Dimpled membrane for foundations and perimeter paving: what it is for and how to install it

Waterproofing a foundation is only half the job. The waterproofing also has to be protected from stones in the backfill, soil pressure and constant moisture. That is exactly what dimpled (drainage) membrane is for. We explain why you need it, which way up to lay it and how to install it properly on a foundation and under perimeter paving.

What dimpled membrane is

It is a polymer sheet supplied in rolls, with rows of pressed-out dimples. The dimples create an air gap between the membrane and the surface, through which water drains away and the structure can "breathe", while the sheet itself takes the soil pressure.

The Gartec "Akvashchyt" membrane is polyethylene with a density of 400 g/m², in 2 × 20 m rolls (40 m²). We make 500 and 600 g/m² membranes to order.

What dimpled membrane is for

• Protecting foundation waterproofing. During backfilling, stones and crushed rock easily damage bitumen or sheet waterproofing. The membrane takes the impact instead.
• Drainage. Water seeping through the soil runs down the gap between the dimples to the drainage system instead of standing against the wall.
• Load distribution. The membrane spreads the load from the weight of the backfilled soil evenly.
• Plinths and basements. It protects basement and plinth walls from moisture.
• Perimeter paving and paths. Under perimeter paving and garden paths, the membrane acts as a waterproofing and drainage layer and cuts off capillary moisture from the base.

Which way up to lay dimpled membrane

This is the most common question, and the answer depends on the job:

Where it is laid	Which way the dimples face	Why
Vertically on a foundation, protecting the waterproofing	Dimples towards the wall	The smooth side takes the soil, the gap next to the wall is ventilated
Wall drainage with geotextile	Dimples towards the soil, geotextile on the outside	Water runs down the channels, the geotextile keeps them from silting up
Under perimeter paving, a path or a screed	Usually dimples facing down	A flat top surface for sand, concrete or paving

If in doubt, follow the design or consult the builder in charge of the project: the method depends on how the waterproofing and drainage are designed.

Membrane and drainage: how they work together

Dimpled membrane does not collect water or carry it anywhere by itself; it only provides a path for water to run down the wall. If the soil on the plot is clay or the water table is high, a drainage pipe wrapped in geotextile and bedded in crushed stone is usually laid at the bottom, next to the foundation footing. Water runs down the membrane to the pipe and on to a soakaway or drainage well. On sandy soils, where water drains away quickly, the membrane often serves as a protective layer without separate drainage. Which arrangement to choose is decided by the design or by a builder who knows the soil on your plot.

How much membrane you need

Work out the area: foundation perimeter × height of the below-ground part. Add an allowance for overlaps and folds at the corners, usually 10–15%. The roll is 2 m wide, so a wall up to 2 m high can easily be covered with a single horizontal strip with no lengthwise joints. For example, a foundation with a 40 m perimeter and a depth of 1.5 m needs a 40 m strip plus the overlap allowance, about 44–46 m, i.e. three 20 m rolls trimmed to height.

What you will need for installation

• rolls of membrane with an allowance for overlaps;
• a utility knife and a tape measure;
• a hammer drill and wall plugs with large washers for fixing the top edge;
• a termination (clamping) bar;
• a level or string line to get the top edge straight;
• if needed, geotextile and a drainage pipe if the membrane is part of a wall drainage system.

It is easier to cut the membrane on a flat area next to the trench rather than in the trench itself. Wear gloves: the edges of the sheet can be sharp after cutting.

Installing membrane on a foundation, step by step

1. Prepare the wall. The waterproofing must be completely finished and dry. Clear sharp stones, rebar and debris out of the trench.
2. Cut the sheets. Measure out the membrane with an allowance for a fold at the bottom and for overlaps. It cuts with an ordinary utility knife.
3. Start at a corner. Wrap the membrane round the corner with some extra, so that the joint does not fall right on the corner.
4. Overlap the sheets. Adjacent sheets are overlapped, usually by 10–20 cm, so that the dimples of one sheet nest into the dimples of the other. Position vertical joints so that water cannot get under the sheet.
5. Fix at the top only. The membrane is fixed with plugs and large washers at the top, above the level of the future backfill. Never puncture the waterproofing below ground level: the backfill will hold the membrane in place.
6. Close the top edge. Cover the top edge with a termination bar so that water and soil cannot get behind the sheet.
7. Backfill in layers. Backfill gradually, compacting as you go, without dumping large stones right against the membrane.

Membrane under perimeter paving

1. Remove the topsoil and grade the base with a fall away from the house.
2. Compact the soil.
3. Lay the membrane with the sheets overlapped and turn its edge up onto the plinth so that water does not get into the junction.
4. Spread and compact a sand or crushed stone bed.
5. Lay the surface: concrete or paving slabs. The edge of the perimeter paving is easily held with a concrete kerb, which keeps the base and paving from spreading.

The membrane is used in the same way under garden paths: it carries water away from the base and stops it from being washed out.

Membrane under a garden path

Under garden paths, the membrane carries water away from the base and stops the sand bed from being washed out. The sequence is simple: remove the topsoil to the depth of the path, compact the soil, lay the membrane with overlapped joints, spread and compact the bed, then lay the surface. For example, Brick paving slabs measuring 200 × 100 mm, 40 or 60 mm thick. Edging is set along the sides of the path so that the base and paving do not spread.

Common mistakes

• Fixing with plugs below ground level. Every hole in the waterproofing is a potential leak.
• No top bar. Water and soil get behind the membrane and the gap silts up.
• Joints without an overlap. Water gets into the seams and the protection only partly works.
• Backfilling with large stones. Even a membrane can be damaged if stones are dropped from a height right next to it.
• Membrane without drainage where the water table is high. The membrane channels water downwards, but a drainage system then has to take it away.

Prices and ordering

The price of Akvashchyt membrane is per roll in the catalogue; from 10 rolls it drops automatically. The cost of installation depends on the area, the depth of the foundation and the amount of earthwork, and is worked out by your contractor. Construction companies get wholesale prices, invoices for businesses and delivery to site: see the wholesale terms.