How to Install Deck Support Beams
Support beams are the horizontal backbone of your deck structure, transferring the load from joists down through the posts to the footings. Getting this right is critical for a safe, long-lasting deck.
Quick Summary
2-4 hours
Moderate
1-2 recommended
Understanding Beam Function and Sizing
Beams carry the weight of the deck from the joists and transfer it to the posts. They run perpendicular to the joists and parallel to the ledger board. The beam size you need depends on three main factors: the span between posts, the joist span (how far joists extend from ledger to beam), and the type of lumber used.
Most residential decks use doubled 2x10 or 2x12 beams made from pressure-treated lumber. "Doubled" means two boards fastened together, which is stronger than a single 4x board of the same dimensions due to fewer defects spanning the full width.
| Beam Size | Max Span (6' joist span) | Max Span (8' joist span) |
|---|---|---|
| Doubled 2x8 | 6 feet | 5 feet |
| Doubled 2x10 | 8 feet | 7 feet |
| Doubled 2x12 | 10 feet | 9 feet |
Pro Tip: These are general guidelines. Always check your local building code and lumber span tables for your specific species and grade of lumber. Southern Yellow Pine has different span ratings than Douglas Fir or Hem-Fir.
Tools and Materials You'll Need
Essential Tools
- Circular saw or miter saw - For cutting beam lumber to length
- Drill/driver with impact setting - For driving structural screws
- Socket wrench set - For tightening carriage bolts
- 4-foot level - Essential for checking beam alignment
- Heavy-duty clamps - For holding beams in position during fastening
- Tape measure (25'+) - For accurate measurements
Materials
- Pressure-treated 2x10 or 2x12 - Rated for ground contact recommended
- 1/2" x 6" carriage bolts - With washers and nuts, 2 per post minimum
- 3" structural screws - For laminating beam boards together
- Post caps (optional) - Simpson Strong-Tie BC series or equivalent
- Construction adhesive - For additional bond between beam boards
Step-by-Step Installation
Step 1: Select and Inspect Lumber
Choose the straightest boards available at the lumber yard. Beams need to be as straight as possible since any curve or twist will affect the entire deck frame above.
- Sight down the edge of each board - reject any with significant crown or bow
- Look for splits, large knots, or checks that span more than 1/4 of the board width
- Match boards with similar crowns so they can be oriented the same direction
Step 2: Measure and Cut Beam Boards
Measure the distance between the outer edges of your end posts and add the desired overhang on each end. Most beams extend 1.5 to 12 inches past the posts, depending on your design.
- 1.Measure post-to-post distance at the top of the posts
- 2.Add overhang amount for each end (typically 1.5" minimum past post face)
- 3.Cut both beam boards to the same length - measure twice, cut once
- 4.Label the boards if you have multiple beams
Step 3: Position First Beam Board
This is where helpers really earn their keep. Lifted 2x10s and 2x12s are heavy and awkward to position precisely.
- 1.Orient the crown - Place the beam with any crown (curve) facing up
- 2.Lift onto posts - Rest the board on top of the posts or against the side
- 3.Align with marks - Position the top edge at your height reference marks
- 4.Clamp temporarily - Secure the board to each post with C-clamps
Safety Warning: Never work alone when lifting beams. A 16-foot 2x12 weighs over 70 pounds and is very difficult to control. Use stepladders or scaffolding for safe access, never stand on the posts themselves.
Step 4: Check Level and Adjust
Before adding the second beam board, verify the first one is level. This is your last chance to make easy corrections.
- Place a 4-foot level on top of the beam at multiple points
- For longer spans, use a string line or laser level
- If off-level, shim under the low end or trim the high post
- Verify the beam is plumb (vertical) against each post
Step 5: Add Second Beam Board (Sandwich Method)
The sandwich method is the strongest way to connect beams to posts. The post is captured between the two beam boards, creating a mechanical connection that resists lateral forces.
- 1.Apply construction adhesive to the face of the first beam board
- 2.Lift the second board onto the opposite side of the posts
- 3.Align the top edges of both boards precisely
- 4.Clamp the assembly tightly at each post location
Pro Tip: For sections of beam between posts, drive 3" structural screws every 16" in a zigzag pattern. This lamination creates a stiffer, stronger beam than two loose boards.
Step 6: Install Carriage Bolts
Carriage bolts provide the structural connection between the beam and posts. This is a critical safety connection - don't skimp on hardware.
- 1.Drill bolt holes - Use a 1/2" spade bit or auger bit to drill through both beam boards and the post
- 2.Stagger the bolts - Place one bolt 2" from the top edge and one 2" from the bottom, not directly above each other
- 3.Insert carriage bolts - Tap them through from one side with a hammer
- 4.Add washers and nuts - Place a washer against the wood, then thread on the nut
- 5.Tighten firmly - Use a socket wrench to snug the nut until the washer just begins to compress the wood
Don't Over-Tighten: Stop when the carriage bolt head is fully seated and the washer on the nut side just touches the wood. Over-tightening crushes the wood fibers and weakens the connection.
Common Mistakes to Avoid
Undersized Beams
Using beams that are too small for the span is a structural failure waiting to happen. The deck may feel bouncy and eventually sag.
Solution: Always consult span tables in your building code. When in doubt, go one size larger.
Notching Posts Too Deep
Some builders notch posts to create a ledge for beams. Deep notches remove too much material and weaken the post.
Solution: Use the sandwich method or post caps instead. If you must notch, never remove more than 50% of the post width.
Relying on Nails Only
Nails can work loose over time and don't have the shear strength needed for beam-to-post connections.
Solution: Always use through-bolts or approved structural screws. Nails are not acceptable for this connection per modern building codes.
Poor Bearing on Posts
If the beam doesn't make full contact with the post top or side, load transfer is compromised.
Solution: Ensure posts are cut flat and level. Use shims if necessary, but only code-approved materials (not softwood scraps).
FAQ
Should I use 2x10 or 2x12 for deck beams?
It depends on your span. For posts up to 8 feet apart with typical joist spans, doubled 2x10s work. For longer spans or heavy loads, doubled 2x12s are safer. Check your local code's span tables for your specific lumber species.
How do I attach beams to posts?
The sandwich method (post between two beam boards) with carriage bolts is the strongest approach. Alternative options include post caps and through-bolts or approved structural screws. Never use only nails.
How far should beams extend past posts?
Minimum 1.5 inches past the post face for proper bearing. Maximum cantilever is typically 1/4 of the span between posts. For an 8-foot span, the beam can extend up to 2 feet past the end post.
Do I need to notch posts for beams?
No, and it's generally not recommended. Notching weakens the post. Modern codes prefer beams that sit on top of posts (with post caps) or sandwich the post. If you notch, never remove more than 50% of the post cross-section.