How to Tank a Cellar
Cellar tanking can transform a damp, unusable cellar into dry, valuable space. This guide covers everything you need to know — from choosing the right tanking method to ensuring a lasting, watertight result.
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Understanding Cellar Tanking in Suffolk, Norfolk & Essex
Tanking a cellar means applying a waterproof barrier to the internal surfaces — walls and floor — to prevent water from passing through the structure into the cellar space. It is one of the oldest and most intuitive forms of waterproofing: you coat the surfaces with a material that water cannot pass through.
The challenge with cellar tanking is that the waterproof coating is on the inside of the structure, meaning that water pressure pushes against the bond between the tanking and the wall. This is the opposite of what happens with external tanking, where water pressure actually pushes the tanking onto the wall. For this reason, the bond between the tanking material and the substrate is absolutely critical, and surface preparation is the most important factor in determining whether a tanking job will succeed or fail.
Challenges Specific to Cellar Tanking in Suffolk, Norfolk & Essex
Cellars present specific challenges that make tanking more demanding than tanking a purpose-built basement:
Irregular Construction
Victorian and earlier cellars were built with whatever materials were locally available — soft red brick, flint, chalk, stone, or a combination. Walls are often uneven, with irregular mortar joints and areas of rubble fill. This makes it difficult to achieve a consistent bond with tanking slurry, and may require a preliminary rendering coat to create a suitable substrate.
Limited Access
Many cellars are accessed via narrow staircases, hatches, or steep steps. Moving materials, equipment, and waste in and out can be difficult and time-consuming. This can affect both the cost and the choice of tanking system — large membrane rolls, for example, may need to be cut to smaller sizes before being brought into the space.
Low Ceiling Height
Cellars often have lower ceiling heights than basements. Any tanking or waterproofing system adds thickness to the walls and floor, reducing the usable space further. Where ceiling height is limited, thinner systems (such as a cementitious slurry rather than a thick cavity drain membrane with plasterboard) may be preferred to preserve maximum headroom.
Existing Moisture Levels
Cellars are often very damp by the time tanking is being considered. Active water ingress, saturated walls, and standing water on the floor are common. Some tanking products can be applied to damp substrates, but not to actively wet surfaces. The cellar may need to be pumped dry and allowed to stabilise before tanking can begin. Alternatively, a cavity drain system can be installed regardless of the moisture level.
Cellar Tanking Methods in Suffolk, Norfolk & Essex
Cementitious Tanking Slurry
This is the most common tanking method for cellars. A cementitious powder is mixed with water (and sometimes a liquid latex additive) to create a thick slurry that is brushed onto the walls and floor in multiple coats. The slurry penetrates into the pores of the masonry and cures to form a rigid, waterproof barrier.
Typical application involves two to three coats, each applied at right angles to the previous coat to ensure full coverage. A total dry film thickness of 2-3mm is normally required. The slurry must be applied to a dampened (but not saturated) substrate, and each coat must set sufficiently before the next is applied. In cold weather, curing times are extended and additional protection may be needed.
Multi-Coat Render Systems
For cellars with very uneven walls, a multi-coat waterproof render system may be more appropriate than a thin slurry. This involves applying a scratch coat of waterproof render to the wall to create a flat surface, followed by a finishing coat incorporating a waterproof additive. The render system is thicker than a slurry coat (typically 10-15mm total) and can accommodate surface irregularities more easily.
Combination Approach: Tanking Plus Drainage
In many situations, the most reliable approach is to combine tanking with drainage. The walls are tanked to prevent capillary moisture migration, while a perimeter drainage channel is installed at the base of the walls to collect any water that does manage to penetrate (or that enters through the floor) and direct it to a sump pump. This belt-and-braces approach provides a safety net — if the tanking is ever compromised at any point, the drainage system catches the water before it can cause damage to the finished interior.
Surface Preparation for Cellar Tanking in Suffolk, Norfolk & Essex
Surface preparation accounts for a significant proportion of the time and cost of a cellar tanking job, and is the single most important factor in determining the long-term success of the treatment. The steps involved are:
- Remove all existing plaster, render, paint, and coatings back to bare masonry or concrete. Tanking must bond directly to the structural material, not to a secondary layer that may itself be poorly bonded
- Rake out defective mortar joints to a depth of at least 15mm and repoint with a compatible mortar. Loose, crumbling, or sandy mortar will not provide an adequate key for the tanking
- Repair cracks, holes, and voids with a compatible repair mortar. Large voids may need to be filled with brick and mortar before tanking can be applied
- Remove any efflorescence (salt deposits) by brushing and washing. Salts on the surface will prevent the tanking from bonding properly
- Plug any active leaks with fast-setting hydraulic cement (waterstop) before applying the tanking. Water flowing through the wall during tanking application will prevent the slurry from curing and bonding correctly
- Form a coved fillet at the wall-to-floor junction using waterproof mortar. This eliminates the sharp right-angle where tanking is most vulnerable to cracking and water entry
- Dampen the substrate immediately before applying the tanking. A damp surface provides a better key for the slurry than a dry surface, and prevents the porous masonry from sucking moisture out of the tanking before it can cure properly
What to Do After Tanking Your Cellar in Suffolk, Norfolk & Essex
Once the tanking has cured, the cellar is ready for finishing. How you finish the cellar depends on its intended use:
Storage or Utility Use
The tanked walls can be left exposed or painted with a breathable masonry paint. Shelving can be fixed to battens rather than directly through the tanking. A simple floor finish (painted concrete or vinyl) can be laid over the tanked floor. Ensure ventilation is adequate to prevent condensation on stored items.
Habitable Room Conversion
For a bedroom, office, playroom, or other habitable use, the walls will need to be dry-lined with insulated plasterboard, proper lighting and electrical installations fitted, heating provided, and ventilation installed that meets building regulations. You will likely need building regulations approval and possibly planning permission, especially if changing the building's footprint or use.
