Substrates & Preparation
Screeding Over an Existing Asphalt Floor
Mastic asphalt floors, once common as a damp-proof and level base, are usually treated as a stable substrate to float over rather than bond to.
Mastic asphalt turns up as a flooring or damp-proofing layer in older buildings, particularly in commercial and Victorian-era properties, valued historically for its inherent waterproofing qualities. When it's found during a refurbishment, the question is usually whether it can be retained and floated over rather than removed, since asphalt removal is labour-intensive and disruptive.
Bonding directly to asphalt is generally avoided given the material's own thermal movement and softness in warm conditions, so a floating approach is the more common route.
Asphalt Substrates — the points that decide it
- 1Mastic asphalt softens somewhat with temperature and has different movement characteristics to concrete, making direct bonding unreliable
- 2A floating liquid screed over insulation or a separating membrane is the more common approach over existing asphalt
- 3The asphalt's own condition should be checked for cracking, delamination or contamination before building up new layers over it
- 4Where asphalt was originally installed as a damp-proof layer, its continued waterproofing role should be understood before altering the floor build-up
- 5Existing falls in an asphalt floor (common in older wet areas) need accounting for in the new floor's levelling strategy
Why floating suits asphalt substrates
Asphalt behaves quite differently to concrete under temperature change, softening and moving more than a rigid mineral substrate. A liquid screed bonded directly to it would be vulnerable to that movement transferring into the cured screed, so floating the new screed over a separating layer avoids relying on a bond to a substrate with its own distinct behaviour.
Checking the asphalt's role in the existing building
In older buildings, an asphalt layer may have been installed specifically to control rising damp or ground moisture. Before covering it with insulation and a new screed, it's worth understanding whether it's still performing that role and whether the new floor build-up needs its own additional moisture strategy, rather than assuming the asphalt alone will continue to manage everything beneath the new finish.
Asphalt Substrates — your questions
Found asphalt under an old floor covering?
We'll assess whether it can be retained and advise on the right floating build-up for your liquid screed.
Or call our landline on 01392 237700
In short
Mastic asphalt floors, once common as a damp-proof and level base, are usually treated as a stable substrate to float over rather than bond to.
Found asphalt under an old floor covering?
We'll assess whether it can be retained and advise on the right floating build-up for your liquid screed.