How to Build Deck Stairs

Deck stairs are just exterior stairs, so the rise and run maths is identical. The two things that make them different are measuring the drop to (often uneven) ground, and anchoring the bottom on something solid.

Step 1: Measure the total rise correctly

This is where deck stairs go wrong. The ground usually slopes, so do not just measure down at the deck edge:

1. Rest a straight board (with a level on it) on the deck surface, running out to where the stairs will land.
2. At the landing point, measure straight down to the ground.
3. That vertical drop is your total rise - deck surface to the ground at the foot of the stairs.

Step 2: Work out rise and run

Feed the total rise into the stair calculator: it returns the number of risers, the riser height and the tread depth, and draws the flight so you can check it before cutting. Outdoor steps often use a slightly lower riser and deeper tread than indoors for a comfortable, safe descent.

Step 3: Cut the stringers

Mark and cut the stringers exactly as for interior stairs - framing square, step off the flight, and drop the stringer by one tread thickness at the bottom. Full method: how to cut stair stringers. Use three stringers for a standard width, more for wide stairs or composite treads.

Step 4: Set a solid footing at the base

The stringers must land on a concrete pad or footing, not soil:

• Pour a small pad or footings where the stairs land.
• Fix the stringers to it with a treated cleat or post anchors, keeping timber off wet ground.

Without a footing, the base settles, the stringers move, and the stairs pull away from the deck.

Step 5: Fix the top, then treads

Hang the stringers off the deck rim joist (a ledger or hangers), check everything is square and level, then fit the treads - and add a handrail per your local code.

While you’re at it

Estimating the rest of the deck too? Use the decking calculator and see how many deck boards do I need. For the stair geometry itself, the stair calculator is the place to start.