Deck Building 4 - Determining Level and Length of Ledger

Margin of Error: 1/4"

Most Common Mistakes

  1. Using badly bowed board
  2. Choosing wrong height or location
  3. Not using redwood, cedar, cypress, or pressure-treated stock
  4. Not following codes

If you are attaching a deck to your existing home, you will probably use a ledger board. You bolt this board to the house and hang the deck on it, thereby attaching the deck securely to the house. If you plan a freestanding deck (not attached to the house), you do not use a ledger. In some areas a deck attached to a house will be taxed, but if it is separated by even an inch or two it will not.

Here we consider the more common approach to attaching a deck to an existing home. If your deck is freestanding, everything discussed here will still apply but the ledger is not bolted to the house. With freestanding decks, added bracing may be needed to stabilize the deck.

Before discussing the ledger itself, let’s talk about its location. The location of the ledger will be determined by the desired level, or height, of the deck and its placement on the house. I assume you have already decided about the deck design and location, so let's look at the ledger’s height and length.

The Level of the Ledger

The level, or height of the ledger determines the height of the deck. Usually the top of the ledger will be 1 1/2" below the final top surface of the deck (because 1 1/2" decking boards will be nailed on top). Sometimes the deck joists, which support the decking or surface of the deck, are installed so that they sit on top of the ledger rather than hang from it.

In that case the top of the ledger will be considerably lower than the final level of the deck (by the dimension of the joist width plus 1 1/2"). Before you install the ledger you will need to decide whether to hang the joists from the ledger or rest them on top. Whenever possible it is best to hang them from the ledger with joist hangers.

No matter what approach you use, you want to be sure the ledger is low enough so that, once the decking is installed, the level of the deck will beat least 1" below the level of the finished floor inside the house.

This is necessary because you naturally want to step down from the house to the deck, rather than trip over it. Also, if the deck is higher than the floor, water runs from the deck to the house. If you never plan to put a door from the house to the deck, the height of the deck is not as important.

Determine the level of the interior finished floor and transfer this level to the outside of the wall where the deck will be built. You can do this by measuring on the inside and outside from a common reference point, such as a windowsill. (Adjust your measurements if the sill slopes toward the outside.) If this is impossible, you may need to measure up from the foundation wall, accounting for the floor joist, sub floor, and finished flooring materials.

Once you have determined this level, mark it on the exterior wall. This first mark will correspond to the level of the finished floor inside. Make a second mark a minimum of 2 1/2” below this initial mark This will allow you to install 1 1/2” decking and still place the level of the deck 1” below the level of the interior floor. You can place the top of the ledger anywhere below this second mark and be safe.

The Length of the Ledger

The length of the ledger equals the total length of the deck, less 3”. This 3” is so that the joists at either end of the deck will overlap the ledger and be nailed to it. The joists are 1 1/2” thick, hence the 3” reduction in length so that the finished deck will be the designated length.

If possible, you will want to have the ledger be one long piece of stock. However, if the length is over 16’ - 18’, it may be hard to find a straight piece that long. In this case, two pieces will work. Always try to use pieces that are at least 6’ - 8’ long.

Location on the Wall

It is important that the ledger be securely attached. Place the ledger at a point on the wall where, once it is attached, the lag screws attaching the ledger will penetrate something solid such as wall studs or floor joists. Ledgers are often placed at the same level as the floor joists of the first floor, and this automatically solves the problem. In this case, the lag screws will penetrate the band joists.

If you are installing the ledger at some other level, see if there is anything solid behind the wall that the lag screws can be screwed into, If there is nothing, you may need to attach the ledger with bolts. First place some blocking behind the wall, to secure it.

Tip: In areas of heavy rain or snow, place a metal flashing on top of each deck board before the deck boards are applied. The metal flashing helps keep water from getting trapped between the decking and the joists and causing rot.

Choosing the Ledger

When selecting a piece for the ledger, be sure it is straight with little bow (a curve in the board). If the ledger is curved, the deck will have a corresponding curve upward or downward. Some minor bow can be forced out as you apply it, but anything more than that (say, 1/2” of curve over 12’ of board) will throw the deck off.

Be sure the board, as with all decking materials, is of redwood, cedar, cypress, or pressure-treated lumber. After you have decided on the final placement of the ledger, in regard to both its height and location on the wall, you are ready to install it.

First, however, check to see if there are any obstructions, such as hose faucets, dryer vents, gas or water pipes, electrical wires, and so on. Any such obstructions may have to be relocated; or, sometimes, the ledger can break, leaving a gap where obstructions occur.

Remember that everything below the ledger will be underneath the deck and therefore less accessible. You may need to call in an electrician or a plumber to rearrange some wires or pipes if you cannot do this yourself. Never play with the electrical or gas system unless you are confident that you know exactly what you are doing.

Mark on the ground the location of any underground pipes or wires before you begin digging the foundation holes so that you can be sure that you will not be disturbing them. If you cannot locate any pipes or wires, dig slowly and be prepared to change the location of the foundation piers if necessary.

Tip: To help keep rain water from running down the wall from flowing into the structure through the lag screw holes, squirt some silicone caulk in the holes before screwing in the lag screws.