How to Install Deck Rim Joists
Rim joists form the perimeter frame of your deck, connecting the ledger board to the beam and defining the deck's exact footprint. Getting this step right ensures a square, solid foundation for all the joists and decking that follow.
Quick Summary
1-2 hours
Easy to Moderate
Squareness
Understanding Rim Joist Anatomy
A deck frame has three types of rim joists that work together to create the perimeter box. Understanding each one's role helps you build a stronger, squarer frame.
End Rim Joists (Side Rim Joists)
These run perpendicular to the ledger and house, connecting the ledger to the outer rim joist. They define the width of your deck and carry the load of all field joists that connect to them with hangers.
Outer Rim Joist (Header Joist)
This runs parallel to the ledger board at the far edge of the deck, sitting on top of the beam. It connects the two end rim joists and provides a termination point for all field joists.
Ledger Board (House-Side Rim)
Already installed in a previous step, the ledger functions as the fourth side of your perimeter frame, anchored to the house.
Pro Tip: All rim joists should be the same size lumber as your field joists - typically 2x10 for most decks. This ensures the top surfaces are flush, which is essential for flat decking boards.
Tools and Materials You'll Need
Essential Tools
- Circular saw - For cutting rim joists to length
- Drill/driver - For structural screws and pilot holes
- 25-foot tape measure - For measuring diagonals during squaring
- Speed square - For marking square cuts and checking corners
- 4-foot level - For verifying level across the frame
- Clamps - For temporarily holding rim joists during fastening
Materials
- Pressure-treated 2x10 or 2x8 - Same size as field joists
- 3" structural screws - For attaching rim joists to ledger and each other
- Corner brackets (A35 or similar) - For ledger-to-rim joist connection
- Hurricane ties - For securing rim joists to beam
- Joist hanger nails (1-1/2") - For installing hardware
Step-by-Step Installation
Step 1: Calculate End Rim Joist Length
The length of your end rim joists determines the deck's depth from house to outer edge. Measure carefully to ensure both sides are identical.
- Measure from the outer face of the ledger to the outer edge of the beam
- Subtract 1.5 inches (the thickness of the outer rim joist that will cap the end)
- For a flush outer edge, this dimension should match at both ends of the ledger
Example: If the distance from ledger face to beam outer edge is 10 feet (120"), your end rim joists would be 120" - 1.5" = 118.5 inches long.
Step 2: Cut the End Rim Joists
Select straight lumber for rim joists - any bow or curve will make squaring the frame difficult.
- 1.Mark your cut line using a speed square for a perfect 90-degree cut
- 2.Cut both end rim joists to exactly the same length
- 3.Stack them together and verify the ends align perfectly
- 4.Label them "Left" and "Right" based on your view from the house
Step 3: Attach End Rim Joists to Ledger
The connection at the ledger must resist both downward loads and outward pulling forces. Use approved hardware for this critical joint.
- 1.Position the rim joist - Butt the end of the rim joist against the end of the ledger
- 2.Verify alignment - Top edges must be flush; outside faces should be flush
- 3.Install corner bracket - Position an A35 or similar bracket in the inside corner
- 4.Nail the bracket - Use all nail holes with proper joist hanger nails
- 5.Add structural screws - Drive two 3" screws through the rim joist into the ledger end
Pro Tip: Pre-drill the screw holes to prevent splitting, especially within 3" of the board end. Use a bit slightly smaller than the screw shaft.
Step 4: Secure End Rim Joists to Beam
The beam end of each rim joist rests on top of the beam. This connection needs to resist uplift from wind and the lateral forces that could push the deck away from the house.
- Position the rim joist so it sits squarely on top of the beam
- The outer face should be flush with the outer edge of the beam (or slightly inside)
- Install a hurricane tie on each side of the rim joist where it meets the beam
- Alternatively, toe-screw with at least three 3" structural screws
Step 5: Measure for Outer Rim Joist
The outer rim joist runs between the ends of the two side rim joists. Its length determines the deck width, and precision here affects squareness.
- 1.Measure between the outer faces of the two end rim joists
- 2.This should match the length of your ledger board
- 3.If it doesn't match, adjust the end rim joists before cutting the outer rim
Step 6: Install Outer Rim Joist
The outer rim joist caps the ends of the side rim joists and sits on top of the beam.
- 1.Position on beam - Set the outer rim joist on the beam, spanning between the end rim joists
- 2.Check flush - Top edges must be flush with the end rim joists
- 3.Fasten to end rim joists - Drive three to four 3" structural screws through the outer rim joist into the end of each side rim joist
- 4.Secure to beam - Install hurricane ties or toe-screw to the beam below
Important: Don't fully tighten all fasteners yet. You'll need some play to square the frame in the next step. Just snug them up enough to hold the frame together.
Common Mistakes to Avoid
Using Undersized Lumber
Rim joists carry significant loads where field joists connect. Using 2x8 rim joists with 2x10 field joists creates a weak point.
Solution: Always use the same size lumber for rim joists as for field joists. The tops must be flush for proper decking installation.
Skipping Corner Hardware
Relying only on screws at the ledger corners leaves the deck vulnerable to pulling away from the house.
Solution: Install corner brackets (A35 or similar) at every rim joist-to-ledger connection. These provide mechanical resistance to lateral forces.
Not Checking for Square
Installing field joists in an out-of-square frame causes problems that compound with every board you install.
Solution: Always check diagonals before finalizing rim joist connections. The next step in your checklist covers this in detail.
Misaligned Rim Joist Heights
If rim joists aren't flush with the ledger at the top, decking boards will have gaps or sit unevenly.
Solution: Check level across each corner as you work. Use a straightedge to verify tops are flush before fastening.
FAQ
What size lumber should rim joists be?
Rim joists should match your field joists - typically 2x10 or 2x8 for residential decks. This ensures flush top surfaces for flat decking installation. Never use smaller lumber for rim joists than for field joists.
How do I attach rim joists to the ledger?
Use corner brackets (Simpson A35 or similar) plus structural screws driven at an angle. The bracket handles lateral forces while screws provide additional holding power. Fill all nail holes in the bracket.
Do rim joists need joist hangers?
Not traditional joist hangers. At the ledger, use corner brackets. At the beam, use hurricane ties or structural angles. The outer rim joist is face-fastened to the ends of the side rim joists with structural screws.
How do I know if my deck frame is square?
Measure the diagonals from corner to corner in both directions. If both measurements are identical, your frame is square. Even a 1/2" difference means you need to adjust before installing field joists.