To repair subfloor damage correctly, the first point to check is the type of defect. A small surface imperfection does not need the same treatment as a deeper damaged area, a missing piece of material or a zone that must be rebuilt before the next floor layer. In many cases, the decision comes down to two options: patching for light surface correction, or repair mortar for deeper filling and reshaping.
Patching or repair mortar: what is the difference?
Patching and repair mortar do not answer the same type of subfloor defect. For light surface correction, the repair should remain close to the surface and should not be treated in the same way as a deep or large repair. RENO C715 EASY FLASH is designed for patching and smoothing minor surface imperfections before the installation of most floor coverings. It can be used from feather edge up to 10 mm, and occasionally up to 15 mm for filling impacts. The aim is to correct the surface without rebuilding the floor in depth.
A repair mortar is used when the damaged area needs more material, more thickness or reshaping. RENO C950 QUICK is a rapid drying repair mortar for interior and exterior walls and floors. It is suitable for deep or large repairs in a single layer, filling holes, reprofiling stairs, and making or reshaping slopes.
This is the key distinction. If the defect only affects the surface, the use of patching may be enough. RENO C715 EASY FLASH is recommended for patching and smoothing minor surface imperfections before the installation of most floor coverings. If the defect has depth, missing material or a shape that needs to be rebuilt, repair mortar is the more suitable route.
When patching is the right choice
A light surface correction is generally suited to minor surface irregularities where no deep filling or reshaping is required. It is used as a feather finish when the subfloor needs local correction rather than deeper repair.
This approach fits situations where the floor surface needs to be made smoother before the next installation step, but where there is no large void, missing section or high-thickness area to rebuild. In this case, the repair remains close to the surface.
Patching should not be treated as a substitute for repair mortar when the damaged subfloor has depth. Once the repair involves larger holes, reshaping or rebuilding material, the method needs to change.
When repair mortar is required
Repair mortar is used when the damaged subfloor cannot be corrected with a thin surface finish alone. This is the case when a hole is deeper, when material has been removed, or when the area needs to be reshaped before the floor build-up can continue.
RENO C950 QUICK is designed for deep or large repairs in a single layer. It can be used for filling holes, repairing damaged areas, reprofiling stairs and making or reshaping slopes. It is suitable for common building substrates in new build and renovation, including cementitious screeds and concrete, calcium sulphate screeds, wooden boards, sanded asphalt screed, magnesia and xylolite screeds, old ceramic tiles, old waterproof adhesive residues, and old painted or resin floors.
Large holes and high-thickness repairs
Where the damaged area is too deep for a surface correction, a repair mortar gives the thickness needed to rebuild the surface before a levelling compound or adhesive application. RENO C950 QUICK can be applied from 1 to 50 mm and up to 50 mm in a single application. It is also classified as C30 1 for compressive strength and F6 for flexural strength 2.
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1 C30 |
2 F6 |
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| Refers to the compressive strength classification of the mortar. | Refers to the flexural strength classification of the mortar. |
Underlay and floor build-up preparation
When a floor build-up is planned, the damaged zone should be brought back to the required level before installation continues. A small surface defect can remain a patching job, but a deeper hole or missing area needs repair mortar to rebuild the surface.
RENO C950 QUICK can be applied into holes, damaged areas or surfaces that need treatment, then spread firmly and evenly with a smoothing trowel to the desired thickness. Once sufficiently firm, it can be trimmed and smoothed to prepare the area for the next floor layer.
How the condition of the subfloor affects the repair
The condition of the subfloor matters as much as the size of the hole. Before applying a repair mortar, the substrate must be sound, dry and free from materials that could reduce adhesion, such as dirt, oil or grease.
Any weak or unstable layer should be removed. This includes release agents, loose adhesives, loose compounds and paint residues. Loose material and dust should also be vacuumed before the repair.
The type of substrate also affects preparation. Ceramic tiles must be firmly bonded to the substrate. If they are not, they need to be refixed or removed. Dense, smooth and metallic substrates need degreasing and grinding before repair. Where RENO C950 QUICK is used, the primer should match the substrate. For concrete slabs and cement-based screeds, recommended primers include GRIP A310 PROJECT, GRIP A500 MULTI or GRIP A700 UNIVERSAL depending on absorbency. For calcium sulphate screeds, GRIP A310 PROJECT or GRIP A700 UNIVERSAL can be used. For ceramic tiles, terrazzo, natural stone, wooden subfloors, resin or painted subfloors and metallic subfloors, primer selection should also be adapted to the surface.
What to check before covering the repaired area
The repaired area should be ready for the floor covering planned on the project. Covering time varies with the floor covering selected and the conditions on site.
With RENO C950 QUICK, the waiting time varies by floor covering. Tiles can be installed after 1 to 3 hours, while PVC and carpet can follow after 2 to 5 hours. Floor finishes that need a longer delay such as parquet, rubber or floor paint, require 12 to 24 hours before installation.
ALTERNATIVE IF USING RENO ST:
With RENO C920 ST, the waiting time varies by floor covering and application thickness. Up to 50 mm, tiles can be installed after 1 to 3 hours, while PVC and carpet can follow after 2 to 5 hours. Floor finishes that need a longer delay, such as parquet or rubber, require 12 to 24 hours before installation. Above 50 mm, the curing time is 24 hours.
For projects where environmental documentation is required, the FDES 3 can also support product assessment alongside the technical repair criteria. These timings help align the repair with the site schedule. When the next floor layer is planned soon after the repair, the chosen method should support both the required repair depth and the covering time needed before installation continues.
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3 FDES |
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| Refers to a French environmental and health declaration sheet used to document environmental and health data for a construction product. |
Choosing the repair method before installation
The repair method should reflect how much of the subfloor needs to be corrected. Patching is suited to small imperfections and feather finishing, where the surface needs local adjustment. Repair mortar is used when the damaged area has depth, when material has been removed or when the surface must be rebuilt before adhesive and floor covering installation.
To repair subfloor damage with the right approach, match the method to the condition of the area and the next floor layer. For more information on repair mortars, visit the Bostik Academy.
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