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.

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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

How to use the stair slope calculator in 6 steps.

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

How DeckMath calculates this — IRC 2021 sources.

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 slope questions, answered.

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.
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.
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″.
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.
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.
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.
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.
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.
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.
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.

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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.

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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

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