Dimple Drainage Board
Dimple drainage boards, also known as dimple mats and drain membranes, provide below-grade hydrostatic pressure relief to extend waterproofing membrane lifespans. They are also used for green roofs, retaining walls and plaza decks.
They keep moisture away from a foundation and building site to prevent water, soil vapor and harmful chemicals from entering the structure.
Benefits
Dimple drainage mats are much less costly than traditional aggregate or sand water drainage layers, and they can be placed directly over waterproofing membrane systems. They are also easy to install, lightweight and require no highly skilled labour or special tools to cut or shape.
These HDPE sheets are the first line of defense against moisture, directing groundwater towards footing drains and keeping backfill materials from touching foundation walls. Construction moisture and air vapor from inside the building can condense, and fall freely through the dimples and down to perimeter drainage systems as well.
As an added benefit, dimpled membranes can span the unavoidable cracks in concrete foundations more effectively than fluid-applied membranes. This reduces callbacks, saves time and profit, and protects the structural integrity of the structure.
While perforated drain sheets are used for all types of applications, they’re especially effective for retaining walls, green roofs, planters and commercial projects with a basement. They also make a great choice for blindside and tunneling work where it’s impossible to place aggregate or gravel, and they have a high compressive strength that’s superior to air gap membranes. In fact, the most commonly used product, Polyflow®, is rated to more than 40,000 psf. This makes it a good alternative to traditional aggregate drainage layer products for all below-grade wall situations.
Installation
Because builders can’t control the quality of their building site (or the weather), they must build in a way that will prevent Plastic Dimple Drainage Board below-grade leaks that compromise the integrity of the foundation. The best way to do that is with dimple board waterproofing solutions, such as Polyguard Dimple Board Wall Drainage Systems.
Like air gap membranes, dimpled membranes span the unavoidable cracks and joints in concrete walls, directing moisture to basement interior drains. They also protect against water seeping from cove joints and through concrete block and poured concrete walls. They work by preventing groundwater from entering the basement, directing it to a footing drain and away from the critical wall/footer cold joint. Dimple boards are also effective at stopping construction moisture, as well as the condensation of water vapor from inside the home.
Dimpled membranes can be installed in any weather, allowing construction to proceed quickly. They can also withstand backfilling, unlike fluid-applied membranes that must be cured before backfilling. Dimple boards are also durable and have an expected lifespan of 50 years or more.
Most dimpled membranes have a non-woven filter fabric on one side, which keeps soil out of the drainage space while letting water flow freely through the perforations. Contractors must carefully lay the membrane, making sure it is properly seated over the concrete pour and that the core flange edges are facing up. The membrane is then secured with a non-woven geotextile to ensure that the seams are sealed before backfilling.
Maintenance
The dimples on the plastic drainage mat promote air flow through the wall, reducing moisture buildup and extending the life of the below-grade waterproofing membrane. This allows for water and harmful contaminants to be drained away from the foundation, preventing damage caused by hydrostatic pressure.
In addition to promoting air flow, the plastic drain board also reduces or eliminates ponding water at the foundation footer. This prevents the foundation from becoming saturated and damaged, and it helps to keep the soil away from the poured walls.
When used with a proper system of drains, the dimples on a dimpled drainage mat move any water that does collect, whether from condensation or a leak, toward a foundation drain. High quality drainage membrane The drains convey the water to daylight, a storm system, or a sump pump, keeping the foundation structure dry.
Compared with aggregate or sand filled systems, a dimpled drain sheet is easy to install and does not require highly skilled workers. The sheets are placed directly over the spray or peel-and-stick waterproofing membrane systems, and they can be attached vertically or horizontally. The sheets are lightweight and can be easily cut to size using simple hand tools, making them a great option for any project. They can even be used on commercial projects requiring drainage up to 50 feet below grade.
Applications
Dimpled drainage membranes are used in a wide variety of applications. Their composition varies between manufacturers, but most consist of a polypropylene or high-density polyethylene sheet molded into a series of raised knobs, or dimples. The membranes’ dimple height, sheet thickness, and compressive strength vary as well. Most contain a non-woven geotextile that ties into the dimples to retain soil particles and keep them from blocking the drainage membrane. Many also incorporate a perforated or needle punched surface, which allows moisture to penetrate the membrane, and many are available with a range of flow rates, permeabilities, and drainage capacities.
Unlike fluid-applied membranes, which must be applied when the weather is dry and clear, dimple membranes can be installed in any conditions. “This allows contractors to work later in the winter and earlier in the spring,” says Greidanus.
In horizontal applications, such as golf greens and athletic fields, or in planters and green roofs, the dimple surface of the drainage membrane acts as a water retention layer for vegetation, while excess water falls through the perforations and is channeled to drains. In vertical applications, such as basement walls or tunnels, the perforated or needle punched drainage membrane carries hydrostatic pressure away from the wall, allowing it to breathe and protecting the waterproofing membrane.
It also keeps moisture vapor and other gases, such as radon, outside the basement, where they don’t cause problems. It can be installed all the way to the base of the footer, where it guides moisture directly into a perimeter drain and away from the critical wall/footing cold joint.
