How to Mount 4x4 Posts on Deck Corners

Written by mariah elaine
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How to Mount 4x4 Posts on Deck Corners
Mount posts firmly for a strong railing system. (deck with a view image by Bruce Shippee from

The vertical 4-by-4 posts are the major support pieces for a deck railing system. There are several ways to mount them, though your local building codes may have very specific requirements. Usually, the corner posts are mounted in the same manner as the surrounding posts. With careful planning, attention to detail and the right hardware, you can correctly mount your supports. Your secure posts will be the start of a strong and safe railing system for your deck.

Skill level:

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Things you need

  • Tape measure
  • Pencil
  • Carpenter's square
  • Circular saw
  • Hammer
  • Chisel
  • Post level
  • Clamps
  • Power drill
  • Countersink bit
  • Drill bits
  • Galvanised or stainless steel lag screws
  • Socket wrench

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  1. 1

    Measure the width of the rim joist plus the decking thickness and make note of the total number.

  2. 2

    Measure the distance obtained in Step 1 along the face of a 4-by-4 post, starting at the end of the post. Mark the endpoint with a pencil.

  3. 3

    Make a straight line across the post at the mark, using the carpenter's square and pencil.

  4. 4

    Draw a 1½-inch line down each side of the post from each end of the line across the face of the post, using the carpenter's square and pencil. Extend the lines vertically, along the side and down to the bottom of the post.

  5. 5

    Set the depth of the circular saw to 1 1/2 inches, following the manufacturer's instructions.

  6. 6

    Cut along the straight line on the face of the post, using the saw and the carpenter's square as a guide.

  7. 7

    Set the circular saw blade to its maximum depth, following the manufacturer's instructions.

  8. 8

    Cut along the vertical line up to the horizontal cut, using the circular saw.

  9. 9

    Pound on the notched material to break it free, using the hammer. Clear away any remaining material, using the hammer and chisel.

  1. 1

    Attach the post level to a 4-by-4 post. Set the post on the edge of the longest side of the deck. Place the notch against the rim joist and align it flush with the corner of the deck.

  2. 2

    Plumb the post with the post level and clamp it in place, using the clamps.

  3. 3

    Trace a reference line along each side of the tail piece onto the rim joist, using the pencil.

  4. 4

    Remove the clamps and lay the post on a level surface with the notched side down.

  5. 5

    Drill two staggered countersunk holes in the face of the tail piece, using the power drill and countersink bit. Drill a pilot hole through the centre of each counterbore, using the power drill and drill bit.

  6. 6

    Reposition the post on the deck, aligning it with the tracing lines and clamp it securely in place. Insert a lag screw in each hole and tighten firmly, using the socket wrench.

Tips and warnings

  • Mitre the bottom of the post for a decorative and finished look.
  • Attach your posts with carriage bolts, washers and nuts for a stronger connection.

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