Foundation waterproofing protects every surface of a building in contact with the ground from water and moisture. It forms a continuous tanking envelope that begins beneath the raft and continues up the outer face of the retaining walls. Any break in that envelope weakens the whole system.
Why it starts at foundation level
Groundwater reaches the structure from below first. If waterproofing is applied only to the wall face and the underside of the foundation is left open, water rises between the blinding and the raft and reaches the slab by capillary action. The result is damp in the ground floor slab and mould beneath the floor finish.
For this reason the membrane is laid over the blinding, the raft is cast on top of it, and the envelope is completed by continuing up the wall face.
Order of work
Blinding and surface preparation. The surface receiving the membrane must be even, free of sharp projections and clean. Stone projections in the blinding puncture the membrane.
Primer. Binds surface dust and lets the membrane adhere.
Horizontal waterproofing. Two layers of bituminous membrane are laid over the blinding with staggered laps. Sufficient overhang is left at the wall line to be joined later to the vertical work.
Protective screed. A screed is placed over the membrane so reinforcement and site traffic do not puncture it.
Raft concrete. Cast once waterproofing and protection are complete.
Vertical waterproofing. After striking the wall formwork the outer face is cleaned, primed and the membrane applied, lapped onto the overhang left in the horizontal work.
Protection and drainage. Drainage and protection board follow, then the drainage pipe and gravel fill at the foot of the foundation.
Weak points of the envelope
Horizontal to vertical junction. The most critical point in the system. Without sufficient overhang left in the horizontal work the joint cannot be made and water enters exactly there.
Wall to raft construction joint. A cold joint forms. A waterstop or swelling profile is required.
Pipe and earthing penetrations. Every element leaving the foundation is a hole. Sleeves and detail tape are needed.
Lift pit. The deepest point of the building, exposed to the highest water pressure. It must be treated as a separate detail.
Upper termination. The membrane must run at least thirty centimetres above ground level and be mechanically terminated.
Where the foundation cannot be reached
With bored pile or diaphragm wall construction the outer face is inaccessible. Bentonite panels or pre applied membrane systems placed in the formwork before casting are used instead. Bentonite swells on contact with water to form a sealing gel.
Frequently asked
Can foundation waterproofing be added to an existing building
Horizontal waterproofing cannot be added later. If the outer wall face can be reached by excavation, vertical waterproofing and drainage can be added. If not, injection and negative side solutions come into play.
Are waterproofing and thermal insulation done together
On the outer wall face waterproofing goes first, thermal insulation over it. The insulation must be of a non absorbent type.
What do the regulations say
Under the Turkish building waterproofing regulation, waterproofing is mandatory for building elements exposed to water and ground moisture, and waterproofing details are expected to be drawn at design stage.
In new build, foundation waterproofing should be planned at design stage. In existing buildings the method is determined by assessing excavation access and water load on site.