Chicago Back Porch Stairs: Rot, Repair, Rebuild

Chicago back porch stairs fail in a predictable order: the treads soften, the bottom of the stringers rots, and the connection at the top works loose. Swapping treads and railing pickets is the small job. Replacing stringers is the big one, and the City treats it as structural work that always needs a permit.

Why do back porch stairs rot before the rest of the porch?

Wood decays when it stays wet. Serious decay needs the wood's moisture content above roughly 30%, and the USDA Forest Products Laboratory says the wetting comes from rain, condensation or wet ground contact, not from humid air alone (USDA Forest Products Laboratory). Back stairs get all three. They sit in the weather, they hold meltwater in every joint, and their feet land on or near the ground.

On Chicago back porches the damage usually starts in four places:

• The bottom of each stringer (the angled board that carries the treads), where it meets a landing, a slab or soil.
• The tread to stringer connection, where water gets in around screws and nails.
• The top connection, where the stair meets the porch frame and water runs down behind the rim.
• Under paint, where a thick coat hides soft wood for years.

Freeze and thaw does the rest. Water in a crack freezes, widens it, and lets in more water the next time.

What counts as a repair, and what needs a permit?

The Department of Buildings (DOB) draws the line on its porch and deck safety page:

Read the second row carefully. The no-permit allowance for treads applies if you have not received a violation. If a violation is open, DOB says nonstructural repair can go through an Express Permit instead. Rules change, so confirm the current version with DOB before you start.

The practical result: new treads on sound stringers is a repair. New stringers is a structural job, even if the porch above is staying.

How do you tell if the stringers are bad?

You can check most of this yourself, with the stairs unloaded and a flat screwdriver.

• Probe the bottom 12 inches of each stringer. Wood that takes the screwdriver tip easily is rotted.
• Look at the notches or brackets where the treads sit. Cracks, dark stains and mushy spots mean water has been sitting there.
• Press on each stringer with the heel of your hand while someone stands on the stairs. Flex or movement is not normal.
• Check the top connection for gaps, rust streaks or fasteners that have backed out.
• Look at what the feet sit on. A stringer bearing on soil or a crumbling pad is a problem by itself.
• Count how many treads are soft. One or two is a repair. Most of them usually means the stringers are wet too.

Probing and looking is DIY-safe. Jacking, shoring or cutting stringers on a second or third floor stair is not. If the stairs flex when someone walks them, stop using them and call.

What goes wrong with the materials?

Three material problems show up again and again on rotted stairs.

Wrong treatment grade. The wood industry standard sets use categories for treated lumber. Ground contact needs Use Category 4A or higher, and some thin lumber is treated for above-ground use only (AWPA). Wood sold for the porch deck is not automatically right for a stringer foot. DCA 6, the American Wood Council's deck guide, also requires ground-contact lumber to be treated wood suitable for that use, and field cuts to be treated with preservative (AWC DCA 6).

Wrong fasteners. Screws, bolts, nails and washers used with preservative-treated wood must be hot-dipped galvanized, stainless steel, silicon bronze or copper, and connectors must be galvanized or stainless (AWC DCA 6). Plain steel screws rust, stain the wood and let go.

Cut ends left bare. Every notch and cut end exposes untreated core wood. Those are the first places water enters.

A rebuilt stair that gets these three right lasts. A stair that gets them wrong is on the same path as the one it replaced.

Do back stairs need footings?

The stairs on a Chicago porch are part of what DOB inspects. DOB says inspectors check joists, beams, columns, stairs, hand and guard rails, ledger boards, connections and footings (City of Chicago). DOB's May 2011 porch guidelines say footings must have the bottom at least 42 inches below grade, but that document predates the 2019 codes, so treat the number as 2011 City guidance and check with DOB for your stair (2011 DOB guidelines). We cover that question on its own in do porch stairs need footings.

What about railings on the stairs?

The City's maintenance standard says handrails and guards must be firmly fastened and capable of supporting normally imposed loads (City of Chicago). A wobbly rail on a back stair is a violation waiting for a complaint, and it is also how people get hurt. Guard height is its own topic: see porch railing height in Chicago.

Should you replace wood stairs with concrete or steel?

Sometimes. Concrete stairs make sense when the stair lands at grade and you want the bottom to stop rotting. Steel can replace wood, but the City's Express permit does not cover replacing noncombustible material with combustible, and replacing wood with steel under Express needs drawings from an architect or structural engineer (City of Chicago). Check how the options compare in concrete or wood porch stairs.

When to call someone

Call when a stringer probes soft, when stairs move under load, when the stair is the only way out of a unit, or when you got a notice. The porch repair or replace guide covers how the larger decision gets made. Our porch team can look at the stair and the porch above it together, tell you straight whether it is a repair, and handle the drawings and permit if it is not. The full picture is in our Chicago porch guide.

Common questions

This article explains general rules and common situations in Chicago. It is not legal advice, and City rules change. For a specific property, check with the Chicago Department of Buildings or a licensed professional.