Substrates & Preparation
Dealing with a Damp Substrate Before Liquid Screed
Persistent damp in a substrate needs diagnosing and controlling at source before insulation and screed are installed, rather than relying on the new floor build-up to mask it.
A damp substrate is a symptom, not a starting point — the underlying cause needs identifying before any floor build-up is designed around it. Whether it's rising damp through an old solid floor, a failed or missing DPM, or seasonal groundwater affecting a ground-bearing slab, covering it over with insulation and screed without addressing the source risks trapping moisture where it can cause longer-term problems.
This is a different question to residual construction moisture in a new slab, which is a normal and expected part of the drying process rather than a defect.
Diagnosing before designing the floor build-up
Damp readings on an old solid floor could point to several different causes — none of which are solved simply by choosing a particular insulation or screed product. Establishing whether the moisture is coming from below (rising damp, groundwater), from a failed damp-proof course, or is residual construction moisture that will naturally reduce over time, changes what the correct floor build-up should look like.
Building a new DPM into the floor
Where an existing floor lacks a functioning damp-proof membrane, one can often be introduced as part of the refurbishment, correctly lapped and detailed at floor perimeters and any penetrations. This lets a floating screed sit above the membrane, isolated from the substrate's own moisture condition, provided the source of any ongoing water ingress has also been addressed rather than simply covered.
Damp Substrates — the points that decide it
- Rising damp, a missing or failed DPM, and seasonal groundwater ingress are different causes that need different remedies
- Moisture meters and more detailed testing (such as relative humidity probes) help distinguish surface dampness from a genuine ongoing moisture issue
- A new DPM can sometimes be introduced as part of the floor build-up even where the original construction lacks one, subject to the specific project
- Floating a screed over a new, correctly lapped DPM is a common way to isolate the new floor from a damp substrate below
- Persistent damp affecting walls as well as the floor may indicate an issue that needs addressing beyond the floor build-up alone
- Any organic growth or salts on the substrate surface should be treated as a sign to investigate further, not just cleaned off and covered
Got a damp reading on an existing floor before renovation?
We'll talk through what testing makes sense and how the floor build-up should be designed once the cause is understood.
Or call our landline on 01392 237700
Damp Substrates — your questions
In short
Persistent damp in a substrate needs diagnosing and controlling at source before insulation and screed are installed, rather than relying on the new floor build-up to mask it.
Got a damp reading on an existing floor before renovation?
We'll talk through what testing makes sense and how the floor build-up should be designed once the cause is understood.