Dimpled Drainage Sheet

What a dimpled drainage sheet does

A dimpled drainage sheet — also called a dimple board or dimpled membrane — is an HDPE sheet pressed with cylindrical or semi-cone dimples. The studs hold a continuous air gap against the structure, so water collected behind a foundation wall drains down to a footing drain instead of building hydrostatic pressure, while the sheet protects the waterproofing membrane behind it. Liquid and gas pass freely between the dimples, which is why a dimple board for wall drainage is the standard detail on basement and foundation walls.

How the dimple board is made

The board is made from high-density polyethylene, the same impermeable resin used in geomembranes, so it combines a drainage function with a waterproofing function in one sheet. The HDPE is extruded into a flat film and then vacuum- or press-formed so that rows of cylindrical or semi-cone dimples are raised across the surface; the depth of those dimples sets the size of the drainage gap. Because the polymer is HDPE, the finished dimpled membrane is anti-aging, corrosion-resistant and root-resistant, giving it a long service life buried against soil or concrete.

Dimple board for foundation and basement walls

As a foundation dimple mat the sheet is fixed dimples-toward-the-wall, creating a drainage cavity that carries seepage to the perimeter drain; this is the dimpled membrane for basement walls detail used for both new build and remedial damp-proofing. The same dimple board waterproofing membrane works on roof gardens, planters and green roofs, where it stores a little water at the dimple base while draining the excess. A nonwoven geotextile facing can be bonded on so the dimple drainage mat filters soil fines and will not clog.

Installation and connection

The sheet is light and flexible, so a single worker can roll it out and fix it fast to a wall or deck — there is no heavy equipment to bring on site. On a basement or foundation wall drainage mat install the dimples toward the structure and lap the edges so the drainage cavity stays continuous; at the base, the cavity should discharge into a footing or perimeter drain. For a geotextile-faced board, keep the filter fabric facing the soil so fines are screened out before water reaches the dimples. On green roofs and planters the board is laid dimples-down over the waterproofing as a combined drainage and protection layer.

Typical applications

Plain or geotextile-faced, the board serves foundation waterproofing, basement and inside-and-outside basement walls, garage-roof and roof-garden greening, vertical greening, football pitches and golf courses, plus municipal works such as airports, road embankments, subways, tunnels and landfills. In water-conservancy and traffic projects it acts as a seepage-control and slope-protection layer for reservoirs, dams, roadbeds, railways and highways. Across all of these the dimple drain board does the same job: it relieves water pressure and protects the layer behind it.

Range, alternatives and specification

The sheet is supplied plain or geotextile-faced, in a range of dimple heights and compressive grades for foundation waterproofing, basement walls, tunnels, landfill seepage protection and roadbed or slope protection layers. Because there is no single public standard for dimple boards, SIGMA dimple height, thickness, compressive load and discharge capacity are confirmed against production data — never guessed. For a punched 3D drainage core with a similar function see our geonet; where a stiffer pipe is needed to carry the collected water see our rigid permeable drain pipe; and to gather the drained water at the base of the wall pair it with our corrugated drainage pipe. Browse the full drainage range for related products.