Dealing with Failed Waterproofing Systems 2

Case Study: Dealing with Failed Waterproofing Systems 2

Job type: Waterproofing buildings
Location: North West, UK

Series of new houses (semi’s) with concrete basements and brick cavity wall construction built over (we’ve remedied 2x houses).
Basements tanked externally with a sheet membrane barrier tanking system.

Internally direct plastered walls were damp at high level - pattern of dampness indicated water ingress at high level.

Issues investigated, assessed and remedied by Trace.

Most water ingress occurs into basements at low level, usually because water perches and pressures against the structure at low level (read this regarding perched water table).

This was an interesting case because it was a high level issue, so what was going on?

Well, concrete basements are ideal in that concrete is an inherently water resistant material, but no one wants to see the concrete structure, so a common design is for concrete retaining walls to stop just below ground level, and brickwork is built on top (so you just see brick).

This means that a small % of the below ground waterproofing is installed onto permeable masonry (not concrete), AND the membrane is usually connected to the DPC/cavity tray above ground, so you have a fiddly detail where the outer leaf of brick is built on top of the tanking membrane, and the membrane runs vertically upwards in the cavity.

This isn’t an area where you want water pressure to come to bear, because if it does and defects are present, the result will be ingress. This is what was happening here.

Trace investigated by digging down externally, and what we found was that the ground came quite a lot higher than the concrete retaining wall (about 375mm).

Basement structures are usually backfilled around with granular (free draining stone) which is to encourage any water to drain straight down, rather than pressuring on the structure. Here topsoil had been laid above the stone. Over time soil had washed down into the stone, essentially filling in the gaps.

Water could then stand/perch on this and pressure on the wall. It was moving through the tanking and brickwork formed above the concrete, but just below ground level.

To remedy we:

  • Excavated around the top of the basement structure down to a level well below the top of the concrete wall, also forming a number of deeper areas (soak-aways) around the perimeter
  • Added geo drainage membrane over for protection
  • Added land drains linking to the deeper soak-aways
  • Backfilled with granular stone

Simplistically we just addressed the drainage details externally and fixed the problem.

External repairs like this aren’t common, but it can be a practical solution as long as there is understanding of what such an approach does and does not achieve.

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