Subfloors- This section describes and covers the correct preparation to British Standards and Manufacturer requirements for floorcoverings.
Concrete / Sand Cement subfloors must incorporate a working Damp Proof Membrane. (See DPM’S for more information) Concrete / Sand Cement subfloors will require a smoothing compound to be applied before most floorcoverings can be installed (See Smoothing Compounds for more information) The surface of the subfloor must be structurally sound and free from contaminants such as dust, plaster droppings, paint, and any water-softenable adhesives or loosely adhered materials. Any remaining contaminates must have sufficient cohesive strength to receive a smoothing compound.
Cement Flow Screed subfloors are becoming more common. Many use a curing agent that must be removed from the surface. Cement Flow Screed subfloors must incorporate a working Damp Proof Membrane. (See DPM’S for more information)Cement Flow Screed subfloors will require a smoothing compound to be applied before most floorcoverings can be installed (See Smoothing Compounds for more information) The surface of the subfloor must be structurally sound and free from contaminants such as dust, plaster droppings, paint, and any water-softenable adhesives or loosely adhered materials. Any remaining contaminates must have sufficient cohesive strength to receive a smoothing compound.
Anhydrite Flow Screed subfloors are a modified Gypsum based subfloor that dry with a surface laitance that must be mechanically removed with specialist grinding equipment. Anhydrite subfloors must incorporate a working Damp Proof Membrane. (See DPM’S for more information) Anhydrite subfloors will require a smoothing compound to be applied before most floorcoverings can be installed (See Smoothing Compounds for more information). Cement-based smoothing compounds require a barrier layer to be applied to the mechanically ground Anhydrite surface to stop a reaction occurring and failure of the smoothing compound. The surface of the subfloor must be structurally sound and free from contaminants such as dust, plaster droppings, paint, and any water-softenable adhesives or loosely adhered materials. Any remaining contaminates must have sufficient cohesive strength to receive a smoothing compound.
Asphalt subfloors Asphalt is known by many names in the UK to include – Mastic Asphalt, Pitch, Bitumen, Tar, Asphaltum etc. Asphalt was one of the first liquid DPM systems. Asphalt can also be classed as a subfloor, however, it will most likely have a concrete base underneath it. Asphalt is laid at approximately 20 – 35mm deep. It can be laid much deeper if being used to raise floor heights. Asphalt is very popular in domestic properties around the 1960’s -1970s. Many Asphalt floors have been damaged over the years by carpet gripper being nailed to the Asphalt, building works through to movement over the years causing cracking. If the Asphalt is cracked or damaged, then it will most likely mean that the DPM is no longer working. A suitable repair must be carried out or an appropriate liquid DPM applied.
It is essential that Asphalt has a suitable smoothing compound applied before most floorcoverings are installed. The Asphalt is required to be sound, dry and degreased to remove all surface contaminates.
Quarry Tile subfloors were very popular pre-1965. Pre-1965 they would have been installed directly to earth meaning there will be no Damp Proof Membrane. (See DPM’S for more information). Quarry tiles were the finished floor and weren’t designed to be covered up. They were designed to be used as a decorative floor finish. Quarry tiles will require to be cleaned, degreased with all surface contaminates removed. Loose tiles will have to be addressed and then a suitable smoothing compound applied. For pre-1965 properties or floors reading about 75% RH (See Moisture Testing for more information) a suitable liquid DPM will be required or removal of the Quarry tiles and a new subfloor base installed to current building regulations.
Magnesite subfloors were used mainly between 1920-1960 and are a rare type of subfloor. Usually, they are a reddish-pink colour but could be tinted to create other colours. Normally they would be laid at approximately 10-25mm thick and normally over a concrete base. Due to the age of properties, they were used in, they will normally not have a Damp Proof Membrane under them (See DPM’S for more information) Magnesite is a breathable subfloor that allows moisture to pass through them, however, if you was to install a none breathable flooring over the top such LVT, Vinyl, Laminate and even modern-day carpet underlays they will quickly deteriorate and fail. They also contain Chloride that can migrate and corrode gas and water pipes along with reinforcements. Magnesite also has a limited life span with many Magnesite subfloors now reaching the end of their life cycle. Unfortunately, Magnesite will require to be uplifted and replaced with a modern-day subfloor floor before new floorcoverings other than breathable carpets are installed.
Wooden / Timber subfloors such as floorboards and chipboard require to be covered with a suitable fabricated underlay such as Plywood or Hardboard to BS8203 2017 or a specialist preparation system designed for timber subfloors before most floorcoverings are installed. Timber bases should be sound, rigid, level and dry. The timber should be at equilibrium moisture content, i.e. the state of dryness attained in normal service conditions, at the time it is covered. BS8203 2017 states that Plywood should be an approved Flooring Grade plywood to conform to BS EN636 and Hardboard must be Flooring Grade HB.H oil tempered to conform to BS EN 622-2.
It should be noted that these types of Plywood and Hardboard are specialist products that are not readily available from builders and timber yards and tend to be only available through specialist flooring suppliers. Checks should be carried out to confirm the products have the relevant certification to prove they are a suitable flooring grade material.
Plywood and Hardboard must be fixed to the timber subfloor using the correct fastenings to BS8203 2017 which are ring shank nails or screws only with a maximum spacing between fixings of 150mm across the centre and 100mm around the perimeter of the products.
DPM’S (Damp Proof Membranes) became a mandatory requirement in 1965. Common DPM’s are installed under the subfloor and normally a Polyethylene sheet that is required to be a minimum 1200-gauge thickness. All joints should be overlapped with sealed joints and tied into the Damp Proof Course (See DPC’s for more information). Sheet based DPM’S should not be visible at the surface of the subfloor. Where an existing subfloor meets a new subfloor the DPM’s should be linked and sealed together under the subfloor. Failure to do so results in a breach in the subfloors DPM and will require to be rectified before a floorcovering can be installed.
Surface applied DPM’s are normally used when an existing subfloor doesn’t have a sheet type DPM under the subfloor or when a breach of the sheet DPM renders the DPM inoperative. Surface DPM’s are also applied to new subfloors as a fast track system so floorcoverings can be installed before the subfloor has finished naturally drying. See table 1 for approximate drying times of solid subfloors. Note that drying times are the length of time the residual moisture to drop to below 75% RH as of British Standards BS8203 2017. Hardening time is the time required for a solid subfloor to be gain the specified strength which will be under 28 days. The two drying times are often confused. Subfloors moisture content should always be measured with the appropriate equipment to British Standards. (See Moisture Testing for more information)
| Table 1 Subfloor Thickness | Approximate Drying Time (To Below 75% RH) |
| 50mm Depth | 50 Days |
| 70mm Depth | 90 Days |
| 90mm Depth | 130 Days |
| 110mm Depth | 170 Days |
| 130mm Depth | 210 Days |
| 150mm Depth | 250 Days |
| 200mm Depth | 350 Days |
Moisture Testing of subfloors is essential to be carried out no matter what age the property is. New solid subfloors, for example, will be too wet to install most floorcoverings on for the first few months after they were installed and won’t effectively start to dry until the property is watertight (roof, windows and doors installed) along with other wet trades completing their works such as plastering the walls. Do not confuse drying times with hardening times. Most floors will fully cure and harden in under 28 days, but to be dry enough to receive a new floorcovering will take much longer. To British Standards BS8203 2017, a moisture test should be carried out on all floors no matter how old the subfloors maybe, due to the structural Damp Proof Membrane may be missing or simply not working (See DPM’S for more information). To carry out a moisture test to British Standards, a Flooring Hygrometer should be set up for a minimum duration of 72 hrs to determine the subfloors true moisture content. A moisture reading of below 75% RH is required before a floorcovering can be installed. Alternatively, a fast track Liquid DPM can be applied to the subfloor so that floorcoverings can be installed before the subfloor has fully dried. (See DPM’S for more information)
Experienced flooring Retailers / Installers may opt to use a professional electronic moisture reader to take instant moisture readings to determine if they will need to set up a flooring hygrometer to BS8203 2017. Please note that a professional meter costs hundreds of pounds and should not be confused with a cheap indication meter that many other trades use. These are far from accurate.
See Table 1 for approxamate drying times of soild subfloors.
Smoothing Compounds are normally a cement-based product designed to create a smooth finish over a range of subfloors. Smoothing compounds go by many names such as Latex or Levelling compound. These products are not levelling compounds, they are for smoothing subfloors. You can get a subfloor level with the compounds also, but you would normally use a vast quantity and will normally result in steps being created in doorways, hence a subfloor would not normally be made ‘level’ but flat and smooth only. Subfloors should be smooth and flat before most floorcoverings are installed. With floorcoverings such as LVT or Vinyl floorcoverings, any imperfections in the subfloor will mirror through the flooring product. Ideally, the flooring installer is trying to get a smooth flat finish to meet a regulation known as SR1 (See Surface Regularity for more information). However, in older properties, it should be understood that achieving such a smooth and flat finish isn’t always possible without extensive building works being required. If the subfloor can’t be smoothed sufficiently then the choice of floorcovering is more limited. Many manufacturers stipulate that SR1 is required for a warranty to be valid with their floorcoverings.
It is also essential that the correct type of smoothing compound is used to suit the subfloor type, condition, moisture levels, floorcovering, type of traffic the area will receive etc. Using an incorrect smoothing compound will lead to the new floorcovering under preforming or worse, failing.
SR (Surface Regularity) is a measurement to determine the smoothness of the subfloor. The test is undertaken using a 2-meter long straight edge that is placed in multiple locations around the area requiring the new floorcovering. Measurements are taken under the straight edge to determine the size of any dips or high spots. For most floorcoverings a maximum deviation of up to 3mm over a 2-meter distance is the maximum permissible. This is known as SR1.
It is unlikely the subfloor will meet SR1 (no more than 3mm over 2 meters) without a floorcovering specialist using a smoothing compound to achieve this (See Smoothing Compounds for more information)
New subfloors should ideally be left with a minimum finish of SR2 or better ready for the flooring covering specialist to apply their smoothing compounds. SR2 is no more than 5mm deviations over a 2-meter distance. Hence then a smoothing compound is used to achieve the SR1 rating.
See Table 2
| Table 2 Surface Regularity CLASS- | Maximum Permissible Deviation Over 2 Meters | Application |
| SR1 | 3mm | High Standard (Domestic & Commercial Floorcoverings) |
| SR2 | 5mm | Normal Standard for Domestic and Commercial Subfloors |
| SR3 | 10mm | Low Standard |
Under Floor Heating (UFH) is becoming more and more popular. UFH can be efficient and means you don’t have radiators taking up wall space and so on. However, UFH can cause many issues with floorcoverings if the UFH isn’t commissioned correctly along with the flooring installer not carrying out the correct process while installing the new floorcovering. For your floorcovering to preform correctly there are some strict rules that must be adhered to with UFH systems. The UFH system should be fully commissioned and correctly set up. To British Standards BS8203 2017, the UFH should never heat the subfloor above 27c. This reading is to be taken at the surface of subfloor so when a floorcovering is installed over the top, it should read slightly lower at the surface of the floorcovering.
Water feed UFH systems should not be run until the screed manufacturers minimum hardening time for UFH has been achieved. The temperature should be slowly increased to help dry the subfloor and then lowered in temperature and switched off ready for moisture testing. The UFH must NOT be run while moisture testing. Moisture test results are required to be lower than 75%RH for UFH systems as they will increase their moisture emission rate while they are heating. FloorSkills recommends that the subfloor should read less than 70% RH when turned off. (See Moisture Testing for more information)
When installing floorcoverings to British Standards BS8203 2017 the UFH system should be turned off for 48 hrs before and after the installation. However, the area must still be heated by other means to a minimum temperature of 18c along with a minimum subfloor temperature of 10c. FloorSkills recommends that with UFH a minimum subfloor temperature of 16c should be maintained at all times. This is due to an occupied heated property with UFH will very unlikely have a subfloor temperature of below 16c when in normal use. Water UFH pipes to British Standards BS8203 2017 should have a minimum of 25mm thickness of screed covering the pipes.
Electric UFH is normally wires that are installed over the surface of the subfloor. These systems are not as efficient as water feed UFH and FloorSkills recommends that electric UFH systems are used to take the chill off the floor and not to try and heat the property without some other form of heating source. Electric UFH wires must be covered with a minimum of 9mm of smoothing compound over the top of the wires. This will result in a total depth of approximately 12mm of smoothing compound required. (See Smoothing Compound for more information). Before a smoothing compound is applied over the wires, moisture tests and a liquid DPM may be required before the wires are installed. (See Moisture Testing & DPM’S for more information).