What's the IRC code maximum for stair angle?

IRC R311.7 doesn't specify a max angle directly — it limits rise (7-3/4″ max) and tread depth (10″ min). At those limits the angle works out to atan(7.75/10) = 37.75° — that's the IRC ceiling. Anything steeper than 38° is non-compliant for new dwelling-unit stairs. Older homes commonly have 42°+ basement stairs (8.25″ × 9″ = 42.5°) but those wouldn't pass an inspection today. ADA accessibility (1:12 ramps, 4.76°) is the other end of the scale.

What angle should my deck stairs be?

Sweet spot: 30-35° (rise 6.5-7.5″ × tread 10-12″). Comfortable for adult adults and meets code. Below 25° (rise 5″ × tread 12″) is generous and elderly-friendly but takes a lot of deck space — you need (5 × steps) ÷ rise feet of run. Above 35° feels steep and increases fall risk; over 38° is non-compliant. The classic 7-1/4″ × 11″ at 33° is the most common deck stair angle.

What's the minimum slope for a composite deck for drainage?

1/8″ per foot (1:96) per Trex, TimberTech, AZEK, and Fiberon manufacturer specs. Some go further: Trex recommends 1/4″ per foot in coastal or shaded areas. Below 1/8″ per foot water pools on the cap surface and the warranty may not cover moisture damage. Wood decks tolerate 1/16″ per foot because boards weep through the gaps — but even wood deck builders typically design for 1/8″.

Do all my stair risers have to be the same height?

Yes — IRC R311.7.5.1 limits variance to 3/8″ within a single flight. If your total rise is 36″ and you use 5 steps, each riser is exactly 7.2″ (36 ÷ 5). If you use 4 steps, each is 9″ (over the 7-3/4″ code limit). The calculator does this rounding for you — you give it the target rise per step and it adjusts to a consistent value. Variance over 3/8″ is a common code-violation reason for stair fail-out.

How do I measure my stair angle?

Use a smartphone level app on the stringer's top edge (the cut diagonal). Alternative: measure the stringer length + total rise, then angle = asin(rise / stringer_length) × 180/π. For unbuilt stairs, calculate from rise + tread: atan(rise / tread). The calculator does this from your inputs. Expect 28-37° for residential deck stairs; anything outside that range needs a closer look.

Why does my stair feel too steep even though it's code-compliant?

IRC's 7-3/4″ × 10″ limit is the absolute max — and it's barely comfortable. The 'feels safe' threshold is more like 7″ × 11″ (33°). The Riser+Tread rule of thumb (2 × rise + tread = 24-25″) is the comfort sweet spot: 7+7+11 = 25 ✓, 7.5+7.5+10 = 25 ✓, 8+8+10 = 26 ✗ (too steep). If your stair feels steep, check this — it's usually a 1″ rise too high.

What's the difference between drainage slope and stair slope?

Stair slope is the angle of an inclined walking surface (e.g. 32° for residential stairs). Drainage slope is a very gentle tilt across a deck surface to shed water (e.g. 0.6° for 1/8″ per ft). The calculator handles both modes separately — stair mode for stair geometry + IRC R311.7 compliance, drainage mode for deck-surface slope + IRC R507.11.2 positive drainage. They're measured in degrees but operate at completely different scales.

Can a deck slope away from the house too much?

Yes — over 1/2″ per ft (3% grade) feels like a ramp underfoot and meets the IRC definition of an accessible ramp (where ADA/IRC accessibility rules apply). For decks, 1/8 to 1/4″ per ft is the practical range. 1/8″ per ft is invisible to anyone walking on the deck; 1/4″ per ft is noticeable but acceptable; over 3/8″ per ft people start tripping. The calculator flags excessive slope at 1/2″ per ft.

How is the stringer length calculated?

Pythagorean theorem: stringer = √(total_rise² + total_run²). For a 36″ rise × 44″ run stair (5 steps × 11″ tread, but the top step is the deck surface so total run is (5−1) × 11 = 44″), stringer = √(1296 + 1936) = 56.85″ along the diagonal. Add 1-2″ for the upper landing cut and the lower landing rip-cut → 60″ rough cut length per stringer.

What's a winder stair and when do I use it?

A winder is a turn flight where treads are wedge-shaped instead of perpendicular. IRC R311.7.5.2.1: 10″ tread depth at walkline (measured 12″ in from the narrow side), 6″ minimum at the narrow side. Useful where straight-stair run length is constrained (under a deck, against a wall). Calculator's 'winder' use type applies the relaxed walkline rule. Spiral (R311.7.10.1) is similar but goes 360° around a central post.

IRC R311.7 stair angle · R507.11.2 deck drainage · dual-mode

Stair Slope Calculator

Compute stair angle + slope ratio + steepness category + IRC R311.7 compliance from rise + tread + use type. 4 use types (dwelling-unit, spiral, winder, commercial) with each use's own IRC limits applied. Plus a deck-drainage mode that checks IRC R507.11.2 positive-drainage and manufacturer minimum slope (1/8″ per ft for composite + PVC, 1/16″ for wood). Side-by-side comparison of your stair vs 7 reference angles.

4 stair uses7 reference anglesIRC R311.7 + R507.11.2Drainage modeStringer lengthFree forever

4·Stair use types

7.75″·IRC max rise

1/8″/ft·Composite drainage min

R311.7·IRC compliance

Inputs

Modestair geometry OR deck drainage

Stair use4 use types · IRC R311.7 / IBC 1011

Stair dimensions

Total rise + target rise per step + tread depth

Total risein · deck height above grade

Rise / stepin · target 6.5-7.5″

Tread depthin · 10-12″ comfort

Nosingin · 3/4-1-1/4″ IRC

Stair widthin · 36″ IRC residential

Dwelling unit (most decks)

33.2°Standard (25-35°)

5 steps · 7-1/4″ : 11″56.9″ stringer

Stair angle

from horizontal

Step count

7.20″ each

Total run

3.7 ft

Stringer length

Pythagorean cut

Reference angles · this stair vs common types

Stair type	Rise × tread	Angle	Use
Your stair	7.20″ × 11.0″	33.21°	Standard (25-35°)
Wheelchair ramp (ADA max)	1″ × 12″	4.76°	Accessibility — 1:12 max slope
Wide porch stair	6″ × 12″	26.57°	Generous tread — comfortable
Standard residential	7″ × 11″	32.47°	IRC code-compliant default
Steep residential	7.75″ × 10″	37.75°	IRC limits — code-borderline
Basement stair (tight)	8.25″ × 9″	42.51°	Pre-IRC older homes — non-compliant for new
Spiral stair	9.5″ × 7.5″	51.71°	IRC R311.7.10.1 spiral
Stepladder	12″ × 3.5″	73.74°	Tool — NOT a stair

Stair compliance

Stair passes Dwelling unit (most decks) compliance

IRC R311.7.5.1

5 steps · 7.20″ rise × 11.0″ tread · 33.2° angle · Standard (25-35°) category. Stringer length 56.9″.

Code referenceIRC R311.7.5.1 / R311.7.5.2 / R311.7.5.3 / R311.7.2 / R311.7.1 controls dwelling-unit stair geometry. Local AHJ may impose additional limits. Always confirm with permit office before framing.

Need rise + run + stringer cuts?

Use the Stair Rise-Run Calculator for the inverse problem (find ideal rise + run from your total deck height) or the Stair Stringer Calculator for the marking cuts.

Open

Stair compliance per IRC R311.7 (residential) / IBC 1011 (commercial). Drainage slope per IRC R507.11.2 + manufacturer spec sheets (Trex / TimberTech / AZEK / Fiberon). DeckMath is not a substitute for a permit inspector.

How to use

Three steps. Permit-ready output.

Pick mode

Stair mode for stair angle + IRC R311.7 compliance. Drainage mode for deck-drainage slope (IRC R507.11.2) — composite + PVC manufacturers require 1/8″ per foot minimum slope for water shedding.

Stair mode — total rise

The vertical distance from grade (or lower landing) to the deck surface. Most deck stairs handle 24-48″ of rise. Above 48″ you typically need an intermediate landing per IRC R311.7.3.

Stair mode — target rise per step

Comfortable range: 6-7.5″ per step. IRC R311.7.5.1 hard limit: 7-3/4″ max for dwelling-unit stairs. Calculator rounds your total rise to a consistent rise across all steps within ±3/8″ (R311.7.5.1 variance limit).

Stair mode — tread depth

Excluding nosing. IRC R311.7.5.2: 10″ minimum. Comfort range: 10-12″. The classic 2×8 + 2×8 (≈11″) tread is the most common deck stair tread.

Stair mode — stair use

Dwelling-unit (most decks). Spiral (relaxed: 9-1/2″ max rise). Winder (turn flight). Commercial / public (stricter: 7″ max rise, 11″ min tread, ≤3/16″ variance).

Drainage mode — deck length + slope/ft

Length is the dimension water flows across (typically the deck depth from house to outer rim joist). Slope presets: 1/16″ (wood only), 1/8″ (composite + PVC code min), 3/16″, 1/4″. Calculator returns total drop, angle, and manufacturer compliance check.

How we calculate

The math, fully transparent.

The Stair Slope Calculator computes stair angle, slope ratio, and IRC R311.7 compliance for residential deck stairs, plus a deck-drainage slope mode that checks IRC R507.11.2 positive-drainage requirements for ledger-attached decks. Enter total rise + target rise per step → calculator returns step count, actual rise + tread, angle in degrees, slope ratio, stringer length, steepness category (shallow / standard / steep / non-compliant), and a side-by-side comparison against 7 reference angles (ADA ramp, porch stair, standard residential, basement, spiral, stepladder). Drainage mode: enter deck length + slope per ft → total drop, water-shedding category, and manufacturer min-slope check for composite + PVC + wood decking.

IRC references

IRC R311.7.5.1 — Stair risers: 7-3/4″ max for dwelling units; ≤3/8″ variance within a flight
IRC R311.7.5.2 — Stair treads: 10″ min depth measured between nosing projections
IRC R311.7.5.3 — Nosing: 3/4″ to 1-1/4″ projection where solid risers are used
IRC R311.7.1 — Stair width: 36″ min where treads serve a dwelling unit
IRC R311.7.2 — Headroom: 6'-8″ min above stair walking surface
IRC R311.7.10.1 — Spiral stair: 9-1/2″ max rise · 7-1/2″ min tread at walkline · 26″ min width
IBC 1011 — Commercial stairs: 7″ max rise · 11″ min tread · 44″ min width · ≤3/16″ variance
IRC R507.11.2 — Deck drainage: positive slope away from ledger for water shedding

Stair limits per IRC R311.7 (dwelling-unit residential) + IBC 1011 (commercial). Spiral + winder limits per IRC R311.7.10.1 / R311.7.5.2.1. Drainage slope minimums per IRC R507.11.2 + manufacturer spec sheets (Trex, TimberTech, AZEK, Fiberon, 2026-Q1).

Stair angle from horizontal

angle_deg = atan(rise_per_step / tread_depth) × 180/π

7″ rise × 11″ tread = atan(7/11) × 180/π = 32.47°. The classic IRC-compliant residential angle. Steeper than 38° crosses into 'feels unsafe' territory; over 42° is non-compliant for new construction.

Step count + actual rise

step_count = round(total_rise / target_rise_per_step) · actual_rise = total_rise / step_count

36″ total rise with 7.25″ target = 36/7.25 = 4.97 → round to 5 steps → actual rise = 36/5 = 7.2″ per step. The rounding keeps all risers identical (R311.7.5.1 variance limit ≤3/8″).

Slope ratio

rise : run = actual_rise (in) : tread_depth (in)

7-1/4″ rise : 11″ tread on a code-compliant residential stair. Expressed as 'roughly 7:11' for permit drawings. Some jurisdictions ask for the ratio explicitly; this is the format they want.

Stringer length

stringer_length = √(total_rise² + total_run²)

36″ rise × 44″ run (5 steps × 11″ each tread, minus the top tread which is the deck surface — so total run = (steps-1) × tread = 44″) → √(1296 + 1936) = √3232 = 56.85″ — that's the cut length of each stringer along the diagonal.

Drainage slope angle

angle_deg = atan(slope_per_ft / 12) × 180/π

1/8″ per ft = atan(0.125/12) × 180/π = 0.60°. Imperceptible underfoot but enough to shed water off a composite deck. 1/4″ per ft = 1.19° — sometimes used in pool decks or coastal installs. Above 1/2″ per ft is uncomfortable and starts to feel like a ramp.

Drainage total drop

total_drop = deck_length_ft × slope_per_ft_in

14 ft deck × 1/8″/ft = 1.75″ total drop from house to outer rim. Calculator returns this so you can pre-cut the joists with a tapered cut OR install on a step-tapered ledger to match. Without slope, water pools and accelerates composite + wood deterioration.

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Stair Rise and Run Calculator

Equal-rise riser distribution, run depth, stringer hypotenuse, and angle for any deck height. IRC R311.7.5 + Blondel 2R + T ergonomic check. Permit-ready callout sheet.

Stair Stringer Calculator

Stringer hypotenuse, notch depth, throat (DCA-6 ≥ 5″), stringer count, stock-length pick, brackets + fasteners. IRC R311.7 + AWC DCA-6 cross-validated.

Stair Tread Calculator

Tread board count, total LF + waste, riser boards, fasteners — across 6 materials (PT, composite, Ipe, Cumaru, cedar). IRC R311.7.5 cross-validated.

Deck Stair Calculator

Risers, treads, stringer hypotenuse, and BoM for any deck height — IRC 2021 R311.7 compliant with auto handrail + landing-pad sizing.