Adding a bench to a finished deck can not only make your deck look a lot nicer, but the built-in seating will be a valuable addition to your outside living space all year round.
While this job can be somewhat complicated and time consuming, a do-it-yourselfer with a basic understanding of carpentry should be able to manage it easy enough.
Before you start cutting lumber, check with your local zoning department to see if you need a permit to add a fixed bench to your deck. Also, inquire about local codes for railings if your deck happens to sit 30 inches or higher off the ground.
Tools and Materials: What you'll need:
  • Circular saw
  • Power drill and bits
  • Hammer
  • Measuring tape
  • 2x10 boards
  • 2x4 boards
  • 2x6 boards
  • Galvanized deck screws, 3-inch
  • Carriage bolts, ½-inch

Building a Bench with the Back Attached to a Finished Deck

When working with wood, especially when cutting it, it is very important to wear safety gear. For this job, you should wear safety goggles, hearing protection, work gloves, heavy duty work boots and a NIOSH-approved respirator or dust mask.
1. Cutting the Back and Seat Supports
Back supports should be made according to the following instructions:
  1. Cut the 2x10 boards into 32” lengths. The number you need depends on the length of the bench (supports should be spaced roughly four or five feet apart).
  2. Lay the board flat on a work surface lengthwise.
  3. On the left-hand end, measure along the top of the piece and mark it five inches from the end nearest you.
  4. On the right-hand end, measure along the length of the piece and mark it 15 inches from the right-hand end.
  5. Use a rigid ruler and a pencil to draw a line from the five-inch mark to the 15-inch mark.
  6. Cut along this line with a circular saw.
  7. On the right-hand side of the board nearest you (across from where you made the 15-inch mark), measure 12 inches lengthwise from the end.
  8. Measure in 1 and ½ inches from the 12-inch mark and cut this section away using the circular saw.

Now that you have one back support made, use it as a template for the remainder to save you time.

Every back support requires two 2x4 seat supports. Here’s how to make them:
  1. Cut the necessary number of 2x4s into 24-inch lengths.
  2. Lay one board flat on the work surface lengthwise.
  3. Measure from the right-hand end, the side farthest from you, three inches in and make a mark there.
  4. On that same end, measure 2 and ½ inches away from you on the end and make a mark there.
  5. Use the ruler and a pencil to draw a line from the three inch mark along the side to the 2 and ½-inch mark on the end.
  6. Cut along this line using a circular saw.

Use this seat support as your template for the rest of them.

Before you can start to install the back and seat supports, you need two more pieces per back and seat support – the front leg and the rear brace.
The front leg is a length of 2x4 square-cut to 15 and ½ inches. Make one per support. The rear brace is a length of 2x4 cut to 12 inches plus the thickness of your decking. Also, make one per support.
2. Installing the Back and Seat Supports
Drill pilot holes in the rear brace and secure it to the deck using carriage bolts. The rear brace should extend above the surface of the deck by 12 inches.
Set the notched end of the back support against and in the center of the rear brace and secure it in place using three-inch deck screws. Rest the flat ends of the seat supports on the overhanging sides of the rear brace, check to make sure they are level, and secure them to the back support using three-inch deck screws.
Position the front leg between the seat support arms at the point where the cut area starts. Secure the leg to the seat support arms using three-inch deck screws and then drive a screw on an angle through the bottom of the leg and into the deck.
Follow these instructions for each support you have, and always remember to pre-drill your holes to prevent splintering the wood.
3. Installing the Seat and Back Slats
Lay a 2x4 board across the seat supports, making sure to keep the board straight. Drill pilot holes and then secure the board to the seat support using three-inch deck screws. Use a piece of ½” thick wood as a spacer and set the next 2x4 in place on the seat support and screw it in place the same way. Continue until the seat is finished.
Follow this same process up along the back support. Once the seat and back slats are in place, finish off the bench by installing a 2x6 board along the top of the back to round off any sharp edges.
All that’s left to do now is paint or stain your new bench to match the look of your deck and to protect it from the elements.

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