How to tell if a deck is still safe

Start at the house end

On a deck attached to a house, one board carries the whole thing. The ledger is fixed flat against the wall, the joists hang off it, and everything on the deck transfers through it into the house frame. Unlike a soft board it gives no warning underfoot.

Go along its length with a light. You want bolts or heavy structural screws staggered high and low, not nails alone. Nails were normal on older decks, and they lose grip over the years. Brick is the other trap: veneer is a skin hung on a framed house, not what holds it up, so fasteners through it have to reach the band board behind or they hold nothing.

Then look at how water gets past. Above the ledger there should be metal flashing, tucked behind the siding and lapped over the board so rain sheds outward instead of down behind. Rust streaks under the fasteners, staining on the siding below and soft joist ends say there is none.

Each joist should land in a metal hanger with a nail through every hole, not three of them. Hanger nails are made to be loaded sideways. Deck screws are not.

What the top of the deck reports about the frame

Most of what a homeowner notices is cosmetic: gray furry boards, splinters, cupping that holds a puddle. A few signs are the frame talking through the boards.

• A gap opening between the wall and the first deck board, or the deck sitting lower against the door threshold than it did. The ledger is moving.
• Soft ends where a board crosses a joist. Water sits in that joint, and the joist top under it is wet too.
• Rust colored bleed from every fastener along a run rather than from one. That is hardware giving up, not one bad screw.
• Board gaps packed solid with leaf mulch. They exist so water can leave, and packed ones keep the frame wet all winter.
• Sway rather than spring. Bounce over a long span is ordinary. A deck moving sideways as you rock usually means nothing braces the posts against the beam.

The posts, and what they stand on

Posts rot in one band, the couple of inches either side of the soil line, where wood gets wet often and dries slowly. The face that goes first is the one turned toward the house: shaded, last to dry, nobody sees it. A downspout discharging beside a footing keeps that ground wet, which is a drainage question more than a deck one.

Pushing a screwdriver into suspect wood is the test that settles it. On a deck the places worth probing are few: each post at grade on all four faces, the joist ends at the ledger, the foot of each stair stringer, and any board end that felt soft.

What you want under a post is a footing coming above grade, with a metal base holding the end grain clear of the concrete. End grain drinks. A post set into soil, or standing in a base that holds water like a cup, is not unsafe today. It has a clock running. At the top, a beam sitting on a post is carried by the post. One bolted to the side is carried by its fasteners.

Guardrails and stairs, the parts people fall from

Test a guardrail at the posts, not the middle of a run. Some flex along a long rail is normal. Movement at the base of a post is not, and the base is the joint that counts, because a guardrail earns its keep in the half second when somebody's weight arrives against it sideways. Most wobble is in how the post is held: lag screwed to the outside of the rim joist, or notched so deeply at the frame that little wood is left. That is hardware and blocking at the base, not a new railing.

In residential work a guard is wanted once the walking surface sits more than about thirty inches above the ground, about thirty six inches tall, with no gap a four inch ball will pass. A wider baluster gap on an older deck is worth a thought with small children around.

Stairs carry more load per square foot than the deck and get looked at least. A stringer is a board with triangles cut out of it, so the wood behind each notch is the thin part, and a split with the grain there is what to find. One sitting on soil rots from the foot up. A handrail is something a hand closes around, which a wide flat board is not.

What warrants keeping people off it

Very few findings mean a deck should be closed today. The ones that do are connections that have already started to let go, and the answer is to keep people off it until somebody has looked at it.

Everything short of that is a repair to plan rather than an emergency.

• A ledger held to the house by nails only, or a gap opening between the ledger and the wall.
• A post soft through at the ground line, or no longer bearing on its footing.
• Joists soft where they meet the ledger, or hangers missing, rusted through or pulled away.
• A guardrail post that moves visibly at its base on a raised deck.
• A stair stringer split through at a notch, or treads that flex at the front edge.
• A deck that leans, has dropped at one corner, or sways when you shift your weight.

Doing this yourself

The inspection is a homeowner job start to finish. A flashlight, a long screwdriver and a willingness to lie on your back is the whole tool list, and nobody needs to be paid to tell you whether there are bolts in your ledger.

Several repairs are yours too: raking packed mulch out of the board gaps, replacing a single soft board, re-securing a loose baluster, cutting back the shrub that keeps a post wet. Ordinary tools and a Saturday.

Where it stops is anything carrying load. Ledger fastening, post replacement, footings, joists, hangers and guardrail post connections share one failure mode, which is that somebody gets hurt. Photograph what you find from underneath. That is the half a picture from the back door never shows, and the half we look at first. Deck lighting and outlets are their own trade at any size, and not part of this work.

Close it, plan it, or leave it

A ledger on nails, a post soft through at the ground, joists off their hangers, a guardrail post loose at its base: those are now jobs, and the deck comes out of service until they are done. Not because deck failures are common, but because that kind arrives all at once with people standing on it.

A guardrail post that moves at its base, hardware rusting through, a post with no footing above grade, missing flashing over a sound ledger: this season. Every one is moving in the same direction, which is wetter.

Gray boards, splinters and a finish that has given up can wait. What decides it is height and connections. A deck one step off the back door onto grass turns a failure into a stumble, and forgives a lot. A raised one bolted to a house does not.

Questions that come with this one