PRE
Pre-applied
Fixed before the pour, on the blinding or against the shoring; the concrete is cast against it.
- Shoring or retaining face (soil side)
- Pre-applied membrane
- Wall concrete cast against the membrane
▽ +09.00Comparison
Pre-applied vs post-applied below-grade waterproofing
The question is timing and position: is the membrane installed before the concrete and cast against, or applied to the outer face after the pour? The site decides before the material does.
The two options
POST
Post-applied
Applied to the outer face of the cast concrete, then protected before backfilling.
- Backfill
- Protection board or drainage layer
- Post-applied membrane
- Primer
- Cast and cured concrete wall
System pageBituminous membranes
A basement wall from the outside in. Illustrative drawing, not to scale.
The key difference
With pre-applied waterproofing, the membrane goes in before the concrete: on the blinding under the raft, or against the shoring where there is no working space behind the wall. Fully bonded types are made to bond to the concrete poured against them, so water cannot travel sideways between membrane and structure.
With post-applied waterproofing, the membrane goes on the outer face once the formwork is struck and the surface prepared — which needs access to that face — and it is protected before backfilling. Often both meet in one building: pre-applied under the raft, post-applied on the walls.
Criterion by criterion
Descriptive criteria, not scores: each cell states the usual behaviour; final values follow the product TDS and the approved specification.
| Criterion | PREPre-applied | POSTPost-applied |
|---|---|---|
| Working space | Needs no working space behind the wall; suits tight sites and shored excavations. | Needs access to the outer face of the wall. |
| Construction sequence | Part of the pour sequence: installed and inspected before rebar fixing and casting. | After striking the formwork, repairing the concrete and sealing tie holes. |
| What it is | Often HDPE-based sheets with an adhesive, protective layer; sometimes sheets on the blinding under a protection screed. | Torch-on bituminous sheets, liquid membranes or other products, per the specification. |
| Water pressure & negative side | On the water (positive) side, against the structure. | On the water (positive) side. |
| Protection requirement | Exposed to damage from rebar fixing and foot traffic before the pour; inspected and repaired before casting. | Needs a protection board or drainage layer before backfilling. |
| Detailing | Penetrations, pile heads and membrane edges are prepared with system accessories before the pour. | Penetrations are treated directly on the cast wall. |
| Inspection & testing | The last chance to inspect is before the pour; damage after that is hidden. | Visible until backfilling, so inspection and repair are easier. |
| Repairability | After the pour it cannot be reached from outside; remedies come from inside, by injection or crystalline treatment. | After backfilling it is also hard to reach, so remedies are internal too. |
| Typical applications | Under rafts and ground slabs, and walls cast against shoring on tight sites. | Walls in open excavations, and roofs of underground structures. |
When to choose which
PRE
Choose PRE when
- No working space behind the wall (site boundary or shoring walls).
- Under the raft and the ground slab.
- Where the membrane must bond to the concrete to stop water travelling sideways.
POST
Choose POST when
- An open excavation with access to the outer face.
- Where the membrane should be inspected and repaired before backfilling.
- Roofs of underground structures and tanks.
Either way
- Where the two meet, a transition detail is designed and inspected to the manufacturer’s instructions.
- Construction joints and penetrations need their own details in both cases.
- Below-ground design starts from the water table and the risk of a leak, and often combines more than one type of protection.
General educational content; final values and requirements follow the manufacturer’s TDS and the approved project specification.
Common questions
Is pre-applied always better?
No. It is the answer where there is no working space, and under the raft. Where there is access, post-applied lets the membrane be inspected and repaired before backfilling.
What if a leak appears after the pour or the backfill?
The water path is traced, then treated from inside: crack and joint injection, then a negative-side system such as crystalline or cementitious waterproofing.
References
Standards that cover these materials — named to show what they cover, not to claim that any product complies.
- BS 8102Protection of below-ground structures against water ingress: protection types A, B and C and their design.
Technical review
Still weighing the options?
Send the location, area and surface condition, and our engineers will propose the right system after a review.