Procedures and Provisions in the Use of Underlayments

Prior to application of a smoothing underlayment, it is important that the subfloor is adequately prepared and should be carried out using suitable mechanised preparation equipment. The following notes offer guidance on the procedures and provisions in the use of underlayments.

1. All plaster and paint contamination should be completely removed along with any irregular residues including any barriers to adhesion, builders’ debris, some adhesive residues and any applied layers that may not be compatible with the new floor smoothing underlayment. Areas contaminated with oil or grease may be treated with degreasing agents. However, if they have penetrated the surface, the screed/concrete must be cut back to good material, or the use of an oil resistant primer may be suitable. Consult the manufacturer for further guidance.
2. Any deep holes from removal of partitions or walls may need to be filled first taking into account there may be an issue if not protected from ground bearing moisture, or using a moisture tolerant repair mortar should be considered. Adequate damp proofing may need to be included in the preparation. See sections 5–6 of this guide.
3. Use of mechanised surface preparation equipment, which is effective in removing adhesive residues/construction surface contamination (if necessary) from concrete or screed surfaces. Techniques include mechanical stripping, planning, grinding and contained shot blasting.
4. The choice of underlayment or any floor preparation product should take into account the type of floor finish, type of subfloor and its condition (presence of existing floor smoothing underlayments adhesive residues, etc.) as well as its intended use and the working environment. Typical products are water mixes (powder polymer), two-component products (bag and bottle) and mortars. Manufacturer’s guidance should be sought for specification and correct application methods.
5. The strength required of an underlayment will be a key factor. Where there will be heavy point-loading or hard wheel trolleys, then a stronger material is required. In large areas of concrete there may be movement joints or crack inducement cuts in the base. Under no circumstances should floor smoothing underlayments be applied over these movement joints. The joints should be filled using suitable compressible filler and a proprietary joint should be inserted and the smoothing underlayment, floor covering or any applied layer terminated either side of the joint. Some joints such as crack inducement, saw-cuts or day joints, along with other cracks that are considered by a structural engineer to no longer be subject to movement or shrinkage, may be cleaned out and stitched or filled using a suitable mortar or epoxy resin and covered over with an underlayment and floor covering.
6. Asphalt should be flooring grade and comply with the requirements of BS 8204 and be correct grade for its service condition. Providing the flooring grade asphalt is in good condition, continuous, free from cracks, sound, strong, has not suffered rutting or any sign of softening, and is free from any form of contamination, the surface should be degreased with an appropriate cleaner and rinsed with clean water and allowed to completely dry. The surface, where required, should be primed prior to the application of the floor smoothing underlayment and generally, any thicknesses above 6mm should be avoided, unless advised by the underlayment manufacturer.
7. Wood and composite block floors (Granwood), when installed on a ground bearing subfloor can suffer extensive dimensional change if covered over with a resilient or impermeable floor covering. Under no circumstances should any cementitious smoothing underlayment be applied, as it will eventually break up. It is considered best practice to remove composite block floors, however some manufacturers may give recommendations to apply underlayments on non-ground bearing subfloors. The application of a plywood overlay on composite block floors, while providing a smooth even surface, can additionally result in the blocks being affected by dry rot, and/or delamination of the blocks by the forces exerted from fixing, and is not considered good practise. In most cases, removal of the wood and composite block flooring is advised, and the exposed subfloor should be suitably prepared for the new flooring. Pitch adhesive residues if sound and non-water softenable, can be overlaid with a compatible smoothing compound. Consult the underlayment manufacturer for further guidance.
8. All wooden floors must be structurally sound and able to support the imposed loads – level, smooth, dry and clean. Adequate ventilation should be provided to suspended timber floors at ground level to ensure that the moisture content of the wood is maintained at equilibrium. Worn or uneven floorboards should either be replaced or levelled by sanding, planing or by patch filling with a suitable floor smoothing underlayment or repair mortar before finally covering with a minimum 6mm flooring grade plywood. Sheets should be positioned, with joints staggered and secured at 100–150mm centres (see BS 8203 for details). Fibre reinforced water-based smoothing compounds, some bag and bottle products and feather finish type materials are available which can be applied to wood and wood-based panels after preparation, as above, ensuring that any holes or gaps are filled to avoid the mortar leaking away. These products should be compatible with the background that they are being applied to and have sufficient deformability to cope with any anticipated movement of wood and timber-based panels. Priming before their application is normally recommended. Always consult the underlayment manufacturer for advice and recommendations.
9. When existing floor coverings have been previously removed and a cementitious underlayment had been used under the original adhesive, then ideally the old underlayment should be removed as well as the floor covering. In this situation, a build-up of adhesive and underlayment layers can result in poor levels being achieved, the soundness of the base would be questionable and there will be a potential for indentation from point loading.
   
   NOTE: Having removed an existing floor covering, if it is found that adhesive contamination is not present, then the application of additional layers of underlayment is possible. However, priming will, in all eventuality, be required to avoid pinholing and suction. That said, it is very important to understand what the existing underlayment is and what its hardness or compressive strength value is. It is generally considered not good practice to apply a fresh layer of new underlayment to an existing material that has a higher compressive strength value than the underlayment which is currently in-situ. There could also be chemical incompatibility issues. If doubt exists, or the existing underlayment cannot be identified, it should be fully removed using mechanised surface preparation equipment.
   
10. On ground floor bases, when removing old floor coverings and replacing with a floor smoothing underlayment and new floor covering, consideration must be given to the moisture content of the base, especially where there is a change of the type of floor covering to be installed. Old flooring and adhesives may have exhibited a higher tolerance to ground bearing moisture than the new flooring and preparation products.
11. Steel surfaces should be prepared using contained shot blasting to SA2½ or rotary disc/wire brushed to ST2 as described in ISO 8501-1 and generally primed. However, some bag and bottle products can be applied to the prepared steel without the need to prime. Consult the underlayment manufacturer for further guidance.
12. All subfloors should be dry i.e., ≤75% RH on a surface hygrometer, prior to installing either resilient or textile floor coverings. Where hygrometer readings >75% RH are obtained, a suitable surface applied DPM, or moisture vapour suppressant may be utilised subject to manufacturers recommendations.

For more technical information and advice, visit the CFA Guide to Contract Flooring.