A principal element of deck drainage is to control the rainwater that strikes the decking surface. Even though decking is installed with gaps to allow for expansion, most of the water runs along the deck surface and spills over the edge. For this reason, it is essential that the deck framing and decking be set up with an appropriate slope (See Section 6 for details) that is sufficient to allow the water to flow off the deck. Standing water on a deck deteriorates the decking material and can be a slip hazard.
Water flowing across a residential lot is altered to some extent by the construction of a deck. Usually, surface water is altered only slightly if the deck is constructed above grade and does not have "apron" or side closures that divert the water away from it. There are two exceptions: 1. Where a large amount of site drainage flows toward the deck, the homeowner may want to divert it away from the underside of the deck to avoid a standing water problem under the deck; 2. A large deck area may create significant runoff where the decking surface drains over the side of the deck. This will require that the adjacent ground area be sloped to carry the water away from the underside of the deck and the house. A standard rule of new construction is that it may not alter the surface drainage on a lot in a way that diverts more water onto a neighboring property.
Even though most rainwater is collected by the deck surface and drained away over the side of the deck, water does collect under the deck as well. Although this can result in vegetation and algae growth. It is also detrimental to the life of the decking material and, over time, creates a pond effect that allows more water to accumulate under the deck during rainstorms. To counter this problem, homeowners must create a smoothly graded surface under the deck that slopes toward a yard to an extent sufficient to carry water out from under the deck. Commercially available underdeck drainage systems are available for second floor decks, where it is desirable to capture all the water from the decking above to allow patio areas to be used during rainstorms.
Vegetation and Growth Control
Standing water under a deck creates a potential for vegetation and algae growth under the deck. This is detrimental in several ways, including aesthetic, health and deterioration issues. To prevent this, homeowners should rake the ground beneath the deck smooth during construction, creating a slope sufficient to carry water away from the house and underside of the deck and out into the yard. If planting beds are installed adjacent to the deck, it is necessary to create drainage paths through the beds to enable the groundwater to escape. After creating the slope, install black plastic over the ground, and weight it down with ballast stone (either pea gravel or occasional large stones).
Deck Site Plan
Deck drainage provisions may need to be communicated to the local municipality as part of their zoning or building code requirements. The best way for a homeowner to accomplish this is through the use of a deck site plan. This plan is based on the property survey most homeowners receive with their home purchase closing documents, and should be enlarged to a scale necessary to show the extent of the deck and how site drainage will flow under, off, or around it. Municipal officials are keenly interested in verifying that a new deck is not diverting additional groundwater onto a neighboring property.
Minimum slopes for decking and grading are necessary to ensure that water does not collect (or stand), and is drained to other areas where it will not remain on or under the deck or find its way back into the home through infiltration. For decking, set up a slope of 1/8 inch per foot to drain the deck properly. This means that a 12-foot wide deck would require 1 1/2 inches of total slope to cause water to run off the surface. Ground water requires 1/2 inch per foot of slope to drain properly, though water diverted into a lawn is quickly absorbed under most conditions without significant runoff.
Sheet & Swale Drainage
Site drainage around and under a deck can be achieved in two ways: sheet and swale drainage. Sheet drainage consists of a constant slope across the full width of an area, with the water flowing evenly in one direction. Swale drainage consists of a shallow depression (or ditch) that is created to collect adjacent water and channel it around a deck.