Moisture problems are rarely solved by buying a product first. A cold patch, a high relative scan, damp timber and a high screed RH result describe different conditions. Identify the construction, establish where the moisture is coming from, test the correct material and only then choose a DPM, primer, levelling compound, adhesive and floor covering as one compatible system.
Do not cover an unexplained moisture problem
A surface membrane cannot repair a leaking pipe, defective drainage, flooded cavity, blocked air brick, decaying suspended floor or water entering under pressure. Stop and obtain an experienced flooring or building survey where readings are high, inconsistent or linked to visible damp, salts, mould, decay, movement or an unknown floor construction. Always follow the current flooring, adhesive and preparation-product instructions, as their limits take priority over this general guide.
Quick moisture, DPM and primer selector
| What do you need to know or do? | WWFS product or route | What it does not prove |
|---|---|---|
| Rapidly scan a finished floor, screed or wall without pinholes | Protimeter Aquant 3 | Its RF reading is relative. It locates patterns and hotspots, but it is not a screed pass-or-fail RH test. |
| Scan non-invasively, then investigate with pins | Protimeter SurveyMaster 3 | Pin readings in non-timber materials are normally comparative Wood Moisture Equivalent readings, not their actual percentage moisture content. |
| Take pin readings in timber and general building materials | Protimeter Digital Mini 3 | The coloured dry, at-risk and wet bands do not override species correction or the floor manufacturer's moisture limits. |
| Measure flooring, boards, joists or plywood with timber-species correction | Protimeter TimberMaster 3 | One surface reading cannot prove that a whole batch, joist depth or concealed void is dry. |
| Obtain an equilibrium RH result from concrete or screed | F. Ball Digital Hygrometer with butyl sealing tape | A handheld RF scan cannot replace this test where an RH result is required. |
| Suppress residual moisture in a suitable prepared concrete or sand/cement base within an F. Ball system | STOPGAP F77 | It does not repair an active water source, unstable base, suspended timber floor or unidentified screed. |
| Prime or suppress residual moisture within a compatible Bona wood-floor adhesive system | Bona R540 | Its moisture-barrier use is for approved cementitious bases. Calcium-sulphate and timber-board uses are primer only. |
| Bind dust and prime a dry, absorbent base before a compatible Bona silane adhesive | Bona D501 | D501 is not a DPM and must not be chosen where moisture suppression is required. |
| Prime a dry, prepared calcium-sulphate or anhydrite screed before a suitable STOPGAP compound | STOPGAP P121 | P121 controls absorbency and promotes the approved build-up. It does not make a damp anhydrite screed ready to cover. |
| Prime an absorbent or approved general substrate before a STOPGAP smoothing compound | STOPGAP P131 | Dilution and suitability depend on the exact surface and compound. |
| Create a textured bond coat on a prepared non-absorbent surface | STOPGAP P141 | It is an adhesion primer, not a moisture barrier. |
| Provide a joined vapour-control layer beneath an approved floating floor | Duralay Timbermate Excel or Silentfloor Gold, with vapour tape | A vapour-barrier underlay is not permission to float flooring over a wet, contaminated or unsuitable base. |
Identify the subfloor before testing it
The visible surface may only be the top layer. A grey floor could be structural concrete, sand/cement screed, a smoothing compound over old adhesive, a cement-faced board or a calcium-sulphate screed. The correct test, moisture limit, preparation and primer depend on the actual construction.
Ground-bearing solid floor
Concrete or screed supported by the ground. Establish whether there is an effective structural DPM, the age and thickness of the base, and whether the moisture is residual construction moisture or continuing ingress.
Suspended mineral floor
Beam-and-block, concrete planks or another suspended structure may carry a screed or levelling layer. Test the upper mineral layer correctly, but also confirm the structural build-up and any ventilation or service-void issues.
Suspended timber floor
Floorboards or sheet material supported on joists over a ventilated void. Inspect the void, air paths, joists, sleeper walls, services, insulation and ground moisture before deciding that a primer or surface product is relevant.
Upper-floor construction
An upper floor is not automatically dry. Bathrooms, heating pipes, roof leaks, wet trades, screeds and trapped construction moisture can still affect timber or mineral bases.
| Subfloor type | Key checks | Normal moisture route |
|---|---|---|
| Structural concrete or sand/cement screed | Age, DPM, laitance, power-floated surface, strength, cracks, contamination and UFH | Indicative RF survey followed by sealed-box equilibrium RH testing where required |
| Calcium-sulphate, anhydrite or gypsum screed | Positive identification, laitance removal, drying, UFH commissioning and the screed supplier's limit | Approved RH or CM method for that screed. Do not borrow a cement-screed conversion or DPM route. |
| Proprietary rapid-drying screed | Exact brand, batch, thickness, installation date and curing record | Use the screed manufacturer's stated test and time-to-cover criteria, not a generic days-per-millimetre estimate. |
| Plywood, chipboard, OSB or floorboards | Grade, thickness, fixing, movement, joints, ventilation and evidence of leaks or decay | Species/material-appropriate pin readings across a planned grid, with deeper investigation where needed |
| Ceramic, terrazzo, asphalt or resin surface | Bond, contamination, cracks, hidden DPM and what is beneath it | Investigate the underlying construction. A dry-looking impervious surface can conceal moisture beneath. |
| Old smoothing compound or adhesive residue | Adhesion, water sensitivity, bitumen, asbestos risk and moisture beneath the old system | Do not rely on surface scanning alone. Obtain a professional preparation and moisture specification. |
Simple absorbency check
A clean water-drop test can help show whether a prepared mineral surface is absorbent or non-absorbent. It does not identify the screed, measure moisture, confirm strength or prove that a DPM exists. Use it only as one part of primer selection after the floor has been identified and prepared.
Which moisture test should you use?
| Instrument | Measurement | Best WWFS use | Important interpretation |
|---|---|---|---|
| TimberMaster 3 | Pin measurement in wood, with calibration choices for up to 150 timber species | Flooring, floorboards, plywood, joists and other wood products where an informed timber MC reading is required | The published pin range extends from 7.9% to 99%, but Protimeter states that readings above 30% are relative. Species and temperature still matter. |
| SurveyMaster 3 | Pin measurement plus non-invasive RF scanning to approximately 19mm depth | Survey a complete area, then investigate selected locations with pins | The search scale is relative. It cannot be reported as screed RH or actual percentage moisture. |
| Digital Mini 3 | Pin range of 7.9% to 99% WME | Timber checks and comparative readings in a broad range of building materials | WME is not the true moisture percentage of screed, plaster or masonry. Use the appropriate wood calibration when interpreting timber. |
| Aquant 3 | Non-invasive RF scale, approximately 60 to 999, scanning to around 19mm | Rapidly locate relative changes beneath finished surfaces without pinholes | Ideal for finding where to investigate next, not for issuing a screed pass result. |
| F. Ball Digital Hygrometer | Equilibrium relative humidity of air sealed against the subfloor | Recorded RH assessment of concrete, sand/cement and approved screed bases | The hood must remain airtight and stabilise in the specified conditions. Room RH is not the same result. |
Non-invasive RF scanning
Protimeter Aquant 3 scans without pinholes, while the SurveyMaster 3 combines a non-invasive search mode with pin measurement. They are valuable for mapping changes, comparing apparently dry and suspect areas, and deciding where further tests are needed. Metal, services, salts, density changes, different floor layers and nearby objects can affect comparative readings.
Treat the result as a moisture pattern, not as “the screed is 220% wet”. The numerical scale is relative and must not be converted into a screed RH or moisture percentage.
Pin measurement in timber
The TimberMaster 3 is the most specific WWFS route for professional timber testing because it provides wood-species calibrations. Take readings in multiple boards, packs and areas, orient the pins as instructed, account for timber temperature where necessary and use a hammer electrode when the investigation requires readings below the surface.
The Digital Mini 3 and pin mode of the SurveyMaster are useful for timber and comparative building-material checks. In wood, the instrument can provide a percentage moisture-content reading within its method. In plaster, screed, masonry and many board products, the displayed value is normally a comparative WME result rather than the true moisture percentage of that material.
Sealed-box equilibrium RH testing
The F. Ball Digital Hygrometer measures the relative humidity of a small volume of air sealed against the concrete or screed and allowed to reach equilibrium with it. This is different from measuring the room's ambient RH and different from quickly placing a handheld meter on the surface.
- Choose representative test positions, including any suspect or slower-drying areas.
- Prepare the test surface as required, removing anything that prevents the butyl tape making an airtight seal.
- Seal the hood completely to the base. Leaking room air can invalidate the result.
- Protect it from direct sunlight, heaters, draughts, traffic and accidental movement.
- Leave it in place for the period required by the current method, commonly until readings have stabilised over the specified test window.
- Record the RH, temperature, position, time, substrate, UFH status and test conditions.
RH, CM% and timber MC% are not interchangeable
Relative humidity from a sealed hygrometer, percentage moisture content from a timber pin meter and carbide-method CM% are different measurements. Do not use an online conversion chart to turn one into another unless the screed or flooring manufacturer has approved that exact method and limit.
How to take readings that are useful
A single reading in the middle of the room is rarely enough. Plan the survey so it can reveal patterns and be repeated.
- Record a floor plan and number each test position.
- Include external walls, doorways, service routes, kitchens, bathrooms, previous leaks, cracks and the lowest areas.
- For timber flooring, sample different packs and boards rather than only the top board of one open pack.
- For a timber subfloor, compare boards, joists where accessible and any sheet overlay. Investigate concealed construction where readings or condition justify it.
- Use clean, undamaged pins and the correct species or calibration setting.
- Record room temperature and ambient RH alongside the material reading.
- On UFH, identify separate zones and pipe layouts, and record the commissioning and operating condition at the time of test.
- Repeat suspect readings rather than selecting the lowest result.
- Keep dated photographs and readings with the flooring installation records.
When testing a screed that will receive a bonded wood floor, an RF scan is an efficient first survey. The formal decision should then be based on the test required by the flooring, adhesive and preparation system. When testing timber, consider both the actual reading and its relationship to the room conditions and the other timber that will be joined to it.
What does the result mean?
F. Ball's current hygrometer guidance states that most floor coverings are normally assessed at no more than 75% RH, while solid timber floor coverings require a lower 65% RH limit. Wood floors are specialist products, so the flooring and adhesive manufacturers' stated limit still controls. Do not assume that every engineered floor is automatically acceptable at 75% RH.
| Result | What it can mean | Next action |
|---|---|---|
| RH is within every specified system limit | The base may be dry enough, but moisture is only one requirement | Continue checking strength, flatness, contamination, cracks, UFH and primer/adhesive compatibility. |
| RH is above the floor or adhesive limit but within an approved membrane limit | Residual construction moisture may be manageable on a suitable cementitious base | Select either natural drying or a complete approved surface-membrane system. Confirm the cause and structural DPM first. |
| RH varies greatly across the floor | Uneven drying, a leak, different substrates, a failed DPM, services or local contamination may be involved | Map the pattern, investigate the construction and do not average the readings into a pass. |
| Timber is consistently higher near a wall or void | Ventilation, ground level, leak, condensation or bridging may be affecting the area | Investigate and correct the source before covering the timber. |
| Flooring moisture differs materially from the timber subfloor or expected room equilibrium | The materials may move differently after installation | Pause, condition the materials correctly and follow the flooring manufacturer's acceptance criteria. |
| Non-invasive scan is high but pin/RH confirmation is not | Metal, density, salts or another layer may be influencing the scan | Use the scan to direct further investigation. Do not diagnose damp from the relative number alone. |
New does not mean dry
A new slab may contain substantial construction moisture even when a structural DPM is present. Drying depends on thickness, mix, curing, ventilation, temperature, humidity and whether other layers have already restricted evaporation. Generic drying estimates are useful only for early planning. They are not a substitute for testing.
Old does not mean dry
Older ground floors may lack an effective DPM, while leaks, raised external ground, drainage problems and later alterations can introduce new moisture. Old bitumen adhesive or tiles may have tolerated moisture that a modern wood-floor adhesive system will not. Unknown black residues can also present an asbestos risk, so do not sand or grind them without the correct assessment.
DPM, primer and vapour layer are different products
| Product type | Main job | Typical position | Common misunderstanding |
|---|---|---|---|
| Structural DPM | Forms part of the building construction to resist ground moisture | Within or beneath the ground-floor build-up | A surface primer does not automatically replace a missing or failed structural membrane. |
| Liquid surface moisture membrane | Suppresses moisture vapour within stated limits on an approved, prepared mineral base | On top of concrete or screed before compatible preparation and flooring products | It is not a liquid cure for leaks, hydrostatic pressure, damp timber or every type of screed. |
| Flooring primer | Controls absorbency, binds residual dust or promotes adhesion | Between a prepared substrate and an approved compound or adhesive | A primer labelled for a substrate is not necessarily a DPM on that substrate. |
| Vapour-barrier underlay | Reduces vapour movement beneath a compatible floating floor when joints and perimeters are detailed correctly | Between the dry, suitable base and floating floor | It does not make a wet base suitable or replace a bonded-floor DPM system. |
Choosing between STOPGAP F77 and Bona R540
Both products can manage moisture on suitable bases, but they belong to different systems and are not interchangeable tins of “liquid DPM”. Choose according to the measured result, substrate and every product that will follow.
| Product | Best suited to | Published limits and system points | Do not use it as |
|---|---|---|---|
| F. Ball STOPGAP F77 | Prepared, absorbent concrete or sand/cement screeds in a compatible F. Ball floor-preparation system | One-coat two-part epoxy route up to 98% RH. F. Ball states up to 90% RH directly over correctly commissioned underfloor-heating screeds at its specified coverage. Cure is approximately three hours at 20°C. | A membrane for timber boards, an automatic anhydrite treatment, a crack-bridging layer or a cure for an active leak. |
| Bona R540 | Compatible Bona silane or polyurethane wood-floor adhesive systems on approved bases | One-component polyurethane. Current Bona guidance uses two coats within the stated interval for moisture-barrier duty on suitable cementitious substrates, up to the stated CM or RH limit. It can also be a primer or lightly reinforce an assessed mineral surface. | A structural DPM. On calcium-sulphate screeds, chipboard, OSB and similar dry board substrates, it is a primer only, not a moisture barrier. |
STOPGAP F77 route
- Confirm a suitable concrete or sand/cement base and obtain recorded RH readings.
- Remove laitance, surface hardeners, weak material, oil, adhesive contamination and anything that would prevent adhesion. Mechanically prepare power-floated concrete.
- Repair or pre-smooth the base only with a system approved beneath F77, keeping pinholes and weak areas out of the finished preparation.
- Mix the complete pre-measured resin and hardener thoroughly. Do not split kits by eye.
- Apply at the coverage rate required for the recorded RH and UFH condition. A thin, missed or pinholed coat is not a membrane.
- Allow it to cure, then prime or overcoat within the current F. Ball system window. The product that follows determines whether a primer such as P131 or P141 is required.
Bona R540 route
Bona's current application guidance states that two coats can suppress residual construction moisture in suitable cementitious substrates up to 5 CM% or 95% RH. Consumption and curing depend on the measured condition, surface and Bona adhesive used afterwards, so confirm the current UK technical sheet or obtain Bona technical approval for the final specification.
- Confirm the substrate and recorded moisture result are within the current Bona moisture-barrier specification.
- Prepare a level, sound, contaminant-free surface and vacuum it thoroughly.
- Apply the first coat evenly without pools or excessive material. Over-application can cause foaming.
- For moisture-barrier duty, apply the specified second coat within Bona's stated interval after the first is ready for foot traffic.
- Observe the curing time for the exact Bona adhesive. The waiting period is not identical for every adhesive.
- Complete the bonded flooring installation inside the system window stated in the current R540 and adhesive data sheets.
These are professional reactive products
STOPGAP F77 is a two-part epoxy system and Bona R540 is a reactive polyurethane product. Read the current technical and safety data sheets before opening them. Use the specified training, ventilation, gloves, eye protection, application tools and disposal controls. Once mixed or cured, these products cannot be treated like ordinary water-based primer.
Which primer should you use?
A primer is selected for the prepared surface and the product being applied next. “Concrete primer” is not enough information. The same floor may need one product before a STOPGAP smoothing compound and a different product before a Bona adhesive.
| Primer | Correct role | Typical prepared substrates | Key limitation |
|---|---|---|---|
| STOPGAP P121 | Specialist acrylic primer for suitable STOPGAP smoothing compounds on dry calcium-sulphate and anhydrite screeds | Mechanically prepared, laitance-free calcium-sulphate screed that has reached its required moisture limit | Normally a specified multi-coat route. Follow the current dilution and coat schedule. It is not a DPM. |
| STOPGAP P131 | General-purpose primer to control absorbency and promote adhesion | Approved absorbent concrete, sand/cement screed, plywood, hardboard and chipboard; also certain prepared non-absorbent surfaces | Used diluted on many absorbent routes and neat on approved non-absorbent routes. The exact ratio comes from the compound and primer data sheets. |
| STOPGAP P141 | Fast-drying acrylic bonding primer that leaves a hard, textured surface | Prepared ceramic tiles, treated power-floated concrete and STOPGAP waterproof surface membranes | For non-absorbent surfaces. It does not control moisture and should not be substituted for P121 on anhydrite. |
| Bona D501 | Vapour-permeable dust binder and primer before compatible Bona silane adhesives | Dry, sound absorbent cementitious bases and approved dry wood-based boards | Not a DPM. If moisture suppression or substrate reinforcement is required, assess R540 or another specified Bona system instead. |
| Bona R540 as a primer | Polyurethane primer with strong penetration, adhesion and limited surface-reinforcement capability | Approved absorbent and non-absorbent mineral bases, plus selected dry board substrates within Bona's system | Primer suitability does not grant DPM suitability. Calcium sulphate and wood-based boards are primer-only substrates. |
Calcium-sulphate screed is its own route
Identify it positively, allow it to dry to its specified limit, mechanically remove laitance, vacuum thoroughly and use a build-up approved for calcium sulphate. STOPGAP P121 is the dedicated F. Ball primer sold by WWFS for suitable STOPGAP compounds on this base. Bona R540 may be used as a primer where Bona permits, but current Bona guidance states that calcium-sulphate screed is primer only, not an R540 moisture-barrier substrate.
Suspended timber floors need a different investigation
A suspended timber floor relies on its construction and ventilation to keep the structural timber safe. Coating the room-side boards does not repair damp in the void and can restrict drying or conceal decay.
Check the source before the surface
- Confirm air bricks and ventilation paths are open on opposing sides where the construction relies on cross-ventilation.
- Check whether extensions, paving, render, insulation or stored material have blocked air paths or raised external ground levels.
- Inspect the subfloor ground, sleeper walls, damp-proof courses, wall plates, joist ends and accessible services.
- Look for plumbing leaks, wet insulation, condensation, fungal growth, insect damage, softness, staining and salts.
- Check that sheet overlays and floorboards are secure, suitable and not trapping an unresolved moisture problem.
- Where the void cannot be inspected safely, use an experienced surveyor or relevant building professional.
Test the timber, not the room air alone
Use a TimberMaster 3 or suitable pin meter to compare boards and structural timber at multiple locations and depths. Record ambient conditions, but do not treat normal room RH as proof that concealed joists are dry. A SurveyMaster 3 or Aquant 3 can help map a pattern before pin investigation.
What products may follow once it is dry?
A dry, sound plywood, chipboard or OSB surface may use STOPGAP P131 before an approved STOPGAP preparation route, or Bona D501 or Bona R540 as a primer within a compatible Bona adhesive system. These are preparation uses on dry substrates. They are not permission to seal damp boards or joists.
Never pour a cement smoothing compound straight over moving floorboards
Movement, deflection, loose fixings, board joints and ventilation must be resolved first. The subfloor-preparation system may require a flooring-grade plywood overlay, secure fixings, reinforced compound or another designed build-up. Continue with the WWFS subfloor-preparation guide and levelling-compound selector.
Moisture testing over underfloor heating
A heated screed must be commissioned and tested as a heated floor, not treated as an ordinary screed that happens to contain pipes. Obtain the screed and heating records before installation.
- Confirm the screed type, thickness, age and manufacturer's drying requirements.
- Complete the prescribed commissioning and heating cycle before the final moisture assessment.
- Allow the floor to reach the stable test condition required by the test and flooring systems.
- Record each heating zone, temperatures and the condition of the system during testing.
- Do not drill for an in-screed probe unless the service layout and test method make it safe. The WWFS F. Ball hygrometer is a sealed surface-box test.
- Maintain the flooring manufacturer's maximum floor-surface temperature after installation, commonly 27°C for compatible wood flooring, but confirm the exact product limit.
STOPGAP F77 is approved by F. Ball directly over suitable commissioned cementitious UFH screeds up to 90% RH at the specified coverage. Bona R540 is suitable within approved heated-floor Bona systems, but the current R540, adhesive and flooring limits must all be checked together.
For the heating operation, floor selection and installation restrictions, read the complete WWFS underfloor-heating and engineered-wood guide.
Build the whole floor system before ordering
The moisture result is the first decision, not the last. Write the proposed layers in order and confirm that every manufacturer permits direct contact with the layer below.
| Stage | Question to answer | Useful WWFS guide |
|---|---|---|
| 1. Construction | Is the floor solid, suspended mineral or suspended timber, and what layers are present? | How to prepare a subfloor |
| 2. Moisture | What is the source, test method, recorded result and acceptance limit? | This guide |
| 3. Mechanical preparation | What laitance, weak material, contamination or old residue must be safely removed? | Subfloor-preparation guide |
| 4. Moisture control | Can it dry naturally, or is an approved surface membrane required? | F77 and R540 sections above |
| 5. Primer and levelling | Is the prepared surface absorbent, non-absorbent or calcium sulphate, and which compound follows? | Levelling-compound selector |
| 6. Installation | Will the floor be bonded, floated or mechanically fixed? | Installation-method guide |
| 7. Adhesive | Which adhesive is compatible with the prepared surface, flooring and UFH? | Wood-flooring adhesive guide |
Floating floors and vapour-barrier underlays
Duralay Timbermate Excel and Silentfloor Gold contain Interfloor's Vapourstop barrier. On an approved solid base, install the barrier in the stated direction, closely butt the underlay and seal every seam with vapour tape in accordance with the current fitting instructions.
This is a floating-floor detail, not a substitute for diagnosing damp. The subfloor must still be sound, sufficiently dry, level and suitable for the flooring. Check UFH thermal resistance separately, as the best acoustic underlay is not automatically the best heated-floor underlay.
Common moisture, DPM and primer mistakes
- Calling an RF number “percentage moisture”. Non-invasive search readings are relative and require confirmation.
- Using a timber pin meter to pass a screed. WME can identify a pattern, but it is not the required equilibrium RH result.
- Testing one convenient position. Moisture can vary around walls, services, cracks and different floor zones.
- Choosing a DPM because the floor looks dark. Colour and surface temperature are not moisture measurements.
- Assuming a new slab is dry because it has a structural DPM. Construction moisture still has to leave or be managed within an approved system.
- Assuming an old slab is dry because it has been uncovered for years. It may lack a DPM or be affected by later changes and leaks.
- Applying F77 to the wrong substrate. Its published standard route is suitable prepared concrete and sand/cement screed. Other bases need a written specification.
- Using R540 as a DPM over anhydrite or wood-based boards. Those are primer-only uses in current Bona guidance.
- Using D501 as moisture protection. It is a primer and dust binder, not a membrane.
- Confusing P131 and P141. P131 is the broad general-purpose route; P141 creates a textured key on prepared non-absorbent surfaces.
- Priming damp calcium sulphate with P121. It must first be identified, dried and mechanically prepared.
- Sealing damp suspended timber from above. Correct the water source, ventilation and structural condition first.
- Spreading a membrane too thinly. Coverage is part of performance. Misses and pinholes break the barrier.
- Mixing brands without checking the interfaces. The membrane, primer, smoothing compound, adhesive and flooring must form one approved build-up.
Moisture, DPM and primer FAQs
Can I use a Protimeter on concrete instead of a hygrometer?
Use a non-invasive or pin meter to survey and compare the floor, but use the screed test required by the flooring and adhesive system for the installation decision. The F. Ball Digital Hygrometer provides the equilibrium RH route stocked by WWFS.
Is 75% RH always safe for engineered wood?
No. It is a common general threshold for many floor coverings, not a universal engineered-wood approval. Check the exact flooring, adhesive, primer and subfloor specifications. F. Ball publishes a lower 65% RH limit for solid timber floor coverings.
Can STOPGAP F77 go over anhydrite?
Do not assume so. F77's standard published base-preparation route names concrete and sand/cement screeds and instructs users to obtain technical advice for other subfloors. A dry calcium-sulphate screed normally requires its own preparation, moisture limit and compatible primer/compound route.
Can Bona R540 be used over calcium-sulphate screed?
Current Bona guidance lists calcium-sulphate screed as a suitable primer substrate only. Do not use that statement as permission to use R540 as a moisture barrier on a damp anhydrite screed.
What is the difference between Bona D501 and R540?
D501 is a vapour-permeable primer and dust binder for dry, approved bases before compatible Bona silane adhesives. R540 is a reactive polyurethane primer that can also provide moisture suppression on approved cementitious bases within its measured limit and coat schedule.
Do I need P131 or P141 over a cured F77 membrane?
It depends on the smoothing compound, application timing and current F. Ball system. P141 is specifically designed to create a textured bond surface on non-absorbent bases including STOPGAP waterproof membranes. P131 is also used on approved non-absorbent surfaces in stated systems. Follow the F77 and next-product data sheets rather than selecting by colour.
Should I apply a DPM over timber floorboards?
Not as a cure for damp. Inspect and repair the suspended-floor ventilation, ground moisture, leaks and timber condition. A dry timber or sheet subfloor may later need a primer for an approved preparation or adhesive system, but that is a different job.
Does a vapour-barrier underlay mean I can float over a damp slab?
No. The slab and flooring must still meet the system requirements. A correctly joined Vapourstop underlay helps control vapour beneath a suitable floating floor, but it does not repair active damp, contamination or an unsuitable base.
Related WWFS guides
- How to prepare a subfloor for engineered wood flooring
- Which floor-levelling compound should I use?
- Which wood-flooring adhesive should I choose?
- Which wood-floor installation method should I use?
- Which wood-floor fixing should I use?
- Underfloor heating and engineered wood flooring
- How to repair loose or squeaking wooden floorboards
- How to repair and re-lay parquet blocks with old bitumen
Need help identifying the route?
Send WWFS clear photographs of the subfloor and room, the property age, floor construction, screed or board type, UFH details, test instrument, dated readings and the wood flooring and adhesive you plan to use. Those details are far more useful than asking for “a primer for concrete”.