Margin of Error: 1/4“
Most Common Mistakes
- Joists not set at right angle from the wall.
- Joists not placed level.
- Joists not nailed to cover exposed ends of the ledger.
- Not pointing the crown skyward.
Now that the ledger is permanently attached to the wall, you can install the two outermost joists at either end of the ledger. At this time these joists are being used as reference for locating the foundation pier holes. The outer joists are set in place, lines are drawn between them at the prescribed distance from the wall, and on these lines the location of the foundation pier holes are marked. These marks are then transferred down to the ground and the holes dug. The distance from the house to the supporting girder will depend on the size of your deck, the type of wood being used as joists, and the size and spacing of the joists.
This is a rather simple way of locating your pier holes. It usually works well, unless the lot slopes steeply. Some builders use other techniques, including setting up what is known as batter- boards and layout lines. I think this method using the two outer joists as references is perhaps easiest for the do-it-yourselfer.
Choose two of your straightest joists to install at either end of the ledger. As you look down the joists you will notice that they have a small bow, or crown. Almost no piece of wood is ever perfectly straight.
Once installed, this crown should always point up toward the sky. This is called “crowning a joist” In time the bow will settle down if gravity is working with it. So be sure to crown your joists before installing. Don’t worry about cutting the joists the proper length at this time. Let them run wild, since they will be cut when you apply the decking.
When you nail these outer joists to the ledger, do so with the joists covering up the exposed end of the ledger. Have one person nail the joist into the ledger using 3 or 4 16d (penny) HDG (double hot dipped galvanized) nails, while another person supports the other end of the joist in a more or less level position. After the end is nailed to the ledger, drive a temporary 2 x 4 stake into the ground that will hold the floating end level and at a right angle to the ledger. To do this, after one person has nailed the joist to the ledger and while the other person is supporting the other end, place a framing square at the intersection of the joist and the ledger. Once this is approximately 90 degrees, drive a stake in the ground to support the joist there, place a 4’ - 8’ level on the joist to be sure it is level, and nail the joist to the stake. At this point none of these measurements are too accurate. The next step, using a 3 - 4 - 5 triangle, will insure the accuracy of your right angle.
Tip: You can apply waterproofing coats to redwood at twelve- to eighteen-month intervals to eliminate the darkening effect and preserve the beautiful buckskin color.
To insure this accuracy, use the old mathematical formula that states, in a right-angled triangle the sum of the squares of the sides equals the square of the hypotenuse. (In case you have forgotten your high school geometry, the hypotenuse is the side opposite the right angle.) For example, if the two sides of a triangle are 3 and 4 (feet, inches, miles, etc.) and the hypotenuse is 5, then the angle across from the hypotenuse is a true right angle (3 squared + 4 squared=5 squared, or 9 + 16=25). This is also referred to as the 3 - 4 - 5 triangle. This remains accurate if you multiply each side by the same number, for example, 6 - 8 - 10, or even 300 - 400 - 500.
Back to our deck. Our goal is to assure that these two outside joists are at a true right angle to the ledger. Measure out along the ledger from the outside edge 8’ and mark it. Measure along the joist 6’ and mark. Now measure between these two marks. If it is 10’ (we are using a 6 - & - 10 triangle) then you are at a true right angle. If not, readjust your stake and joist until it is exactly 10’ between the marks. Once you have it exact, nail some temporary cross braces from the ledger to the joists at an angle to keep the joists in place. You should now have your two outer joists level, at true right angles from the ledger, and temporarily supported by stakes.