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How to Calculate Outdoor Deck Pitch

Outdoor decks make great places to throw parties or just to enjoy the fresh air. However, if your deck doesn't have enough pitch, it can weaken your house by letting rainwater puddle against the wall and rot it. You may barely notice the pitch or slope of a deck, but if your deck has tongue-and-groove flooring or other solid flooring without drain spaces between the boards, the pitch should be at least 1/16 inch lower for every foot in width, to encourage rainwater to run away from the house. Measure the pitch to be sure.

Calculate the pitch of your deck to make sure it drains rainwater
  1. Lay a straight board across the deck perpendicular to the house, edge up, with one end against the house. If the board is as wide as the deck, that's ideal but a shorter board will do as long as it's not warped and you measure carefully. Sight along the edge to make sure it's not warped. If you don't have a board, tie a string to a brick or book to weight it down, place the weight on the deck against the house so the string starts from underneath the weight and ask someone to hold the string tight at the outer edge of the deck.

  2. Lay a carpenter's level on the top edge of the board or hold it against the string, barely touching. Raise the end of the board or the string that's away from the house until the level shows perfectly level. Measure from the surface of the deck up to the string or the bottom of the board in inches, using a measuring tape. Take the measurement at the edge of the deck or at the end of the board if it's not as long as the deck.

  3. Measure the distance in feet from the house wall to where you measured the string or board.

  4. Divide the first number by the second number. For example, if the string was 1 inch off the deck and you measured it at the edge of a deck that was 10 feet wide, you'd divide 1 by 10 to get 1/10. The fraction is the pitch of the deck, expressed as inches per foot.