An 18" deck needs 3 risers at 6" and 2 treads: a 20" run on 10" treads, cut from 8 ft 2x12s with 6-1/8" of throat. Gentler: 4 risers at 4-1/2" on 11" treads, a 33" run (2R + T = 20.0").
finished to finished
fewest legal
nosing to nosing
3/4–1-1/4 under 11″
DCA 6 max 18
18" total rise
Hook the tape once at mark 0, where the bottom riser line meets the top edge of the 2x12, and pencil every mark from that same hook. Each step is 11-11/16"− along the edge; marks are rounded once each, so nothing drifts.
| Done | # | Mark | Tape | Decimal | mm |
|---|---|---|---|---|---|
| 0 | Bottom riser line | 0" | 0.000" | 0 mm | |
| 1 | Tread 1 nose | 11-11/16"− | 11.662" | 296 mm | |
| 2 | Top plumb cut | 23-5/16" | 23.324" | 592 mm |
Then from mark 0 measure 5" down the bottom riser line and square across for the level heel cut.
Shortest run, steepest legal stair
Gentler climb, longer run
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| 0 | Bottom riser line | 0" | 0.000" | 0 mm | 0" |
|---|---|---|---|---|---|
| 1 | Tread 1 nose | 11-11/16"− | 11.662" | 296 mm | 11-11/16" |
| 2 | Top plumb cut | 23-5/16" | 23.324" | 592 mm | 1' 11-5/16" |
Step-off 11-11/16"− (11.6619") · bottom cut 5" under 1" treads · last mark 1' 11-5/16" at the top plumb cut.
| 0 | Bottom riser line | 0" | 0.000" | 0 mm | 0" |
|---|---|---|---|---|---|
| 1 | Tread 1 nose | 11-7/8" | 11.885" | 302 mm | 11-7/8" |
| 2 | Tread 2 nose | 23-3/4"+ | 23.770" | 604 mm | 1' 11-3/4" |
| 3 | Top plumb cut | 35-5/8"+ | 35.655" | 906 mm | 2' 11-5/8" |
Step-off 11-7/8" (11.8849") · bottom cut 3-1/2" under 1" treads · last mark 2' 11-5/8" at the top plumb cut.
05 / Field guide
This sheet works an 18" porch or low deck two ways. Option A uses the fewest risers the IRC allows, 3 at 6" on 10" treads. Option B adds one riser and uses 11" treads for a gentler 4-1/2" step. Both come with hook-once tape marks for the stringer edge, below the calculator.
18" ÷ 7.75 = 2.32, rounded up to 3 risers, each 6" (6.0000", 152.4 mm). On 2 treads of 10" the stair lands 20" (1' 8") out from the deck and climbs at 31.0°; 2R + T scores 22.00".
Each step is 11-11/16"− along the stringer edge. Notches cut 5-1/8" deep, leaving 6-1/8" of throat in a 2x12; the cut length is 2' 1-7/8", so buy 8 ft stock.
One more riser drops each step to 4-1/2" (4.5000"). With 11" treads the run grows to 33" (2' 9"), 13" more than Option A, and the angle eases to 22.2° with 2R + T at 20.00".
The diagonal is 11-7/8", the throat 7-1/16", and the stringer cut length 3' 1" (8 ft).
Take the tread thickness off the bottom riser: 6" becomes 5" under 1" 5/4 or composite treads, 4-1/2" under 1-1/2" 2x6 treads. Every finished step then measures 6".
At the top, the last tread notch sits 7" below the finished deck for 1" treads. Hung flush from the rim, the stringer edge meets the rim face 1" below the finished deck surface.
With 3 risers no handrail is required (the IRC asks for one at four or more).
Under 30" of total rise the open sides need no guard, though many owners add one.
The 20" run is within the 6'-0" a cut 2x12 stringer may span under DCA 6, so no mid-support is needed.
Questions
3 risers at 6" with 2 treads, the fewest the IRC allows. For an easier climb, 4 risers at 4-1/2" with 3 treads.
About 2' 1-7/8" along the board for Option A, so 8 ft 2x12s; Option B needs 3' 1" (8 ft).
20" (1' 8") on 10" treads, or 33" (2' 9") on 11" treads with 4 risers. Leave a landing at least 36" deep beyond the last riser.
Not by code: under 30" of rise and fewer than four risers, neither a guard nor a handrail is required.
Not under AWC DCA 6, which asks for 2x12s. A 2x10 would keep only 4-1/8" of throat on Option A, under the 5" minimum.
Raised decks, basements and storeys