If you visit any building site while basements are under construction, you will see the external walls clad in a tough, black or brown plastic in rolls, covered in raised bumps (like an egg carton).
Builders call it an "egg-crate" membrane. Its technical name is High-Density Polyethylene (HDPE) Drainage Membrane. The biggest misconception is that many owners (and even some contractors) believed this plastic is the waterproofing. Huge mistake! The dimple board alone waterproofs nothing (the roll joints leak). Its role is purely protective and drainage.
Why should you never backfill a basement without one?
When the bulldozer dumps soil, the mix contains stones, scrap metal, broken bricks and roots. The tough plastic of the dimple board absorbs impacts and scratches, acting as "armour" that protects the delicate bituminous or cementitious waterproofing (or XPS insulation boards) behind it.
Here lies the design magic. The raised bumps ("dimples") create an air gap between the soil and the wall. When rainwater percolates through the soil and reaches the wall, it doesn't stick to the waterproofing. It finds the gap created by the bumps and, by gravity, falls rapidly downward! Where does it end up? Right inside the perforated drainage pipe (French drain) at the base of the foundations.
This is the question that has caused the most arguments on building sites. The correct answer depends on the type of membrane you bought:
The general rule: the bumps (dimples) face the wall (or the XPS insulation if fitted). This creates the air gap (approximately 8 mm) that lets the wall "breathe" and water run downward. The smooth side faces the soil.
In recent years, the premium solution is membranes with a white fabric (geotextile) bonded onto the dimples. In this case, the fabric side faces the soil! The fabric filters the mud, clean water passes through the dimples and drains downward, and the smooth side rests against the wall protecting it.
Many install the membrane but leave the top open (where the soil line meets the ground floor). What happens then?
Soil, mud and pebbles slip behind the membrane, clog the air gap and neutralise the entire system.
At the top, the membrane must be fixed with special plug fasteners and "sealed" with a dedicated metal or plastic termination profile. This profile acts as a "lid" that prevents debris from falling behind the membrane.
Our basement is ready, coated in thick bituminous emulsion. The soil is about to go in.
We tell the contractor to dump the soil directly onto the bitumen to save the €100 cost of the membrane roll. Stones scratch the bitumen in 10 places. In winter, the wet soil sticks permanently to the wall (holding moisture like a sponge). Water finds the scratches and enters the basement. We built a perfect waterproofing job and destroyed it in the last five minutes.
We unroll a composite dimple board with geotextile all around (fabric facing the soil). We seal the top with the termination profile. The bulldozer dumps tonnes of soil. The plastic absorbs all the shock. In the rain, water passes through the fabric, races down between the dimples and into the drainage pipe. Our wall doesn't even know it's raining!
Final Verdict: The dimple board is not your waterproofing - it is its bodyguard. Never allow backfilling without carefully installing this cheap but absolutely critical membrane. It keeps the soil away, drains water fast and ensures your waterproofing lasts for decades.
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