How do I calculate stair tread lumber?

(1) Find tread count = riser count − 1. (2) Per-tread board LF = (stair width in ft) × (boards per tread). 42″ wide × 1 board (single-board) = 3.5 lf. (3) Total LF = per-tread LF × tread count × waste factor (1.15 for cut stringers, 1.05 for solid). 5 treads × 3.5 × 1.15 = 20.1 lf order. (4) Stock board count = ceil(treadCount × boardsPerTread × waste). 5 × 1 × 1.15 = 5.75 → 6 boards. Add riser boards (1×8 PT) if closed-riser style: 5 × 3.5 = 17.5 lf riser stock.

What tread material is best for outdoor stairs?

Best value: Composite grooved (Trex Transcend, TimberTech AZEK, Fiberon Sanctuary) — $6.50/lf, 30-yr warranty, hidden-clip fastening (clean look), no maintenance, slip-rated 'good'. Best longevity: Brazilian Ipe — $11.50/lf, 75-yr life, hardest commercial decking wood (4× harder than oak), ages to silver. Best budget: PT 5/4×6 or 2×6 — $2.95/lf, 18-yr life with annual sealing. Mid-tier hardwood: Cumaru — $8.95/lf, 50-yr life, 25% cheaper than Ipe with similar density.

Should I use composite or wood for stair treads?

Composite — when you want zero maintenance, can pay $6.50-7/lf upfront, and prefer a uniform color/texture across the deck. 30-yr warranty, never needs staining/sealing, slip-rated good with texturized surface. Wood (PT or hardwood) — when budget is tight (PT) or you want a high-end natural look (Ipe/Cumaru). PT requires re-staining every 2-3 years; Ipe ages to silver if unsealed (most people prefer this). Cedar is a middle-ground softwood but NOT ground-contact, so stringers must be PT.

What thickness should stair tread boards be?

5/4 deck board (1.0625″ actual) is the minimum AWC DCA-6 standard for treads when stringers are spaced ≤ 16″ o.c. (the cut-stringer standard). 2× lumber (1.5″) is allowed and arguably preferred — adds rigidity, less bounce, slightly more material cost (+15-20%). For wider stringer spacing (≥ 24″, solid-stringer style), 2× lumber is required — 5/4 will sag under load. Composite + hardwood treads typically come 1″ thick (matches deck-board lines) and are rated for any stringer spacing.

How wide should each tread board be?

11.25″ (single 2×12 nominal): cleanest look, fits 11″ run + 1″ nosing exactly. Works to 48″ stair width. Two-board pattern (2 × 5.5″ each, 2×6 nominal × 2): standard for wider stairs, more stable across span, mimics deck-board pattern aesthetic. Pre-bullnosed stair-tread stock (12″ wide, factory radius nosing): premium look, no separate nosing build-up needed — but limited to PT lumber and Ipe at most yards.

Do I need riser boards for outdoor stairs?

Closed-riser stairs (with riser board behind each tread) — code default per IRC R311.7.5. Riser blocks debris from falling through, gives a finished look, allows full nosing compliance (3/4″-1-1/4″). Open-riser stairs — allowed only if 4-inch sphere cannot pass through the open space between treads (R312.1.3.1). This is rare for residential decks since most stair geometries leave 6-8″ open. Closed-riser is the safe code-default choice.

How much tread waste should I order extra?

Cut/notched stringers: 15% extra lumber. The notched cuts in the stringer leave 1-2″ unusable scrap from each tread board. Solid stringers (with cleats): 5% extra. Treads bolt straight to cleats, only trim waste at ends. DeckMath's waste factors are conservative and assume professional cuts — DIY cuts can run 20%+ for cut stringers. Always round UP to the next whole stock board.

What stock length should I buy for stair treads?

PT lumber: 8 ft is the smallest commonly stocked. For 42″ stairs (3.5 ft wide), buy 8 ft → 1 tread per board (lots of waste) OR buy 16 ft → 4 treads per board (lots of cuts). DeckMath picks the next stock length ≥ stair width — usually 8 ft minimum. Composite: 12/16/20 ft only — pick to minimize waste (e.g. 16 ft → 4 treads per board × 42″ wide = 4 × 3.5 = 14 ft used, 2 ft scrap = 12.5% waste). Hardwoods: 6/8/10/12 ft — shorter stock, order more boards.

How are stair treads fastened?

PT lumber treads: 3-3.5″ exterior deck screws (Spax PowerLags or Simpson Strong-Drive), 2 per stringer per board. Pre-drill near ends to prevent splitting. Composite grooved: hidden clips (CAMO Marksman or TigerClaw) screwed into stringer groove — clip slides into board side groove. Composite square-edge: 3″ stainless deck screws + color-matched plugs (CAMO Hex Plugs). Hardwood: stainless screws ONLY (carbon-steel rusts at the Ipe phenol-tannin contact), pre-drilled pilot holes mandatory (Ipe is 4× harder than oak — splits regular screws).

How long do stair treads last?

PT lumber: 18 yrs with annual sealing/staining; 12 yrs un-maintained (silver-grey weathering, eventual checking). Cedar: 22 yrs (natural oils slow rot). Composite (Trex/TimberTech): 25-30 yrs (manufacturer warranty 25-30 yrs against rot/fading/splintering — actual life often longer). Hardwood Ipe: 50-75 yrs (the longest-lasting deck material commonly available — natural Class A fire rating). Cumaru: 50 yrs. Maintenance: PT + cedar need annual seal/stain; composite + hardwood need only a power-wash 1× per year.

stairs · tread BoM · IRC R311.7.5

Stair Tread Calculator

Tread board count, total LF + waste, riser-board count, and material-tier price comparison across 6 deck-stair materials: PT lumber, grooved + square-edge composite, Ipe + Cumaru hardwoods, and cedar. Pick board pattern (single 2×12, two-board 2×6×2, or pre-bullnosed stair-tread stock) and stringer style (cut adds 15% waste, solid 5%). Every tread + nosing cross-validated against IRC 2021 R311.7.5.2 + R311.7.5.3. Pairs with stair-stringer-calculator (cut layout) and stair-rise-run-calculator (pure geometry).

6 materialsIRC R311.7.5Cost comparisonCut vs solid waste3 board patterns

6·Materials compared

10″·IRC min run

15/5%·Cut/solid waste

75 yr·Ipe max lifespan

Inputs

Stair dimensions

Tread countrisers − 1

Stair widthclear

Tread runIRC ≥ 10″

Nosingoverhang

Effective tread depth12″

Total tread LF20.1 lf

Boards needed6 × 8′

Riser boards5 (17.5 lf)

Tread material

Board patternboard layout per tread

Stringer style

State2-letter

Results

IRC R311.7.5 compliant · Pressure-treated 5/4 or 2× (SYP) · 18-yr life

Tread boards

8′ stock

Total tread LF

15% waste

Risers

17.5 lf

Project total

Low $198 · Northeast

Tread run 11″ ≥ IRC min 10″

IRC R311.7.5.2

Each tread is 11″ deep (run only, excluding nosing). Effective foot-room = 12″ including 1″ nosing. IRC R311.7.5.2 requires 10″ minimum tread depth.

Nosing 1″ in IRC range 3/4-1.25″

IRC R311.7.5.3

Nosing of 1″ falls within IRC R311.7.5.3's required 3/4″-1-1/4″ projection for closed-riser stairs.

6 tread boards × 8′ stock

AWC DCA-6 tread requirements

20.1 lf of Pressure-treated 5/4 or 2× (SYP) including 15% waste for cut stringer cuts. 1 board per tread row × 5 treads.

5 riser boards (1×8 PT)

AWC DCA-6 riser standard

17.5 lf of 1×8 PT riser stock. Closed-riser style per IRC R311.7.5 (riser blocks debris, hides framing, allows full nosing compliance).

Item

Qty

Total

Pressure-treated 5/4 or 2× (SYP) treads

6 boards × 8′ stock · 5 treads × 1 board each · 20.1 lf w/ 15% cut waste

6.0 × 8′

$59

Pressure-treated 5/4 or 2× (SYP) treads

6 boards × 8′ stock · 5 treads × 1 board each · 20.1 lf w/ 15% cut waste

6.0 × 8′

—

$59

Riser boards (1×8 PT)

5 risers × 3.5′ wide = 17.5 lf · closed-riser style only

5.0 ea

$32

Riser boards (1×8 PT)

5 risers × 3.5′ wide = 17.5 lf · closed-riser style only

5.0 ea

—

$32

Tread fasteners (PT decking screws)

5 fastener kits · 1 per tread · 1 board attachment per tread

5.0 kit

$16

Tread fasteners (PT decking screws)

5 fastener kits · 1 per tread · 1 board attachment per tread

5.0 kit

—

$16

Materials subtotal

$107

Material comparison — same stair, every material

Material	$/lf	Treads $	Lifespan
Pressure-treated 5/4 or 2× (SYP)selected	$2.95	$59	18 yr
Western red cedar 2×	$4.20	$85	22 yr
Composite grooved (Trex / TimberTech / Fiberon)	$6.50	$131	30 yr
Composite square-edge (face-screwed)	$6.85	$138	30 yr
Brazilian Cumaru (Brazilian teak)	$8.95	$180	50 yr
Brazilian Ipe (Lapacho)	$11.50	$231	75 yr

Treads-only cost across all 6 materials — pick the right tier for your budget + lifespan target. Hardwoods cost 4× PT but last 4× longer with zero stain/seal maintenance.

Cost breakdown

Materials (treads + risers + fasteners): $99 – $108
Labor (cut + fasten + nose): $99 – $121
Subtotal: $198 – $229
Contingency (6%): $14
Project total: $198 – $242

2026-Q1 retail pricing. TREADS ONLY — does NOT include stringers (use Stair Stringer Calculator), handrail, or landing pad (use Deck Stair Calculator for full stair BoM). Regional labor adjusted for Northeast.

How to use

Three steps. Permit-ready output.

Enter tread count + stair width

Tread count = riser count − 1 (top step is the deck edge). For a 36″ deck → 5 risers → 4 treads; 42″ deck → 6 risers → 5 treads; 48″ → 7 risers → 6 treads. Use the Stair Rise & Run Calculator first if you don't know your riser count. Stair width is the clear distance between stringer outer faces — 36″ IRC min, 42-48″ most common.

Pick run + nosing

Tread run is the horizontal depth (excluding nosing) — IRC ≥ 10″. 11″ is the contractor sweet-spot. Nosing projects past the riser face by 3/4″-1-1/4″ on closed-riser stairs (R311.7.5.3). Effective tread depth = run + nosing, so 11″ + 1″ = 12″ of foot-room.

Choose tread material

PT lumber ($2.95/lf): cheapest, 18-yr life, fits 8/10/12/16 ft stock. Composite grooved ($6.50/lf): 30-yr warranty, hidden-clip fastening, 12/16/20 ft. Composite square-edge ($6.85/lf): face-screwed + plugs. Ipe ($11.50/lf, 75 yr) + Cumaru ($8.95/lf, 50 yr): premium hardwoods, stunning grain, ages to silver if unsealed. Cedar ($4.20/lf, 22 yr): visual upgrade over PT, mid-tier.

Board pattern

Single-board: one 11.25″ board (2×12 nominal) — clean look, works to 48″ wide. Two-board: 2 × 5.5″ boards (2×6 + 2×6) — standard for wider stairs (> 48″), more stable, mimics deck board patterning. Stair-tread stock: pre-bullnosed 12″ wide board — eliminates nosing build-up, premium look, only available in PT + Ipe.

Risers + stringer style

Include risers: closed-riser style (riser board behind each tread). Skip risers for open-riser (skip-stop) style — visible from below, requires open spacing ≤ 4″ per IRC R312.1.3.1. Stringer style affects waste: cut stringers (notched) leave 15% scrap from tread cuts, solid stringers (cleated) only 5% trim waste. DeckMath bumps tread LF accordingly.

How we calculate

The math, fully transparent.

The Stair Tread Calculator returns the tread lumber BoM for a deck stair: board count by stock length, total LF + waste factor, riser-board count, fasteners, and side-by-side cost comparisons across six materials (PT lumber, grooved + square-edge composite, Ipe + Cumaru hardwoods, cedar). Pick tread count, stair width, run, nosing, and board pattern (single 11.25″ board, two-board 2×6+2×6, or pre-bullnosed stair-tread stock). DeckMath enforces IRC R311.7.5.2 (10″ min run) + R311.7.5.3 (nosing 3/4″-1-1/4″) and adds the right waste factor for cut vs solid stringers (15% vs 5%). Pairs with stair-stringer-calculator (cut-layout BoM), stair-rise-run-calculator (pure geometry), and deck-stair-calculator (full stair BoM).

IRC references

IRC 2021 R311.7.5.2 — Tread depth: 10″ minimum
IRC 2021 R311.7.5.3 — Nosing: 3/4″-1-1/4″ projection on closed-riser stairs
IRC 2021 R311.7.5.3.1 — Nosing of treads must comply with slip-resistance
IRC 2021 R312.1.3.1 — Open-riser stairs: 4″ sphere can't pass through opening
IRC 2021 R317.1 — Pressure-treated lumber required within 6″ of grade
AWC DCA-6 — Tread board minimum thickness 1-1/16″ (5/4 deck board) for ≤ 16″ stringer spacing

2026-Q1 retail pricing: PT lumber 5/4×6 $2.95/lf · composite grooved $6.50/lf · composite square-edge $6.85/lf · Brazilian Ipe $11.50/lf · Cumaru $8.95/lf · western red cedar $4.20/lf. Stock-length availability varies — PT/cedar 8/10/12/16 ft, composite 12/16/20 ft, hardwoods 6/8/10/12 ft. Waste factor: cut/notched stringers 15% (notched-cut tread offcuts), solid stringers 5% (trim only). Riser boards 1×8 PT $1.85/lf. Fastener kits 1/tread — hidden clips + plugs for composite, stainless screws + pilot bit for hardwood, deck screws for PT/cedar.

Per-tread board LF

perTreadLf = (widthIn / 12) × boardsPerTread

Stair width converted to feet × boards per tread row. 42″ wide × 1 board (single-board pattern) = 3.5 lf per tread. 42″ × 2 (two-board) = 7 lf.

Total tread LF with waste

totalLf = perTreadLf × treadCount × wasteFactor

Cut stringers waste 15% from notched-cut tread offcuts; solid stringers waste 5%. For 5 treads × 3.5 lf × 1.15 (cut) = 20.1 lf order. Solid: 5 × 3.5 × 1.05 = 18.4 lf.

Tread board count by stock length

boards = ceil(treadCount × boardsPerTread × wasteFactor)

Assumes one stock board cuts down to one tread board (most efficient when stair width ≤ stock length). 5 treads × 1 board × 1.15 = 5.75 → 6 boards needed. Pick stock = next stock length ≥ stair width — 42″ width → 4 ft? No — 8 ft is the smallest commonly stocked.

Riser board LF

riserLf = treadCount × (widthIn / 12)

Closed-riser: 1 riser board per tread row, same width as the stair. 5 treads × 3.5 ft = 17.5 lf of 1×8 riser stock.

Effective tread depth

effDepth = treadRunIn + nosingIn

What your foot actually steps on: 11″ run + 1″ nosing = 12″ of foot-room. Open-riser stairs have no nosing, so effDepth = run = 11″ (still IRC-compliant, but feels slightly less roomy).

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Deck Stair Calculator

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

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 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.