Waterproofing an underground car gallery

What Grade 3 means

BS 8102:2022 is the British Standard for protecting structures below ground from water. It sets grades by how dry the space must be.

• Grade 1 suits car parking and workshops, where some seepage or damp may be tolerated.
• Grade 2 allows no water penetration, but tolerates damp from condensation.
• Grade 3 accepts no water ingress and no damp areas, and needs ventilation and dehumidification as well.

A plain garage could be Grade 1. A gallery with finished floors, electrics, charging and a valuable collection is not a plain garage, so we treat it as Grade 3.

Types A, B and C, in plain English

• Type A, barrier protection. A membrane or coating applied to the structure, inside or out.
• Type B, structurally integral. The concrete itself keeps water out. It is watertight concrete, with water bars sealing every joint.
• Type C, drained protection. It accepts that some water may pass the structure. A studded membrane catches it in a cavity, leads it to channels and a sump, and pumps it away.

Two forms of protection

Every system has a way to fail. Types A and B depend on workmanship at joints and service entries. Type C depends on pumps and maintenance. So a Grade 3 gallery never relies on one.

NHBC Standards chapter 5.4 calls for a combined system of two types where Grade 3 is required and more than 600 mm of ground is retained. Our approach is a watertight concrete box, a drained cavity inside it, and a membrane on the buried roof under the garden.

BS 8102:2022 also asks for a waterproofing design specialist from the earliest design stage. We appoint one at concept.

Where basements leak

Leaks are rarely random. NHBC data reported by the Construction Index found that tanking defects caused 66% of basement claims, mostly at joints, floors and service openings. Sedgwick reports that water damage claims are rising as large basement builds increase.

So the joints get special care in the design. Water bars go into every construction joint, with re-injectable hoses as a second line. Services are cast into the concrete as it is poured, never cored through it afterwards.

The drained cavity

Inside the concrete box, a studded membrane lines the walls and floor. Any water that passes the concrete runs down behind it into perimeter channels, which fall to a sump.

The cavity is sealed from the room air, so damp never reaches the gallery. Once the box is complete, it is left for a few weeks so any leaks can be found and injected before the cavity drain goes in.

Sumps, pumps and alarms

Every sump has a duty pump and a standby pump that take turns. High-level alarms report to the building controls and to the owner's phone. At least one pump has its own battery backup, and the generator covers the rest. Our section on standby power explains why.

Groundwater, surface water and any foul drainage are kept in separate sumps. Sump lids never sit under a parking bay, so they can be checked without moving a car.

Below sewer level, drainage is pumped. In London, Kensington and Chelsea requires a pumped device against sewer flooding, and says a non-return valve alone is not acceptable.

Servicing and guarantees

BS 8102:2022 asks for a maintenance schedule for drained protection, with the first service at handover. On a Grade 3 gallery we recommend servicing channels, sumps, pumps, alarms and batteries every six months.

Waterproofing guarantees often depend on that annual service. Under BELOW Care it is booked, logged and reported, so the guarantee stays valid. The BELOW Standard sets dry for life as its first principle.