My deck is sinking. How can I fix it?

A sinking deck means something underneath it has moved or gone soft. Everett's long wet season is hard on footings and posts, so this is a common problem here. Below we explain how to work out what is going on, what you can safely check yourself, and when someone should come out and look at it.

The short answer

A sinking deck almost always means one of three things has happened. A footing has settled into wet soil, a post has rotted at the base, or the ledger board where the deck meets the house has pulled away. The fix is to find out which one it is, hold the deck up on temporary supports, and then repair or replace that part. Lifting the surface boards or pushing shims under a low corner does not fix it. The low spot comes back, and the shim can hide a problem that keeps getting worse.

Most sinking decks can be repaired without tearing the whole deck down. We jack the framing back to level on temporary supports, replace the failed footing or post, and set the load back down on something solid. Whether that turns out to be a small job or a big one depends on what we find once we get underneath.

What usually causes it around Everett

Our climate is hard on the parts of a deck you cannot see. From fall into spring the rain keeps the soil under a deck damp, and much of the ground around Everett and the nearby towns is dense glacial till or clay that holds water instead of draining it. A footing on that kind of soil can settle slowly. This is especially true of a shallow precast block set on the surface, as opposed to a poured footing dug down to firm ground.

Wood posts are the other common cause. A post buried directly in the soil, or set in concrete that holds water around its base, stays wet and rots from the bottom up. At eye level the post looks fine, but it is soft where it meets the ground. Once it loses a little height, that corner of the deck drops.

The third cause is the ledger, the board fastened to the house. If it was nailed on instead of bolted, or if water got behind it because the flashing was missing, the house side of the deck can sag or pull away. This cause matters most, because ledger failure is how decks come down suddenly.

What changes the answer on your property

A few things decide whether a sinking deck is a quick repair or a bigger job.

Where it is sinking. One low corner on a ground-level deck usually points to a single footing or post. A deck sagging along the house wall points to the ledger. A deck that dips across the middle may have undersized or rotted joists and no footing problem at all.

How high it is. A low deck on a flat lot is easier to support and repair. A second-storey deck on a sloped lot, which is common on hillside homes in Mukilteo and parts of Edmonds, puts more load on taller posts and needs more care.

How sound the framing is. If the posts and joists are solid and only one footing moved, we fix that footing. If rot has spread through the joists and beam, new posts just put new legs under a weak frame. At that point Deck Replacement can be a better use of your money than one repair after another.

What the footings are. Precast blocks sitting on the surface settle and shift. Poured footings dug below the frost line rarely do. Knowing which ones you have tells us whether the fix is one footing or all of them.

What you can check yourself

You can learn a lot in fifteen minutes with a flashlight, a screwdriver and a level.

Lay a level or a straight board across the deck in a few directions and note where it drops. Then look under the deck at that spot. See whether the post is sitting square on its footing or has tilted or slid off. Push a screwdriver into the base of each post and into the ends of a few joists. Sound wood resists the tip. Rotted wood lets it sink in easily, or crumbles.

Look at the ledger where the deck meets the house. You should see bolts or structural screws, not just nails, and metal flashing over the top. Watch for a gap opening between the ledger and the siding, rust streaks, or dark staining on the wall below.

Notice whether the ground under the deck stays wet. Standing water, moss on the soil, or a downspout emptying near a post are all signs the footings are sitting in soaked ground.

Do not crawl under a deck that is visibly leaning or pulling away from the house, and do not try to jack it up yourself with a car jack. Framing under load can shift without warning.

When it needs a professional

Call for help if the house side of the deck has dropped or pulled away, if a post is leaning out of plumb, if the deck moves or bounces when you walk on it, or if the screwdriver test found soft wood in more than one place. Keep people off the deck until it has been checked. Do not put heavy loads on it either, like a hot tub, planters full of wet soil, or a crowd for a barbecue.

When we visit, we start with the same checks described above, done from underneath with better access. We look at each footing, post, beam and joist, and at the ledger connection. Then we explain what we found and what we recommend, and you get a written estimate before any work starts.

How we usually fix it

Most sinking decks are a Deck Repair job. We set temporary supports, lift the framing back to level, and replace the failed footing with one dug to firm ground, along with the post above it. The new post sits on a metal base that holds the wood off the concrete so it can dry out between rains. If the ledger is the problem, we fix the attachment and the flashing at the same visit.

If we find soft wood in the joists or beams, that part of the work becomes Rot Repair. We cut out the damaged section, sister or replace the affected members, and find where the water is coming from so the rot does not come straight back. If the frame is past saving, we will tell you so and talk through replacement instead.