How it works
- Number of risers = total rise ÷ tallest riser allowed, rounded up.
- Riser height = total rise ÷ number of risers. Every step comes out the same.
- Treads = risers − 1. The deck itself is the top step.
- Total run = treads × tread depth. This is how far the stairs reach out from the deck.
- Stringer length = √(total rise² + total run²).
Example: a deck surface 36 inches above the landing. 36 ÷ 7.75 = 4.6, so 5 risers. Each riser is 36 ÷ 5 = 7.2 inches, which is 7 3/16 in. There are 4 treads, so the run is 4 × 10 = 40 inches. The stringer is √(36² + 40²) = 53.8 inches, so an 8 foot 2x12 is enough for each one.
Measure the rise where the stairs land
Yards slope. The ground 4 feet out from the deck is often lower or higher than the ground right under the edge. Hold a straight board level out from the deck surface to the spot where the bottom step will sit, and measure down from there.
Common code limits
The International Residential Code, which most US states use as a base, sets these limits for stairs:
- Risers no taller than 7 3/4 inches.
- Treads at least 10 inches deep.
- The tallest and shortest riser in one flight within 3/8 inch of each other.
- Stairs at least 36 inches wide.
- A handrail on stairs with four or more risers.
Your town can be stricter, so check before you cut.
Common questions
How many steps do I need for a 36 inch high deck?
Five risers at 7 3/16 inches each, with 4 treads. The deck surface is the top step.
What is the maximum riser height for deck stairs?
The International Residential Code limit is 7 3/4 inches, with treads at least 10 inches deep. All risers in a flight must be within 3/8 inch of each other. Local codes can differ.
Why is there one less tread than risers?
Because the deck is the top step. You step from the last tread straight onto the deck surface, so the stringer only carries the treads below it.
How many stringers do I need for deck stairs?
A 36 inch wide stair usually gets 3 stringers with 2x lumber treads, or 4 with thinner 5/4 or composite treads. Many composite makers require stringers 12 inches apart or less.
Do deck stairs need a handrail?
Under the International Residential Code, stairs with four or more risers need a handrail on at least one side. Decks more than 30 inches above the ground also need a guard rail.