Code Corner
Straight answers to the code and construction questions people actually ask — each with its section citation, a link to the source, and the calculator that works it out. We cite specific sections and link out; we never reproduce copyrighted code text.
Stairs
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How do you calculate the number of steps in a staircase?
Divide the total rise (floor-to-floor height) by the maximum riser of 7¾ inches and round UP — that's the minimum number of risers (2018 IRC §R311.7.5.1). Then the actual riser height is the total rise divided by that count. Example: 108 inches ÷ 7¾ = 13.9 → 14 risers, each 108 ÷ 14 ≈ 7.71 inches. Treads equal the risers minus one.
2018 IRC §R311.7.5.1 -
How high can a stair flight rise before a landing?
Under the 2018 IRC (§R311.7.3), a single stair flight may rise no more than 12 feet 7 inches (151 inches) of vertical height between floor levels or landings. Climb higher than that and you must break the run with an intermediate landing, at least as deep as the stairway is wide.
2018 IRC §R311.7.3 -
How much can stair risers vary in height?
Within a stair flight, the difference between the tallest and the shortest riser cannot exceed 3/8 inch, and the same 3/8-inch limit applies to tread depths (2018 IRC §R311.7.5.1 and §R311.7.5.2). Uneven risers are a trip hazard, so measure each one after framing — not just the design height.
2018 IRC §R311.7.5.1 -
What is the maximum stair riser height?
For stairs in one- and two-family dwellings the maximum riser height is 7¾ inches (7.75 in) under the 2018 IRC (§R311.7.5.1). The tallest and shortest riser in a single flight may differ by no more than 3/8 inch, so risers must be evenly spaced.
2018 IRC §R311.7.5.1 -
What is the minimum stair headroom?
Residential stairs need a minimum headroom of 6 feet 8 inches (80 inches), measured vertically from a line along the tread nosings to any ceiling or obstruction above (2018 IRC §R311.7.2). The clearance has to hold for the full width of the stairway and continue over the landings.
2018 IRC §R311.7.2 -
What is the minimum stair tread depth?
The minimum tread depth (the run of each step) for stairs in a dwelling is 10 inches under the 2018 IRC (§R311.7.5.2), measured horizontally from nosing to nosing. As with risers, treads must be uniform — the largest and smallest in a flight can differ by no more than 3/8 inch.
2018 IRC §R311.7.5.2 -
What is the minimum width for a staircase?
Residential stairways must be a minimum of 36 inches (914 mm) in clear width, measured above the handrail height and below the required headroom (2018 IRC §R311.7.1). Handrails may project up to 4½ inches into that width on each side, so the clear width can narrow to 31½ inches with one handrail, or 27 inches with handrails on both sides.
2018 IRC §R311.7.1 -
What is the stair nosing requirement?
On stairs with solid risers, treads less than 11 inches deep must have a nosing that projects ¾ inch to 1¼ inches beyond the tread below (2018 IRC §R311.7.5.3). The nosing's leading edge may have a radius up to 9/16 inch or a bevel up to ½ inch, and the projection must be consistent within 3/8 inch across the flight.
2018 IRC §R311.7.5.3 -
What makes a stair comfortable? (the 2R + T rule)
A stair feels natural to climb when two risers plus one tread — 2R + T — total about 24 to 25 inches (the Blondel rule). A 7-inch riser with an 11-inch tread gives 2(7) + 11 = 25 inches, right in the range. It's a comfort rule of thumb, not code, but a stair that meets both the IRC limits and 2R + T rarely feels too steep or too shallow.
Blondel rule of thumb (not a code value) -
When is a handrail required on stairs?
Under the IRC, stairways with 4 or more risers require at least one handrail (§R311.7.8.1). The top of the gripping surface must sit 34 to 38 inches above the stair nosings, and the handrail must run continuously and be graspable along the flight. This is separate from any guard required at an open side.
2018 IRC §R311.7.8
Ramps & ADA
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How big does an ADA ramp landing need to be?
ADA ramp landings must be at least as wide as the widest ramp run leading to them and a minimum of 60 inches long in the direction of travel (2010 ADA §405.7). A landing is required at the top and bottom of each ramp run, and where a ramp changes direction the landing must be at least 60 by 60 inches.
2010 ADA §405.7 -
How high should ADA ramp handrails be?
The gripping surface of an ADA ramp handrail must be 34 to 38 inches above the ramp surface, measured vertically (2010 ADA §505.4). Handrails are required on both sides of any ramp run that rises more than 6 inches (§405.8), and the height must stay consistent along the full length of the run.
2010 ADA §505.4 -
How wide does an ADA doorway need to be?
An accessible doorway must provide at least 32 inches of clear width, measured between the face of the door and the stop with the door open 90 degrees (2010 ADA §404.2.3). Because hardware and the open leaf eat into the opening, that usually means a 36-inch door. Openings deeper than 24 inches need 36 inches of clear width.
2010 ADA Standards §404.2.3 -
How wide is an ADA parking space?
An accessible car parking space is at least 96 inches (8 feet) wide with an adjacent access aisle at least 60 inches wide (2010 ADA §502.2–§502.3). A van space is 132 inches (11 feet) wide, or a 96-inch space paired with a 96-inch access aisle. The access aisle runs the full length of the parking space.
2010 ADA §502 -
What is the ADA accessible counter height?
An accessible sales or service counter must include a section no more than 36 inches above the floor (2010 ADA §904.4). A parallel-approach counter needs at least 36 inches of lowered length; a forward-approach counter needs 30 inches plus knee and toe clearance beneath. Dining and work surfaces run 28–34 inches (§902.3).
2010 ADA §904.4 -
What is the ADA clear floor space for a wheelchair?
The ADA clear floor or ground space for a single stationary wheelchair is 30 by 48 inches (760 by 1220 mm), positioned for either a forward or a parallel (side) approach (2010 ADA §305.3). This is the footprint that has to stay clear at fixtures, controls, and appliances so a wheelchair user can pull up and use them.
2010 ADA §305.3 -
What is the ADA reach range?
For an unobstructed forward or side reach, operable parts must be no higher than 48 inches and no lower than 15 inches above the floor (2010 ADA §308.2.1, §308.3.1). Reaching over an obstruction lowers the maximum — for example, 44 inches for a forward reach over a surface 20–25 inches deep (§308.2.2).
2010 ADA §308 -
What is the ADA toilet seat and grab bar height?
Under the 2010 ADA Standards, an accessible water closet seat is 17 to 19 inches above the floor (§604.4), and grab bars mount with their tops 33 to 36 inches above the floor (§609.4). The side grab bar is at least 42 inches long and no more than 12 inches from the rear wall (§604.5.1); the rear bar is at least 36 inches (§604.5.2).
2010 ADA §604 / §609 -
What is the ADA wheelchair turning space?
A wheelchair turning space under the 2010 ADA Standards is either a 60-inch diameter circle (§304.3.1) or a T-shaped space within a 60 × 60-inch square with 36-inch-wide arms (§304.3.2). Knee and toe clearance under a counter or lavatory may count toward part of the required space (§304.3).
2010 ADA Standards §304.3 -
What is the maximum cross slope for an ADA ramp?
An ADA ramp's cross slope — the slope perpendicular to the direction of travel — may be no steeper than 1:48, about 2.08% (2010 ADA §405.3). That nearly level cross slope keeps a wheelchair from drifting or tipping sideways. It's separate from the running slope, which can be as steep as 1:12.
2010 ADA §405.3 -
What is the maximum rise for a single ADA ramp run?
A single ADA ramp run may rise a maximum of 30 inches (760 mm) before a level landing is required (2010 ADA §405.6). At the steepest allowed running slope of 1:12, a 30-inch rise means a run of 30 feet, after which the ramp needs a 60-inch landing before the next run continues the climb.
2010 ADA §405.6 -
What is the maximum slope for an ADA ramp?
The maximum running slope for an ADA-compliant ramp is 1:12 — one inch of rise for every 12 inches of run (8.33%) — under the 2010 ADA Standards (§405.2). A single ramp run may rise at most 30 inches (§405.6) before a level landing is required.
2010 ADA Standards §405.2
Guardrails
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How high does a deck railing need to be?
A deck railing is a guard: under the IRC it must be at least 36 inches high (§R312.1.2), and it's required whenever the deck walking surface is more than 30 inches above the grade below (§R312.1.1). Commercial decks and balconies follow the IBC's 42-inch guard (§1015.3). The infill must also reject a 4-inch sphere.
2021 IRC §R312.1 -
How much load must a guardrail resist?
A residential guard must resist a single 200-pound concentrated load applied in any direction at any point along its top (2018 IRC §R301.5, Table R301.5). The in-fill — balusters or panels — must separately resist a 50-pound load over a 1-square-foot area. Commercial guards also carry a 50 lb/ft distributed load on the top rail (IBC §1607.8).
2018 IRC §R301.5 -
How tall does a guardrail need to be?
Residential guards must be at least 36 inches high (2021 IRC §R312.1.2); in most commercial buildings the minimum is 42 inches (2021 IBC §1015.3). A guard is required wherever a walking surface, including stairs and landings, is more than 30 inches above the floor or grade below.
2021 IRC §R312.1.2 · IBC §1015.3 -
What is the maximum gap between guardrail balusters?
Guardrail infill must not allow a 4-inch sphere to pass, so the clear gap between balusters must be less than 4 inches (2021 IRC §R312.1.3, 2021 IBC §1015.4). Because a sphere exactly 4 inches must NOT pass, size the spacing to land just under 4 inches — not exactly at it.
2021 IRC §R312.1.3 · IBC §1015.4 -
When is a guardrail required?
A guard is required wherever a walking surface — a deck, balcony, landing, or the open side of a stair — is more than 30 inches above the floor or grade below, measured within 36 inches horizontally (IRC §R312.1.1, IBC §1015.2). Below that 30-inch threshold a guard isn't required by code, though it can still be a good idea.
2021 IRC §R312.1.1 · IBC §1015.2
Framing
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How deep should a beam be? (rule of thumb)
A fast preliminary estimate uses the span-to-depth (L/d) ratio: a wood floor joist runs about L/18–L/20, a glulam/LVL beam L/16–L/20, a steel beam L/20–L/24, and a one-way concrete slab L/24–L/28. For a 20-ft steel beam, L/20 works out to about 12 inches deep. These are rules of thumb for early layout — not span tables and not a code check.
Rule-of-thumb span-to-depth ratios (not a code check) -
How do you calculate the number of studs in a wall?
For the field studs, divide the wall length by the on-center spacing, round up, and add one: a 16-ft wall at 16 inches o.c. is (192 ÷ 16) + 1 = 13 studs. That covers the evenly spaced in-line studs only — add extra for each corner, wall intersection, and rough opening (kings, jacks, and cripples).
Layout geometry (field studs) -
How far apart are floor joists?
Floor joists are most commonly spaced 16 inches on-center (o.c.); 12, 19.2, and 24 in o.c. are also used. There's no single required spacing — the IRC sets the allowable span for a given size, species/grade, and load in its floor-joist span tables (§R502.3), so the spacing you can use depends on how far the joists have to carry.
2021 IRC §R502.3 (floor-joist spans) -
How far apart are roof rafters?
Roof rafters are most commonly spaced 16 or 24 inches on-center (o.c.); 12 and 19.2 in o.c. are also used. There's no single required spacing — the IRC sets the allowable span for a given size, species/grade, spacing, and roof load (including snow) in its rafter span tables (§R802.4), so the spacing you can use depends on the load and the distance spanned.
2021 IRC §R802.4 (rafter spans) -
What is standard stud spacing?
Wood wall studs are standard at 16 or 24 inches on-center (o.c.). 16 in o.c. is the common default; 24 in o.c. is allowed for many walls but typically calls for 5/8-inch gypsum board to keep the drywall from sagging. The allowable spacing depends on stud size, height, and load — set by the IRC (§R602.3).
IRC §R602.3 -
What is the actual size of a 2x4?
A '2x4' is a nominal size — the actual dressed dimensions are 1½ × 3½ inches. Nominal dimension lumber is always smaller than its name once surfaced: 2x6 = 1½ × 5½, 2x8 = 1½ × 7¼, 2x10 = 1½ × 9¼, and 2x12 = 1½ × 11¼ inches. These finished sizes come from the American Softwood Lumber Standard (PS 20).
American Softwood Lumber Standard (PS 20) -
What is tributary area?
Tributary area is the floor or roof area that delivers its load to one support — a beam, column, or footing — bounded by the midlines to adjacent supports. An interior column carries a full bay (column spacing one way × the other); an edge column about half, a corner about a quarter. Multiply by the load per square foot to size it.
Structural load takedown (tributary area)
Roofing
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How do you calculate rafter length?
A common rafter's line length is √(run² + rise²) — the diagonal of the right triangle formed by the horizontal run and the vertical rise. Run is half the building span; rise is run × (pitch ÷ 12). For a 24-foot span at 6:12, run = 12 ft and rise = 6 ft, so line length = √(12² + 6²) ≈ 13.42 ft. Then add the overhang and subtract half the ridge thickness.
Roof geometry (Pythagorean theorem) -
How do you calculate roof area?
A pitched roof covers more than the footprint beneath it, so multiply the plan footprint by the slope factor — √(rise² + 12²) ⁄ 12 — to get the actual surface area. A 6:12 roof has a factor of about 1.118, so a 1,200 sq ft footprint becomes ~1,342 sq ft of roof. Divide the result by 100 to get roofing squares.
Slope-factor geometry -
How many bundles of shingles are in a square?
Most three-tab and architectural asphalt shingles come about 3 bundles to a square (a square = 100 sq ft of roof). Heavier or specialty shingles can run 4 or more bundles per square, so confirm the coverage printed on the wrapper. Order 5–15% extra for waste, starter course, and hip and ridge caps.
Roofing trade convention (check product wrapper) -
How many nails do you need per square of shingles?
Asphalt shingles need at least 4 fasteners per shingle, or 6 in high-wind regions and on steep slopes (2018 IRC §R905.2.5, plus the shingle maker's instructions). At roughly 80 shingles per square, that's about 320 nails per square — or about 480 with 6 nails — which works out to roughly 2 to 2½ pounds of roofing nails.
2018 IRC §R905.2.5 -
What is a roofing square?
A 'square' is the roofing trade's unit for 100 square feet of roof area. Roofers, estimates, and shingle packaging all measure in squares, and most shingles run about 3 bundles per square. It's an industry convention for estimating and ordering — not a building-code value.
Roofing trade convention (not a code value) -
What is the minimum roof slope for asphalt shingles?
Asphalt shingles may be installed only on roof slopes of 2:12 or steeper (IRC §R905.2.2). From 2:12 up to but not including 4:12, a double layer of underlayment is required; at 4:12 and above a single layer is typical. Below 2:12 you need a low-slope (membrane) roofing system instead of shingles.
IRC §R905.2.2 -
Where is ice and water shield required on a roof?
Where ice damming is a concern — the IRC triggers it where the average January temperature is 25°F or colder — an ice barrier must extend from the eave's edge to a point at least 24 inches inside the exterior wall line (2018 IRC §R905.1.2). It's a self-adhering membrane, or two cemented layers of underlayment, replacing normal underlayment in that zone.
2018 IRC §R905.1.2
Concrete
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How deep does a footing need to be?
A footing's bottom must be at least 12 inches below the undisturbed ground surface, and always below the local frost line (2018 IRC §R403.1.4, §R403.1.4.1). Frost depth is set by your jurisdiction — a few inches in the South to 48 inches or more in cold climates — so the required depth varies widely by location.
2018 IRC §R403.1.4 -
How do you calculate concrete for a slab?
For a rectangular slab, concrete volume = length × width × thickness with every dimension in feet, then divide by 27 to convert cubic feet to cubic yards. A 10 × 10 ft slab at 4 inches (0.333 ft) thick is 33.3 ft³ ≈ 1.23 yd³. Add about 10% for spillage, sub-grade irregularity, and over-excavation.
Volume geometry (V = L × W × T) -
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. At typical yields that's about 45 bags of 80-lb mix (≈0.60 ft³ each), 60 bags of 60-lb (≈0.45 ft³), or 90 bags of 40-lb (≈0.30 ft³). Yields vary by product and consolidation, so check the bag — and for anything past a few bags, ready-mix by the yard is usually cheaper.
Manufacturer bag yields (Quikrete / Sakrete) -
How much concrete does a sonotube need?
A round pier or sonotube holds π × radius² × height of concrete. For a 12-inch-diameter tube (a 0.5-ft radius) 4 feet deep: π × 0.5² × 4 ≈ 3.1 cubic feet, about 0.12 cubic yards. Multiply by the number of piers, then add ~10% for waste and any flared base or footing.
Volume geometry (V = π r² h)
Conversions
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How do you convert centimeters to inches?
Divide centimeters by 2.54 to get inches — one inch is exactly 2.54 centimeters (or multiply inches by 2.54 to get centimeters). Example: 30 cm ÷ 2.54 = 11.81 in, about 11¾ in. And 100 cm (1 meter) ÷ 2.54 = 39.37 in.
Unit conversion (1 in = 2.54 cm) -
How do you convert feet to meters?
Multiply feet by 0.3048 to get meters, because one foot equals exactly 0.3048 meters (or divide meters by 0.3048 to get feet). Example: 10 ft × 0.3048 = 3.048 m. A quick field estimate is feet ÷ 3.28, which lands within a few millimeters.
Unit conversion (1 ft = 0.3048 m) -
How do you convert inches to centimeters?
Multiply inches by 2.54 to get centimeters, because one inch equals exactly 2.54 centimeters (or divide centimeters by 2.54 to get inches). Example: 12 in × 2.54 = 30.48 cm. For millimeters, multiply inches by 25.4 instead.
Unit conversion (1 in = 2.54 cm) -
How do you convert inches to decimal feet?
Divide the inches by 12 to get decimal feet — there are 12 inches in a foot. So 6 inches = 0.5 ft, 3 inches = 0.25 ft, and 9 inches = 0.75 ft. For a mixed measurement, convert the inches and add: 8 ft 6 in = 8 + (6 ÷ 12) = 8.5 ft.
Unit conversion (12 in = 1 ft) -
How do you convert meters to feet?
Multiply meters by 3.28084 to get feet, because one meter equals 3.28084 feet (or divide feet by 3.28084 to get meters). A quick mental estimate is 1 m ≈ 3 ft 3⅜ in. Example: 2.4 m × 3.28084 = 7.87 ft, which is about 7 ft 10½ in.
Unit conversion (1 m = 3.28084 ft) -
How do you convert millimeters to inches?
There are exactly 25.4 millimeters in an inch, so divide millimeters by 25.4 to get inches: 100 mm ÷ 25.4 ≈ 3.94 in. For centimeters, divide by 2.54. To go the other way, multiply inches by 25.4. One foot is exactly 304.8 mm, and one metre is about 39.37 inches.
Exact conversion (1 in = 25.4 mm) -
How do you convert roof pitch to degrees?
Convert a roof pitch to degrees with the arctangent of rise over run: angle = atan(rise ÷ 12). A 6:12 pitch is atan(6/12) = 26.57°, and a 12:12 pitch is 45°. To get percent grade instead, use (rise ÷ 12) × 100 — so 6:12 = 50%.
Trigonometry (no code value) -
How do you convert square meters to square feet?
Multiply square meters by 10.7639 to get square feet (or divide square feet by 10.7639 to get square meters). Example: 10 m² × 10.7639 = 107.64 ft². The factor is the length factor squared: 1 m = 3.28084 ft, so 1 m² = 3.28084² = 10.7639 ft².
Area conversion (1 m² = 10.7639 ft²) -
How many cubic feet are in a cubic yard?
There are 27 cubic feet in a cubic yard. A yard is 3 feet, and volume cubes all three dimensions: 3 ft × 3 ft × 3 ft = 27 cubic feet. To convert cubic feet to cubic yards, divide by 27; to go the other way, multiply cubic yards by 27.
Unit conversion (1 yd = 3 ft) -
How many inches are in a foot?
There are exactly 12 inches in a foot. So to go from inches to feet, divide by 12 (30 in ÷ 12 = 2.5 ft = 2 ft 6 in); to go the other way, multiply feet by 12 (3 ft × 12 = 36 in). A yard is 3 feet, or 36 inches.
Definition (1 foot = 12 inches) -
How many square feet are in a square yard?
There are 9 square feet in a square yard. A yard is 3 feet, and area multiplies two dimensions: 3 ft × 3 ft = 9 square feet. To convert square feet to square yards, divide by 9; to go from square yards to square feet, multiply by 9.
Unit conversion (1 yd = 3 ft) -
How many square feet are in an acre?
An acre is 43,560 square feet, which is 4,840 square yards or about 0.405 hectares. It's an area, not a shape: any region totaling 43,560 sq ft is an acre, whether it's a square (about 208.7 ft on a side) or a long strip. A common mental picture is roughly 90% of an American football field, minus the end zones.
Exact conversion (1 acre = 43,560 sq ft) -
What is 1/8 inch scale?
At 1/8-inch scale (written 1/8" = 1'-0"), one-eighth of an inch on the drawing represents one foot in the real building. Because a foot is 12 inches, the drawing is 1:96 (12 ÷ 0.125 = 96). It reads at half the size of 1/4-inch scale, so it is common for whole-floor plans that need to fit on a sheet.
Drafting scale (1/8" = 1'-0") -
What scale is 1/4 inch = 1 foot?
The scale 1/4 inch = 1 foot equals a ratio of 1:48. On the drawing, 1/4 inch represents 1 foot (12 inches), so one inch of paper equals 48 inches — 4 feet — in reality. To find the ratio for any imperial scale, divide 12 by the inches-per-foot: 12 ÷ 0.25 = 48.
Scale ratio (12 ÷ inches-per-foot)
Estimating
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How do you calculate square footage?
Square footage is length × width, with both measured in feet — a 12 × 10 ft room is 120 sq ft. For odd shapes, split them into rectangles and add the pieces. Handy conversions: 1 square yard = 9 sq ft, 1 acre = 43,560 sq ft, and 1 square metre ≈ 10.76 sq ft.
Area geometry (exact conversions) -
How many drywall sheets do I need?
Divide the total wall or ceiling area by one sheet's coverage and round up: a 4×8 sheet is 32 sq ft, so 500 sq ft ÷ 32 ≈ 16 sheets — about 18 with a 10% waste allowance. Larger 4×12 sheets (48 sq ft) cut down on seams. Area is gross, so you can subtract large openings for a tighter count.
Area ÷ sheet size + waste -
How many tiles do I need?
Divide the area to cover by the area of one tile, then add 10–15% for cuts, breakage, and future repairs. A 12×12-inch tile is 1 sq ft, so a 100 sq ft floor needs ~100 tiles bare, or ~110–115 with waste. Buy the extra from the same dye lot, since colors shift between production runs.
Area ÷ tile area + waste -
How much does a gallon of paint cover?
A gallon of interior paint covers roughly 350 square feet in one coat. It's a typical value — real coverage depends on the product, the surface texture, and porosity, so check the label. For two coats, figure the wall area twice; primer on bare or patched surfaces is extra.
Manufacturer coverage (typical ~350 sq ft/gal)