Cellars sit below ground level, making them more susceptible to moisture from the surrounding soil. If you notice musty smells, damp patches, or repeated damage to plaster and paint, it is reasonable to question whether tanking a cellar is the right solution.
Many property owners improve ventilation, redecorate, or apply surface treatments, only to find that penetrating damp returns. This is because cellars behave differently from the rest of a building, with moisture able to move through walls and floors long after rainfall. In most cases, tanking is considered where moisture is entering through walls or floors due to ground contact rather than surface condensation.
This guide explains what cellar tanking involves, how to recognise when it may be appropriate, and what steps to take next to protect your property with confidence.
Tanking out a cellar involves installing a Type A waterproofing system, also known as barrier protection, designed to prevent water and moisture from entering a below-ground space through surrounding walls and floors. The goal is to create a continuous internal barrier that resists moisture pressure from the ground.
This is typically achieved by applying cement-based waterproof coatings to properly prepared masonry or concrete surfaces. The effectiveness of tanking depends on thorough surface preparation, careful detailing at joints and service penetrations, and correct installation, as workmanship defects are a common cause of failure. For an example of a successful tanking installation, check out our Wareham case study.
According to guidance from the Property Care Association (PCA) and BS 8102, waterproofing design should be based on the intended use of the cellar [1]. Habitable spaces should meet Grade 3 standards, where no water ingress or damp is acceptable, while storage areas may be designed to Grade 2, where moisture penetration is prevented but some condensation may be tolerable.
The PCA also advises that below-ground waterproofing systems should be designed following a site-specific risk assessment by a qualified specialist. This ensures factors such as ground conditions and water pressure are properly considered before tanking is specified.
Cellar damp often behaves differently from surface-level damp because it is influenced by conditions beneath the ground rather than short-term weather changes. The Environment Agency notes that groundwater flooding can occur days, weeks, or even months after prolonged rainfall, meaning moisture pressure may continue long after rain has stopped [2].
This delayed behaviour explains why cellar damp can appear persistent or unpredictable. Moisture may continue to migrate through walls and floors even during dry periods, particularly in areas with clay-heavy soils or limited natural drainage.
In some properties, moisture also enters laterally through cellar walls under pressure from damp soil, leading to low-level staining and salt deposits, known as efflorescence. Understanding how and when moisture enters the structure helps determine whether waterproofing is required or whether alternative controls are appropriate.
Some cellar damp issues are temporary, while others indicate moisture moving through the structure. Persistent or recurring symptoms are usually the clearest signal that tanking out a cellar may be necessary.
Common indicators include:
The importance of these signs lies in their development over time. Structural moisture ingress tends to worsen gradually, with damage concentrated at lower wall levels and returning despite repair. In contrast, surface condensation often fluctuates with changes in temperature, ventilation, or occupancy.
Where these signs persist, a professional assessment helps confirm whether tanking is required or whether another form of damp control would be more appropriate.
Tanking is usually considered when the required level of dryness cannot be achieved through partial measures alone. This is common when the cellar needs to remain dry year-round rather than tolerate occasional dampness.
Tanking out a cellar is typically appropriate when the space is intended for:
In these situations, structural waterproofing provides a controlled internal environment rather than ongoing maintenance. Where cellars are being prepared for improvement, tanking is often addressed early as part of the groundwork.
Our team supports this through our basement conversion services, to ensure moisture protection is considered before finishes are installed.
A professional tanking survey establishes how moisture is entering the cellar and whether tanking is the most appropriate solution. Both internal conditions and external influences are assessed to build an accurate picture.
The survey process typically includes inspection of walls and floors, moisture profiling, and review of drainage and ventilation. Based on these findings, a suitable waterproofing system is recommended with a clearly defined scope of work.
Our surveys are carried out by experienced specialists and focus on long-term performance rather than short-term fixes. Examples of completed waterproofing projects can be viewed in our case studies section.
The cost of tanking out a cellar depends on factors such as the size and condition of the space, access requirements, drying time, and whether associated works are needed. A professional survey helps define the correct scope early, reducing the risk of repeated repairs and unresolved moisture issues.
Croft Preservation provides standards-led cellar waterproofing surveys that assess internal conditions and external factors, including drainage and ground levels. With regional coverage across Dorset and Hampshire and a strong record of client feedback, it’s clear our focus is on long-term performance rather than short-term fixes.
Call 01202 737739 or arrange a consultation to book a professional assessment and receive clear guidance on whether tanking is the right solution for your property.
[1] Property Care Association (PCA), BS 8102, “waterproofing design should be based on the intended use of the cellar”: https://www.property-care.org/media/rj2agml1/best_practice_guidane__type_a_waterproofing__june_2022.pdf
[2] GOV.UK, Environment Agency, “groundwater flooding can occur days, weeks, or even months after prolonged rainfall”: https://www.gov.uk/guidance/groundwater-flooding
