Question 1

What is a fillet seal?

Accepted Answer

A fillet seal is a wedge, or cove, of high-strength waterproofing mortar formed along the internal angle where one concrete surface meets another — most often the junction between a wall and a floor slab, but also at the base of upstands, kerbs, plinths and around service penetrations. That 90-degree internal angle is one of the most vulnerable points in any tanking or coating system: it concentrates structural stress and cracking, it is where water collects under pressure, and it forces a membrane or liquid coating to turn a sharp corner where it thins, bridges a gap, or tears under thermal and structural movement. The fillet removes that problem by rounding the sharp corner into a smooth curve, typically 25-50mm radius, so the waterproofing can be dressed over it in one continuous sweep with no stress concentration and no unsupported span. Fillet seals are a defining detail of Type A (barrier) tanking under BS 8102, where a cementitious or epoxy fillet is run continuously around every wall-to-floor and wall-to-wall junction before the slurry or membrane is applied; the fillet both reinforces the joint and creates the geometry the coating needs. The mortar used is usually a rapid-setting, high-strength cementitious repair mortar or an epoxy mortar, and it is often keyed into a chase cut at the junction so it locks mechanically into the structure rather than simply sitting on the surface. Where a junction is actively leaking, the fillet is normally combined with a hydrophilic waterstop or low-viscosity resin injection through the joint so the water path is sealed before the fillet and coating go on. Done well, a fillet seal is the difference between a basement or lift pit that stays dry and one that weeps at the floor line within months. MPS Concrete Solutions forms fillet seals as part of basement tanking, lift pit and car park deck waterproofing across the UK.

Question 2

What is Type A waterproofing?

Accepted Answer

Type A waterproofing, often called 'Barrier Protection,' involves applying a waterproof material to the internal or external surfaces of the structure. This blocks water ingress physically. Examples include external adhesive membranes applied before backfilling (positive side) or internal multi-coat rendering/tanking slurries (negative side). While effective, Type A relies 100% on the workmanship and substrate preparation; a single defect can lead to failure, which is why we often combine it with other systems.

Question 3

What is Type B waterproofing?

Accepted Answer

Type B waterproofing is 'Structurally Integral Protection.' It relies on the design and materials of the structure itself to be water-resistant. This is typically achieved using waterproof concrete (containing hydrophobic admixtures) and waterstops embedded in the construction joints (like hydrophilic strips or metal waterbars). While the concrete itself blocks water, the joints are the weak points, which is why MPS specializes in injection hose systems to seal these joints post-construction if needed.

Question 4

What is Type C waterproofing?

Accepted Answer

Type C waterproofing is 'Drained Protection.' Instead of trying to hold back massive hydrostatic pressure physically, it accepts that some water may penetrate the shell but manages it. It uses a Cavity Drain Membrane (an egg-crate shaped plastic sheet) applied to walls and floors. Water runs behind the membrane into a perimeter drainage channel and is directed to a sump pump station, where it is pumped away. This is often considered the safest method for existing basements as it does not rely on perfect structural integrity.

Question 5

What is 'tanking' a basement?

Accepted Answer

Tanking a basement means applying a continuous waterproof barrier to the walls and floor so that groundwater cannot pass through, effectively turning the below-ground structure into a watertight tank. It is the traditional term for Type A (barrier) waterproofing under BS 8102:2022, and it is usually achieved with a cementitious tanking slurry — a cement-based coating that becomes part of the substrate — or, historically, with bituminous coatings and self-adhesive membranes. Tanking can be applied to the inside face (internal or 'negative side' tanking) or the outside face (external or 'positive side' tanking) of the structure. External tanking is the more robust in principle because it stops water before it reaches the concrete, but on an existing building it needs excavation, which is often impractical or prohibitively expensive, so internal tanking is far more common as a retrofit. The critical challenge with internal tanking is that it has to resist negative hydrostatic pressure — the groundwater is actively trying to push the coating off the wall — so the bond to the substrate is everything. That makes surface preparation absolutely vital: all paint, plaster, render and laitance must be removed back to sound brick or concrete, junctions reinforced with fillet seals, and any live cracks or leaking joints injected with resin before the slurry goes on. Tanking is not the only option under BS 8102 — for habitable space, high water tables or structures where the substrate cannot be guaranteed, a Type C cavity drain membrane system (which manages water to a sump and pump rather than resisting it) is often more reliable, and the most dependable basements frequently combine both for redundancy. Which approach is right depends on the water table, the grade of internal environment required, and the condition of the structure, and it should be specified by a waterproofing design specialist. Costs vary widely with system and size — cementitious tanking is typically the lowest cost per square metre, with full basement projects running into several thousand pounds and up. MPS Concrete Solutions designs and installs BS 8102-compliant basement tanking and cavity drainage across the UK, from single cellars to large commercial basements.

What is a fillet seal?