How to Fix a Sagging Deck: Joists, Posts, Ledger, and Joist Hangers

A sagging deck almost always traces back to one of four problems: rotted or broken joists, a failed post or footing, a pulling ledger board, or failed joist hangers. Each has a clear fix.

A sagging deck almost always traces back to one of four problems: rotted or broken joists, a failed post or footing, a pulling ledger board, or failed joist hangers. Each has a clear fix. The key is getting underneath the deck with a flashlight and a screwdriver to probe the wood before you start buying materials.

What You Need

• Joist hanger hardware (LUS or LUS-series Simpson Strong-Tie)
• Joist hanger nails (1.5 inch 10d or manufacturer-specified)
• 1/2-inch hot-dipped galvanized lag screws (3.5 to 4 inch)
• Post base hardware (ABA series or EPB series)
• Pressure-treated lumber (2x10 or 2x8, ground-contact rated)
• Hydraulic bottle jack and blocking

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Step 1: Full Inspection Before You Buy Anything

Change into clothes you don’t mind dirtying and get under the deck. Bring a flashlight and a flat-blade screwdriver or an awl. You are probing for soft wood — a screwdriver should not penetrate more than 1/4 inch into sound pressure-treated lumber. Anything that feels soft, spongy, or crumbles is rotted.

Check these locations in order:

Ledger board: The ledger attaches the deck to the house. Look for dark staining, soft wood, rusted fasteners, and gaps between the ledger and the house framing. Check the flashing — there should be metal flashing that directs water away from the ledger-to-house joint. Missing flashing is the number-one cause of ledger rot.

Joists: Walk under each bay. Look for cracks, splits, sagging curves, and soft spots. A healthy joist is straight and firm. Check the ends of joists where they rest on the ledger and the outer beam — these end-grain areas absorb moisture and rot first.

Joist hangers: Each hanger should be fully nailed with the correct fasteners in every hole. Missing nails, bent hangers, or hangers that have pulled off the face of the ledger are a common cause of joist sagging that has nothing to do with rot.

Beam and posts: The beam carries joists and transmits load to posts. Check beam splices for separation. Check posts at their bases — if posts are set in concrete rather than on post bases, probe around the base for rot. Post rot often hides inside the concrete collar.

Footings: Visible heaving or settling of the footing piers causes post lean and beam tilt. Step back and sight along the beam — it should be level, not tilted.

Photograph everything before you start work.

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Step 2: Fix Failed Joist Hangers First

Joist hangers are the quickest fix and sometimes the only fix needed for minor sagging.

Look for hangers with missing nails (the hanger is present but nail holes are empty or were fastened with drywall screws — not acceptable), bent or twisted hangers, or hangers where the joist has separated from the hanger seat.

For hangers with missing nails: use the manufacturer-specified fasteners — for Simpson Strong-Tie hangers this is usually 10d x 1.5-inch nails into every hole. Never use drywall screws in joist hangers; they are not rated for shear loads and will snap in a collapse.

For a hanger that has bent or pulled away: remove the joist from that bay temporarily (sister the joist first so the decking above does not drop), remove the old hanger, install a new matching hanger, and reinstall the joist.

For a joist that has sagged within its hanger due to the wood compressing: add a full bearing plate (a piece of 1/2-inch pressure-treated plywood or solid blocking) under the joist seat inside the hanger to restore bearing height.

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Step 3: Sister Damaged Joists

Sistering is the right repair for joists that are cracked, split, partially rotted at the end, or sagging in the middle. It does not require removing deck boards above.

Cut new lumber to the same dimension and same length as the damaged joist. Use pressure-treated lumber rated for above-ground use (UC3B or better). If the damaged joist runs from ledger to beam, the sister must run the same full span.

Slide the sister joist into position alongside the damaged one. Raising it into place under a loaded deck can be done with a hydraulic bottle jack on blocking — jack the decking above slightly to relieve the old joist so the sister can slip in at the correct height.

Set the sister on the ledger and beam at each end. Install a joist hanger at the ledger end. Face-nail the sister to the damaged joist with 16d nails (or 3-inch structural screws) in a staggered pattern every 12 inches along the full length.

If the original joist end has rotted at the ledger: do not try to save it. The sister becomes the new bearing member; the old joist hangs alongside for stiffness only (or is removed if it is mostly gone).

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Step 4: Replace a Failed Post

If a post has rotted at its base, is leaning more than 1 inch out of plumb, or has a visible crack through its cross-section, it needs to be replaced.

First, build temporary support. Place a hydraulic bottle jack on a solid footing (a piece of 3/4-inch plywood spreads the load) under the beam, close to the failing post. Raise the jack until it just contacts the beam, then raise another 1/4 turn to transfer the load. The deck will not visibly move — you are just taking the load off the post.

Remove the post: knock out any toe-nailed fasteners at the base and top, then pry the post free.

Assess the footing. If the concrete footing is sound but the post was set directly in concrete, chip away the old concrete around the base to expose the footing top. Anchor a new adjustable post base (APBX or ABU series) to the footing using appropriate anchor bolts.

Cut the new post to the correct length. Pressure-treated posts for deck use should be UC4B (ground-contact rated) even if they do not touch the ground — the post base area collects water and UC4B provides better long-term protection. Set the post in the base hardware, plumb in both directions with a level, and secure.

Lower the jack slowly to transfer load to the new post. Check plumb again after load transfer — slight settling is normal.

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Step 5: Re-Lag a Pulling Ledger Board

A ledger that has begun to pull away from the house framing needs immediate attention. This is a partial fix if the ledger wood itself is still sound; if the ledger is rotted, it must be replaced (which is a larger project, typically requiring a permit).

For a sound ledger that is pulling due to failed or missing fasteners:

Remove the decking boards over the ledger area to allow access. If the ledger pulled, some joist connections may also be stressed — inspect all hanger connections at the ledger face.

Mark new fastener locations in the pattern required by code for your deck’s span and width (refer to the IRC prescriptive fastener schedule or have your building department review the layout). New lags must land in solid framing — the rim joist or band joist of the house framing behind the sheathing. Avoid plywood or OSB sheathing-only penetrations.

Drill pilot holes through the ledger and house sheathing into the framing. Apply a thick bead of construction adhesive or flashing sealant around each hole location before driving the lag. Drive 1/2-inch hot-dipped galvanized lag screws (minimum 3-inch penetration into solid framing) with a socket wrench — do not use an impact driver at full torque, which can over-drive and crush the wood fibers.

Install or repair flashing over the ledger-to-house joint before replacing decking. Self-adhesive flashing membrane is the most reliable option — lap it up the house wall behind any siding and down over the face of the ledger.

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Step 6: Recheck Footings and Post Bases

While you have the deck jacked up and tools out, check every post base and footing.

Footings that have heaved from frost should be repaired by a concrete contractor — pouring a new footing deeper than the frost line is the permanent fix. Temporary shimming at post bases is not a long-term solution.

Post bases that have corroded fasteners should have the fasteners replaced with new hot-dipped galvanized or stainless hardware. Many post bases have a sacrificial “do not galvanize inside this zone” area — replace any fasteners that show heavy rust.

Check for any wood-to-concrete contact at post bases. Standing water in a post base cup that is not slotted for drainage will rot the post end even on UC4B lumber over enough time. Install post bases with drainage slots where possible.

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Step 7: Inspect Decking Boards and Structural Reassembly

With framing repairs complete, inspect the decking boards above the repaired areas before reinstalling them.

Decking boards directly above severely rotted joists may have been wet for extended periods. Probe them with a screwdriver — any soft spots mean the board should be replaced rather than reinstalled.

Reinstall decking with 3-inch stainless or hot-dipped galvanized deck screws. Stainless costs more upfront but eliminates the rust streaks and fastener corrosion that require repeat maintenance on galvanized hardware.

Gap decking boards at 1/8 inch for drainage. Do not push wet boards tight — they will cup as they dry.

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How to Prevent Future Sagging

Annual deck maintenance prevents the rot cycle from restarting. Sweep the deck clear of leaves and debris in fall — packed organic matter in joist bays is the primary moisture trap. Inspect the ledger flashing every spring. Re-stain or seal the decking every 2 to 3 years to slow water absorption.

Any deck over 30 inches above grade requires a permit in most jurisdictions. If your deck repairs are extensive — full joist replacement, ledger replacement, new footings — pull a permit. A failed deck that was not permitted creates homeowner liability and insurance complications.

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Related Reading

• How to Build a Deck from Scratch — new deck construction from footings to decking
• How to Stain and Seal a Deck — annual maintenance to prevent future rot
• How to Replace Deck Boards — swapping damaged surface boards without touching framing