Check Deck Frame Level and Square
Before you install a single decking board, take time for this final frame inspection. Every high joist, low spot, or out-of-square corner you catch now is infinitely easier to fix than after decking is installed. This is your last chance to ensure a flat, professional-looking deck surface.
Quick Summary
20-45 minutes
1/4" per 10 feet
Joist-to-joist flush
Why This Final Check Matters
Decking boards span across your joists and reveal every imperfection in the frame below. A joist that's 1/4 inch higher than its neighbors creates a visible hump in every board that crosses it. A frame that's racked out of square shows up as tapered gaps at the deck edges.
The time to find and fix these issues is now - while the frame is fully accessible and adjustments are straightforward. Once decking covers the structure, fixes become much more difficult and often require removing boards.
Inspection Timing: If your jurisdiction requires a framing inspection, schedule it before this step. The inspector may catch issues you'd miss and require corrections before approving decking installation.
Tools for Frame Inspection
- 4-foot level - For checking individual joists and short spans
- 8-foot level or straightedge - For checking across multiple joists at once
- 25-foot tape measure - For re-checking diagonals
- String line - For checking long spans where levels won't reach
- Pencil or chalk - For marking problem areas that need attention
- Shims - Plastic or metal structural shims for adjustments
Complete Inspection Checklist
Check 1: Level Across Multiple Joists
This test reveals joists that are higher or lower than their neighbors - the most common cause of uneven decking surfaces.
- 1.Lay your longest level or straightedge perpendicular to the joists
- 2.Position it so it spans at least 3-4 joists at once
- 3.Check the level at the ledger end, middle, and beam end of the deck
- 4.Rock the level/straightedge - if it rocks, a joist is high
- 5.Mark any high or low joists with a pencil note
Pro Tip: An 8-foot aluminum straightedge (available at drywall supply stores) is ideal for this check. It's lightweight, won't warp, and spans enough joists to reveal problems.
Check 2: Level Along Each Joist
This test catches joists that dip or rise along their length - often caused by lumber that's bowed or installed crown-down.
- 1.Place your 4-foot level on top of each joist, along its length
- 2.Move the level from ledger end to beam end, checking at multiple points
- 3.Note any joists that bow up (excessive crown) or sag down
- 4.Sight along the joist top with your eye for a quick visual check
Acceptable Variation
- Good: Slight crown up (1/8" or less) - will flatten under load
- Borderline: 1/4" variation - may show in decking
- Fix required: More than 1/4" - will create visible problems
Check 3: Re-Verify Square
Installing joists and blocking can subtly shift the frame. Re-check that your diagonals still match.
- 1.Measure the diagonal from ledger/rim joist corner to opposite outer corner
- 2.Measure the other diagonal
- 3.Measurements should still match within 1/8"
- 4.If not, identify what shifted and correct before proceeding
Check 4: Inspect All Connections
A thorough inspection of all hardware and fasteners catches issues before they're hidden by decking.
- Joist hangers: Every nail hole filled? No missed holes?
- Ledger connection: All lag bolts tight? Flashing intact?
- Beam-to-post: Bolts tight? No visible gaps?
- Hurricane ties: All installed where required? Properly nailed?
- Blocking: Face-nailed with correct number of nails?
Check 5: String Line for Long Spans
For decks wider than 8 feet, a string line reveals level variations that shorter tools can't detect.
- 1.Stretch a string line from rim joist to rim joist across all the joist tops
- 2.Pull it tight enough that it doesn't sag in the middle
- 3.The string should just touch (or nearly touch) every joist top
- 4.Joists that push the string up are high; gaps below the string indicate low joists
- 5.Check at multiple points along the joist length
Fixing Common Problems
High Joist (1/8" or less)
Slightly high joists can be planed or sanded down to match their neighbors.
Fix: Use an electric hand planer or belt sander along the top edge of the high joist. Work gradually, checking frequently, until it's flush with neighbors.
Significantly High Joist
A joist that's more than 1/4" high indicates an installation issue rather than lumber variation.
Fix: Check that the joist is fully seated in its hanger. Verify the hanger is at the correct height. Look for debris under the joist at the beam. If the joist has excessive crown, consider replacing it.
Low Joist
Low joists need to be shimmed up at the support points.
Fix: At the ledger end, insert structural shims (plastic or metal, not wood) between the joist bottom and the hanger seat. Re-nail the hanger through the shim. At the beam end, shim between joist and beam, then re-fasten.
Crowned or Bowed Joist
A joist with significant bow creates a hump or valley along its length.
Fix: If the crown is up (correct orientation) but excessive, consider replacing the joist. If the joist was installed crown-down, remove and reinstall it correctly. Minor crowns often flatten under the weight of decking.
Out-of-Square Frame
If diagonals no longer match, the frame has racked during joist installation.
Fix: With joists installed, adjustment is difficult. Try loosening blocking and using a come-along or pipe clamp to pull the long diagonal shorter. Re-tighten blocking once square. If the difference is small (under 1/4"), you may choose to proceed and adjust decking at the edges.
Drainage Considerations
While traditional wood decks don't require significant slope (water drains between board gaps), a very slight pitch away from the house is beneficial. This prevents water from pooling against the foundation and helps with overall drainage.
Recommended Slope
- Standard deck: 1/8" per foot away from house (optional but helpful)
- Covered deck: 1/4" per foot for better drainage
- Over waterproofed surface: Follow membrane manufacturer's recommendations
If you want to add slope, now is the time - before decking installation. You can achieve slight slope by shimming the joists at the beam end or by cutting a slight angle into the beam top.
FAQ
How level does a deck frame need to be?
Being within 1/4 inch over 10 feet is acceptable for most residential decks. More important than overall level is that individual joists are flush with each other - high or low joists create visible bumps or dips in the decking.
What if one joist is higher than the others?
For slightly high joists (1/8" or less), plane or sand the top down. For significantly high joists, check hanger installation and joist seating. If the joist has too much crown, consider replacing it with a straighter board.
What if a joist is lower than the others?
Low joists can be shimmed up at the hanger and beam locations using structural shims (plastic or metal). Insert the shim, then re-nail the connection through the shim for a secure fix.
Should a deck have a slight slope for drainage?
Wood decks don't require slope since water drains between board gaps. However, a slight slope (1/8" per foot away from the house) helps prevent water pooling at the foundation and is considered good practice.