available to you, we would always advise you to see technical advice that is available from most manufacturers.

Top ^           Glossary

Engineered floor directly glued to a wood sub-floor (inc floorboards, ply board, hardboard and chipboard)

  1. Run through all the checks in the general sub-floor preparation section.
  2. When gluing to a wood sub-floor you should assess if the floorboards or ply board have been treated, varnished, lacquered, painted or generally had additives applied. It helps the adhesive to fuse if the wood material being adhered to is raw and unsealed. This isn’t always an absolute requirement but does help greatly. If you do decide to apply the adhesive to a pre-finished wood sub-floor, make sure the surface material has a perfect bond. Any flaky material should be removed. If there is flaky material, I would suggest you question the surface as a whole. If the task of removal is to great, at the bare minimum the surface of the sub-floor should be keyed in some way i.e. Using a grinder or course sander.
  3. Hardboard can be used to glue an engineered wood floor to but is not the best sub-floor type. Ply board or wood floor boards are much better sub-floor to consider.
  4. Chipboard sub-floors can be glued to. However, often excessive vertical movement with chipboard sub-floors can cause problems. If the chipboard is springy/bouncy it’s worth considering strengthening this up with a layer of 12 mm thick ply board or replacing the chipboard altogether. Chipboard flooring can contain inhibitors incorporated in the manufacturing process which can prevent correct adhesion of a wood floor adhesive. The presence of inhibitors isn’t easy to deduce. Therefore, think carefully prior to adhering to a chipboard floor. If in doubt, install a minimum 9 mm layer of structural plyboard to the chipboard and adhere to that.

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Engineered floor directly nailed to a solid sub-floor (inc concrete, ashpalt/bitumen, tiles)

  1. Do not consider this!

Engineered floor directly nailed to a wood sub-floor (inc floorboards, ply board, hardboard and chipboard)

  1. A wood floor being secret nailed to a wood sub-floorRun through all the checks in the general sub-floor preparation section.
  2. Please refer to ‘Laminate floor floated over wood sub-floor (inc floorboards, ply board, hardboard and chipboard)‘. That section covers some good information on cupping and crowning and the concept applies to engineered wood being nailed to a wood sub-floor. Please note, not all engineered flooring can be installed with nails.
  3. An engineered floor should not be fitted in the same direction as floorboards if it is to be nailed. Only fit 90 degree’s to the floor boards or install a minimum of 9 mm WBP ply board as a levelling layer – thickness dependent on condition of the floorboards. You can then run the engineered wood flooring in your chosen direction.
  4. If the sub-floor is ply board, nailing the engineered floor is regarded as good practice. That is unless, it is specifically manufacturers as a floating engineered floor! Always be mindful of heating pipes and electrical cables that may run close to the underside of any wood sub-floor you’re nailing into.
  5. Hardboard alone can not be used as a sub-floor to nail into, especially and obviously, if fitted on top of a solid floor. If the hardboard is fitted onto floorboards or chipboard, nailing would be ok. I say ok with slight reservation. It’s always best to nail directly to the original sub-floor. Sandwiching a layer between is not ideal (unless the layer is ply board) as the fixing cleats (nails) will not be used to there maximum. Any slight compression of the middle layer can create a loosening of the cleats, so any compression type underlayment should be avoided.
  6. Chipboard composite flooring should NOT be nailed into as a fixing for an engineered floor. The composite is not stable enough to hold the cleats. A ply board layer of a minimum 12 mm should be installed on top of the chipboard or the chipboard should be removed altogether and ply board installed directly the timber joists.

Top ^           Glossary

Solid wood floor directly glued to a solid sub-floor (inc concrete, ashpalt/bitumen, tiles)

  1. A picture showing adhesive being applied to a solid sub-floorRun through all the checks in the general sub-floor preparation section.
  2. The process of gluing any floor to a solid sub-floor should be considered carefully. Solid wood is an asset to any home. Once glued to the sub-floor, it can be a sound and gorgeous looking floor for life. However, moisture from s solid sub-floor can be extremely damaging.
  3. As well as the general sub-floor preparation guide, it is often advised to use a liquid DPM and then a self levelling compound over the top. Once dry, this gives an excellent surface for wood floor adhesive to bond. This is not always strictly required but should you have any doubt, we would strongly suggest you to utilise these precautions to give you a long lasting installation. Where possible, it is always recommended to use a complete system of liquid DPM, levelling compound and adhesive from the same manufacturer (for instance Ardex). This is to ensure that no unwanted chemical reactions occur at any point between the applied layers. If a full system is not available to you, we would always advise you to see technical advice that is available from most manufacturers.
  4. It is imperative that you consult the adhesive manufacturers literature prior to adhering to a solid sub-floor. Some adhesives are not suitable to be used on solid sub-floors like ashpalt/bitumen due to the lack of porosity. A self levelling compound layer is often advised.
  5. It is always advised to remove ceramic sub-floors. Should this be difficult, an adequate self levelling compound should be applied. Always follow the SLC manufacturers guidelines on priming/installing on a tile sub-floor.
  6. Ashpalt tiles should not be adhered to. When the solid wood floor expands, shear forces are applied. This can result in the tiles becoming loose, resulting in de-bond and the structural integrity of the floor becoming compromised. With the result of bucking/lifting/plank separation.

Top ^           Glossary

Solid wood floor directly glued to a wood sub-floor (inc floorboards, ply board, hardboard and chipboard)

  1. A solid wood floor being glued to a sub-floorRun through all the checks in the general sub-floor preparation section. That section has some good information on cupping and crowning and the concept applies to solid wood flooring.
  2. When gluing to a wood sub-floor you should assess if the floorboards or ply board has been treated, varnished, lacquered, painted or generally had additives applied. It helps the adhesive to fuse if the wood material being adhered to is raw and unsealed. This isn’t always an absolute requirement but does help greatly. If you do decide to apply the adhesive to a pre-finished wood sub-floor, make sure the surface material has a perfect bond. Any flaky material should be removed. If there is flaky material, we would suggest you question the pre-finish as a whole. If the task of removal is to great, at the bare minimum the surface of the sub-floor should be keyed in some way i.e. Using a grinder or course sander.
  3. Hardboard can be used to glue a solid wood floor to but is not the best sub-floor type. Ply board or wood floor boards are a much better sub-floor to consider.
  4. Chipboard sub-floors can be glued to. However, often excessive vertical movement with chipboard sub-floors can cause problems. If the chipboard is springy/bouncy it’s worth considering strengthening this up with a layer of 12 mm thick ply board or replacing the chipboard altogether. Chipboard flooring can contain inhibitors incorporated in the manufacturing process which can prevent correct adhesion of a wood floor adhesive. The presence of inhibitors isn’t easy to deduce. Therefore, think carefully prior to adhering to a chipboard floor. If in doubt, install a minimum 9 mm layer of structural plyboard to the chipboard and adhere to that.

Top ^           Glossary

Solid wood floor directly nailed to a solid sub-floor (inc concrete, ashpalt/bitumen, tiles)

  1. Do not consider this!

Solid wood floor directly nailed to a wood sub-floor (inc floorboards, ply board, hardboard and chipboard)

  1. A contractor nailing down a wood sub-floorRun through all the checks in the general sub-floor preparation section.
  2. Please refer to ‘Laminate floor floated over wood sub-floor (inc floorboards, ply board, hardboard and chipboard)‘. That section covers some good information on cupping and crowning and the concept applies to solid wood flooring being nailed to a wood sub-floor.
  3. If the sub-floor is ply board, nailing the solid floor is regarded as good practice. Always be mindful of heating pipes and electrical cables that may run close to the underside of any wood sub-floor you nail into. The correct length cleats should be used. Generally 38 mm for overlay installations or 50 mm for nailing directly to joists.
  4. A solid wood floor should not be fitted in the same direction as floorboards if it is to be nailed. Only fit 90 degree’s to the floor boards or install a minimum of 12 mm WBP ply board as a levelling layer. Ideally 18 mm WBP ply. You can then run the solid wood flooring in your chosen direction.
  5. Hardboard alone can not be used as a sub-floor to nail into, especially and obviously, if fitted on top of a solid floor. If the hardboard is fitted onto floorboards, nailing would be ok. I say ok with slight reservation. It’s always best to nail directly to the original sub-floor. Sandwiching a layer between is not ideal (unless the layer is ply board) as the fixing cleats (nails) will not be used to there maximum. Any slight compression of the middle layer can create a loosening of the cleats, so any compression type underlayment should be avoided.
  6. Chipboard composite flooring should not be nailed into as a fixing for a solid wood floor. The composite is not stable enough to hold the cleats.

Top ^           Glossary

 

© 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, Sub-Floor preparation Tagged with: , ,
  • Graham Maughan

    Hello Wes,

    Thank you for an excellent summary. I’d appreciate your thoughts on the following problem:

    Flooring across two subfloors. A hallway that starts in conventional timber floorboards but due to a rear extension becomes screed with wet ufh. The screed with ufh will move in a very different way to the timber.

    1. Ideally we would like to lay engineered timber across both but are not sure if it will cope with the temperature changes and movement across the join.

    2. One other option is amtico.

    3. Another, safer but less attractive option is to have two different flooring materials one for each section.

    Have you come across this problem before?

    I’d be very grateful for your thoughts.

    Best wishes

    Graham

  • Hi Graham,

    I have come across this before and ran like the clappers. Although, the projects I looked at were a lot bigger than a hallway so I had a larger co-efficient (expansion ratio in relation to the L/W spans of a room) to think about.

    Could you perhaps give me some further details? The Width/Length of hallway and the size of the UFH area etc? Even a picture may be of some help..

    Regards,

    Wes.

  • Graham Maughan

    Hello Wes,

    Many thanks for the reply. I’ve tried uploading a couple of pix but they wouldn’t upload (only 600kb). I’ll measure tomorrow and get back to you then. All the best, Graham

  • Hi Graham,

    You could always send them direct if that works better – contact@fitmywoodfloor.co.uk .

    Regards,

    Wes.

  • Hi John,

    My apologies for the delay in getting back to you!

    As your property is pre 1960’s, the concrete should be checked to confirm it has an appropriate mechanical/sheet dpm beneath the concrete slab. If not, an appropriate liquid dpm, should be used (Or of course, remove the slab and start again – most people don’t wish to go that far).

    It is perfectly fine for the bricks below the DPC to be damp. That is what they were designed for. The bricks below the DPC will often stay wet/damp (take on board and release moisture) throughout their life. In a situation such as yours, the compound above a liquid DPM (applied to the slab ) should have been dammed up to prevent contact (Both thermal and moisture bridging) with both the bricks beneath the DPC and the original concrete slab beneath the DPM.

    Regards,

    Wes.