How to Build a Deck Slope for a Runoff

Decks with a Ledger Board

• 1

  Take a 2-by-4-inch board and rest one end on top of the ledger board opposite an over-long, uncut perimeter deck post. Place a 4-foot carpenter's level on top of the board. When the board is level, make a mark on the post.

• 2

  Measure down from this level mark 1/16 inch for every foot of deck distance between the ledger and the perimeter post. This is the amount of deck slope required. For example, if your deck is 8 feet wide, measure down 1/2 inch below the level mark and place a second mark.

• 3

  Measure down from the second mark by the height of the joists and place a third mark. For example, if you're using 2-by-6-inch joists, the actual height is 5 1/2 inches, so place the mark 5 1/2 inches below the previous mark. Place a square against the post at this point and mark a cutting line around each side of the post.

• 4

  Cut the post at the line with a reciprocating saw if you're using a double-beam support system. This is where a support board is attached to each side of the post and the tops of the boards are flush with the top of the post. Do not cut the post and continue on to Step 5 if your deck is designed to have beams attached to the tops of the posts.

• 5

  Measure down from the previous mark by the beam's height and make a fourth mark. Place a square against the post at this point and mark a cutting line around each side of the post. Cut the post at the line with a reciprocating saw.

• 6

  Repeat this process between the ledger board and each perimeter post. Once the posts have been cut to ensure the proper slope, the deck's support system and the top boards can be attached.

Decks with Pier Blocks

• 7

  Hammer a corner stake into the ground next to your house once the excavation and ground preparation for your deck have been completed. Mark the height of the deck on the stake. This measurement is the total of the height of the pier blocks, the support joists and top boards. Tie a string around the stake at this mark.

• 8

  Measure straight out from this stake the width of the deck. Hammer a front corner stake into the ground at this location. Pull the string tight and tie it around the stake. Place a string level on the string and adjust the string up or down on the front stake until the string is level.

• 9

  Add 1/16 inch for every foot of deck to determine the amount of deck slope. For an 8-foot deck, this would be a 1/2-inch slope. Place a mark 1/2 inch below the string on the front stake.

• 10

  Measure from the mark to the ground. To maintain the slope from the back of the deck to the front, the measurement must equal the height of the deck as calculated in Step 1. If it does not, use a shovel to remove enough dirt until it equals the deck's height. Set a pier block at each of these corner locations.

• 11

  Repeat the process at the opposite end of the deck, then evenly space pier blocks between the back corner and front corner blocks, according to the deck's design. Repeat the process to ensure the proper slope between the back and front corresponding pier blocks. Once the pier blocks are in place with the correct ground slope, the joists and top boards can be installed.