How do you know if deck joists are damaged?
Signs include a bouncy or sagging floor, boards that won't stay fastened, dark wet staining on joist tops, soft wood when probed, and rusted or pulled-out joist hangers.
The best inspection is from underneath with a flashlight and a screwdriver. Probe the tops of joists, especially under board seams where water collects, and the ends where joists sit in hangers or on beams. Check that every hanger has all its specified nails and isn't heavily corroded.
What is sistering a deck joist?
Sistering fastens a new full-strength joist alongside a damaged or undersized one so the two share the load. It is a common, effective repair when damage is localized and the joist ends are well supported.
A proper sister runs the full span or at least well past the damaged area, bears on the same supports, and is fastened with a structural fastening pattern. Damaged wood should be cleaned off or treated so rot doesn't spread to the new member. If the joist ends themselves are rotted, the sister needs its own hanger or bearing.
How do you fix a bouncy deck?
- Add solid blocking or bridging between joists at mid-span to make them work together.
- Sister joists that are undersized for their span.
- Add a beam and posts to shorten long spans.
- Replace cracked or rotted joists.
- Tighten or replace loose hangers.
How can deck joists be protected from rot?
Joist-top tape or membrane sheds water from the most vulnerable surface. Good ventilation under the deck, clean board gaps that let water drain, and flashing at the ledger all extend framing life. Treated lumber rated for ground contact is a common choice for deck framing because joists are hard to replace and often stay damp.
Do joists need to be changed for composite boards?
Often. Many wood decks were framed at a spacing that is too wide for composite or PVC, especially on diagonal layouts. Adding joists between existing ones is a common part of re-decking.
Check the decking manufacturer's maximum joist spacing for the specific board and layout angle. Stair treads frequently have stricter limits than the main deck surface.
Why do joist hangers fail?
Corrosion, missing fasteners and the wrong fasteners. Hangers must be fastened with every specified nail or screw, and both the hangers and fasteners must be rated for use with treated lumber.
Older decks sometimes have hangers held with only a few nails, roofing nails or drywall screws — none of which carry the intended load. Modern preservatives can corrode ordinary steel, so hot-dip galvanized or stainless hardware is standard. Replacing hangers is usually done one joist at a time with temporary support.
How do you decide between joist repair and replacing the deck?
Count the problems. A few damaged joists on an otherwise sound deck are a repair. When rot appears across many joists, at the ledger and at post tops, the deck has reached the end of its service life and rebuilding the frame — often with new decking at the same time — avoids paying for repairs that will be torn out later.
How does deck joist repair differ across the states we serve?
Joist rot follows moisture trapped under boards. The figure varies most between Washington (95) and Minnesota (60).
| State | Wet days / yr (median) | What it means |
|---|---|---|
| Georgia | 80 | Frequent rain: flashing, drainage and airflow protect deck joist repair |
| Illinois | 74 | Drier spells: sun and checking are as important as moisture for deck joist repair |
| Kansas | 60 | Drier spells: sun and checking are as important as moisture for deck joist repair |
| Maryland | 81 | Frequent rain: flashing, drainage and airflow protect deck joist repair |
| Massachusetts | 82 | Frequent rain: flashing, drainage and airflow protect deck joist repair |
| Minnesota | 60 | Drier spells: sun and checking are as important as moisture for deck joist repair |
| North Carolina | 75 | Drier spells: sun and checking are as important as moisture for deck joist repair |
| Pennsylvania | 79 | Drier spells: sun and checking are as important as moisture for deck joist repair |
| Tennessee | 84 | Frequent rain: flashing, drainage and airflow protect deck joist repair |
| Virginia | 77 | Drier spells: sun and checking are as important as moisture for deck joist repair |
| Washington | 95 | Frequent rain: flashing, drainage and airflow protect deck joist repair |
Figures are medians across our covered communities in each state, from NOAA 1991–2020 climate normals and the Census Bureau's 2020–2024 American Community Survey. Each state and community page explains local conditions in more detail.
