Drainage issues can turn a paver patio into a giant puddle. Unilock pavers, made from tumbled concrete that resembles stone, have notches in their edges that help them lock together and prevent shifting. These notches also hold the pavers apart a consistent distance so water can drain between them, but this only works if they are properly installed. Understanding Unilock drainage issues helps you avoid problems.
The type of soil under your Unilock pavers affects drainage, just like it does in a garden bed. Clay soil drains more poorly than loamy or sandy soils. Walkways and patios generally require a base depth of 4 to 6 inches of gravel and sand, while driveways and other areas subjected to more weight require a depth of 8 to 12 inches. In clay soils with exceptionally poor drainage, you may need a 12-inch base for walkways and patios and 18 inches for driveways. The increased depth allows for deeper base materials, which drain better than surrounding clay and help keep drainage from becoming an issue. Add 2 inches, plus the height of the Unilock pavers, to this measurement to find the complete base depth you'll need to excavate.
Barriers to Success
Plastic sheeting prevents proper drainage and you shouldn't use it under Unilock pavers. Although it may seem like a good way to block weeds, it creates a drainage nightmare when water collects on top of the plastic. Don't use plastic sheeting anywhere under pavers. The plastic usually won't prevent weeds anyway, because weeds seeds can sprout in the cracks between pavers. Filling the paver joints with polymeric paver sand prevents most weed issues without hindering drainage. If you have clay soil, you can place a sheet of geotextile or landscape fabric beneath the paver base so the clay doesn't mix with the gravel and sand. Landscape fabric allows water to pass through it, so it won't cause drainage issues.
Start at the Bottom
Selecting the right base materials is key in preventing drainage problems. Alex Burke, of Burke Environmental, Inc., recommends using coarse sand as the top layer of base material because it drains better than stone dust. Install a layer of pea gravel to the depth determined by you soil and usage type, compacting it with a plate compactor. To ensure even compacting, run the compactor over the gravel base after applying each 3 inches of base material. For example, a 6-inch-deep gravel bed will require two passes of the compactor. A 1- to 2-inch layer of coarse sand made for paver installation over the top of the gravel base helps provide drainage directly beneath the Unilock pavers, while also helping to prevent settling issues later.
Making the Grade
If your patio is sloped the wrong way or it is uneven, you will likely have drainage issues, even if you use the right base materials. Aim for a 1 to 2 percent slope away from the house to prevent water pooling issues. Set this slope when you're installing the gravel and sand base. To create the slope using a 4-foot level, attach a 1/2-inch spacer for a 1 percent slope or a 1-inch spacer for a 2 percent slope to one end of the level and then use the level as usual when leveling the paver base. Double check the slope after the pavers are installed to check there are no dips. Remember to slope the pavers away from buildings so the draining water is directed away from the building.
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