Construction Glossary

Key construction, carpentry, and building code terms defined. A quick reference for contractors, builders, and DIYers.

A

Actual Dimension: The true measured size of a piece of lumber after planing and drying. A nominal 2x4 has actual dimensions of 1-1/2" x 3-1/2". Actual dimensions are always smaller than nominal dimensions because wood shrinks during kiln drying and material is removed during surfacing (planing). When calculating structural loads, clearances, or material fit, always use actual dimensions. Use our Board Foot Calculator →

B

Baluster: A vertical member (also called a spindle) in a railing system that fills the space between the top rail and bottom rail or floor. Per IRC R312.1.3, balusters must be spaced so that a 4-inch sphere cannot pass through the opening between them. Balusters can be made of wood, metal, glass, or composite material. Use our Baluster Calculator →
Bevel Angle: The angle at which the blade tilts away from vertical on a miter saw or table saw. A bevel cut changes the angle across the thickness of the material. In crown molding work, the bevel angle works together with the miter angle to create the correct compound joint. A 0-degree bevel means the blade is perfectly vertical. Use our Compound Miter Calculator →
Birdsmouth Cut: A notch cut into a rafter where it sits on the wall top plate. The birdsmouth consists of two cuts: the seat cut (horizontal, resting on the plate) and the plumb cut (vertical, against the outside of the wall). The depth of the seat cut determines the Height Above Plate (HAP). A proper birdsmouth should leave at least 2/3 of the rafter depth intact for structural integrity. Use our Rafter Calculator →
Board Foot: The standard unit of measurement for hardwood lumber volume, equal to a piece 1 inch thick, 12 inches wide, and 12 inches long (144 cubic inches). Board feet are always calculated using nominal dimensions, not actual dimensions. The formula is: BF = (Thickness x Width x Length) / 144, where thickness and width are in inches and length is in inches. Lumber yards price hardwoods by the board foot. Use our Board Foot Calculator →
Building Code: A set of regulations governing the design, construction, and maintenance of buildings. In the United States, the two primary model codes are the International Residential Code (IRC) for one- and two-family dwellings and the International Building Code (IBC) for commercial and multi-family buildings. Local jurisdictions adopt and sometimes amend these model codes. Always verify which code edition your jurisdiction enforces.

C

Common Rafter: A rafter that runs perpendicular from the wall plate to the ridge board at the full slope of the roof. Common rafters are the most basic roof framing member and form the primary structural support of a gable roof. Their length is calculated using the run (half the building span), the rise per foot of run (pitch), and the Pythagorean theorem. All other rafter types (hip, valley, jack) reference common rafter dimensions. Use our Rafter Calculator →
Compound Miter: A cut that combines both a miter angle (rotation of the workpiece on the saw table) and a bevel angle (tilt of the saw blade). Compound miters are required when joining material that meets at an angle other than 90 degrees, or when installing trim that is tilted relative to the wall, such as crown molding. The two angles work together to create a tight joint at the correct corner angle. Use our Compound Miter Calculator →
Concrete Yield: The volume of mixed concrete produced from a bag of dry concrete mix. An 80-pound bag of standard concrete mix yields approximately 0.6 cubic feet (0.022 cubic yards). A 60-pound bag yields about 0.45 cubic feet (0.017 cubic yards). Ready-mix concrete is sold by the cubic yard. When estimating bag counts, always add 5-15% for waste depending on the project complexity. Use our Concrete Calculator →
Crown Molding: A decorative trim installed at the junction of a wall and ceiling. Crown molding sits at an angle (the spring angle) rather than flat against either surface. Common spring angles are 38 degrees and 45 degrees. Because the molding is angled, cutting corners requires either compound miter cuts (flat method) or positioning the molding upside-down and angled in the saw (nested method). Use our Compound Miter Calculator →
Cubic Yard: The standard unit for ordering ready-mix concrete and bulk aggregates, equal to 27 cubic feet (3 feet x 3 feet x 3 feet). One cubic yard of concrete weighs approximately 4,050 pounds. To convert cubic feet to cubic yards, divide by 27. Most ready-mix companies require a minimum order (typically 1 yard) and charge short-load fees for orders under their minimum. Use our Concrete Calculator →

F

Frost Line: The maximum depth at which soil freezes in winter. Footings must extend below the frost line to prevent frost heave, which can shift and crack foundations. Frost line depth varies by region — from 0 inches in southern states to 72+ inches in northern states. Your local building department specifies the required footing depth. The IRC requires footings to be placed below the frost line per Section R403.1.4.1. Use our Concrete Calculator →

G

Guardrail: A protective barrier at the open side of a floor, landing, stair, or deck that is elevated above the adjacent surface. Per IRC R312.1.1, guardrails are required when the walking surface is more than 30 inches above the floor or grade below. The minimum guardrail height is 36 inches for residential decks and landings, and 34 inches measured vertically from the stair nosing for stair guardrails (IRC R312.1.2). Use our Baluster Calculator →

H

HAP (Height Above Plate): The vertical distance from the top of the wall plate to the top edge of the rafter, measured at the outside face of the wall. HAP is determined by the depth of the birdsmouth seat cut. A minimum HAP of 3-1/2 inches (matching the wall plate width) is standard practice, ensuring the rafter has adequate bearing surface and does not extend below the top plate. Use our Rafter Calculator →
Headroom: The vertical clearance measured from the nosing of a stair tread to the underside of any overhead obstruction directly above. IRC Section R311.7.2 requires a minimum headroom of 6 feet 8 inches for residential stairs, measured along the full length of the stairway. Headroom is one of the most commonly failed inspection items, especially in basement stair renovations where existing framing creates low points. Use our Stair Calculator →
Hip Rafter: A rafter that runs diagonally from the ridge to the corner of the building at the intersection of two roof planes. Hip rafters are longer than common rafters because they travel at a 45-degree angle in plan view while also climbing the roof slope. The hip rafter length uses a multiplier of the square root of (1 + (rise per foot)^2/144 + 1), and requires special check cuts and backing angles at the ridge. Use our Rafter Calculator →

I

IBC (International Building Code): The model building code for commercial, institutional, and multi-family residential buildings. Published by the International Code Council (ICC), the IBC has stricter stair requirements than the IRC: maximum 7-inch riser height, minimum 11-inch tread depth, minimum 80-inch headroom, and minimum 44-inch stair width. The IBC is updated on a three-year cycle. Use our Stair Calculator →
IRC (International Residential Code): The model building code for one- and two-family dwellings and townhouses in the United States. Published by the International Code Council (ICC), the IRC covers all aspects of residential construction. Key stair requirements include: maximum 7-3/4" riser height, minimum 10" tread depth, minimum 6'8" headroom, and maximum 3/8" variation between risers in a flight (IRC R311.7). Updated on a three-year cycle. Use our Stair Calculator →

J

Jack Rafter: A shortened rafter that does not span the full distance from ridge to plate. Jack rafters connect to either a hip rafter (hip jacks) or a valley rafter (valley jacks). They decrease in length by a constant increment determined by the rafter spacing and roof pitch. Jack rafters require a compound angle cut (cheek cut) where they meet the hip or valley rafter. Use our Rafter Calculator →

M

Miter Angle: The angle at which the workpiece is rotated on a miter saw table, measured from the fence (0 degrees = square cut against the fence). For a standard 90-degree inside corner, the miter angle is 45 degrees — each piece gets half the corner angle. For non-90-degree corners, the miter angle equals (180 minus the corner angle) divided by 2. Use our Compound Miter Calculator →

N

Nominal Dimension: The named size of a piece of lumber before planing and drying. A "2x4" is nominally 2 inches by 4 inches, but its actual dimensions are 1-1/2" x 3-1/2". Board foot calculations in the lumber industry always use nominal dimensions. Common nominal-to-actual conversions: 1" = 3/4", 2" = 1-1/2", 4" = 3-1/2", 6" = 5-1/2", 8" = 7-1/4", 10" = 9-1/4", 12" = 11-1/4". Use our Board Foot Calculator →
Nosing: The portion of a stair tread that projects beyond the face of the riser below. Nosing provides a larger landing surface for your foot during descent. IRC R311.7.5.3 requires a nosing projection between 3/4 inch and 1-1/4 inches when the tread depth is less than 11 inches. The nosing projection must be consistent within 3/8 inch between the largest and smallest nosing in a flight. Use our Stair Calculator →

O

On-Center (OC): A measurement from the center of one structural member to the center of the next. Studs framed at 16 inches on-center have their centerlines 16 inches apart. This is different from the clear space between members, which equals the OC spacing minus the member width. Common OC spacings in residential construction: 16" for walls and floors, 24" for some walls and roofs, 12" for heavy loads. Use our Baluster Calculator →

P

Pitch (Roof): The slope of a roof expressed as the number of inches of rise per 12 inches of horizontal run. A 6/12 pitch means the roof rises 6 inches for every 12 inches of run. Pitch can also be expressed as an angle in degrees (6/12 = 26.57 degrees), a percentage (50%), or a slope factor. Standard residential pitches range from 4/12 to 12/12. Pitches below 2/12 require special low-slope roofing materials. Use our Roof Pitch Calculator →
Plumb Cut: A vertical cut made on a rafter or stringer. The plumb cut angle equals the roof pitch angle measured from horizontal. Common plumb cuts include the ridge cut (top of rafter), the birdsmouth plumb cut (at the wall), and the tail cut (at the overhang). On a speed square, set the pivot point on the board edge and align the pitch with the board edge to mark a plumb line. Use our Rafter Calculator →

R

Ridge Board: A horizontal board at the peak of a gable roof to which the upper ends of rafters are fastened. The ridge board is typically one size deeper than the rafters (e.g., 2x8 ridge with 2x6 rafters) so it extends above the rafter backs. Common rafters must be shortened by half the thickness of the ridge board (typically 3/4 inch for dimensional lumber) when calculating their length. Use our Rafter Calculator →
Riser: The vertical face of a step in a stairway, or the measurement of its height. The unit rise is calculated by dividing the total rise by the number of risers. IRC maximum riser height is 7-3/4 inches; IBC maximum is 7 inches. The variation between the tallest and shortest riser in a flight cannot exceed 3/8 inch (IRC R311.7.5.2.1). Consistent riser height is critical for safety — uneven risers are a leading cause of stair falls. Use our Stair Calculator →

S

Slope Multiplier: A factor used to calculate the actual area of a sloped surface from its horizontal projection. For a roof with pitch P/12, the slope multiplier equals the square root of (1 + (P/12)^2). For example, a 6/12 pitch has a slope multiplier of 1.118, meaning the actual roof area is 11.8% larger than the footprint area. Also called the roof area factor or secant of the roof angle. Use our Roof Pitch Calculator →
Spring Angle: The angle at which crown molding tilts away from the wall when installed. Measured between the back of the molding and the wall surface. The two standard spring angles are 38 degrees (also called 38/52) and 45 degrees (also called 45/45). The spring angle determines the compound miter and bevel settings needed to cut corners. Always check the spring angle before cutting — using the wrong angle results in gaps. Use our Compound Miter Calculator →
Stringer: The inclined structural member that supports the treads and risers of a stairway. A stringer has triangular notches cut into it — the horizontal cut for the tread and the vertical cut for the riser. Most residential stairs use three stringers: two on the outside edges and one in the center. Stringer length is calculated using the Pythagorean theorem: length = square root of (total rise squared + total run squared). Standard stringer stock is 2x12 lumber. Use our Stair Calculator →

T

Tread: The horizontal surface of a step that you walk on. The tread depth (or unit run) is measured from the face of one riser to the face of the next, not including the nosing overhang. IRC minimum tread depth is 10 inches; IBC minimum is 11 inches. The number of treads in a flight equals the number of risers minus one, because the top "tread" is the landing or upper floor itself. Use our Stair Calculator →

U

Unit Rise: The vertical height of a single step, calculated by dividing the total rise (floor-to-floor height) by the number of risers. For example, 109 inches total rise divided by 15 risers = 7.267 inches unit rise (approximately 7-1/4"). All risers in a flight must be the same height within 3/8 inch tolerance. The target unit rise should fall between 7 and 7-3/4 inches for residential stairs. Use our Stair Calculator →
Unit Run: The horizontal depth of a single stair tread, measured from the face of one riser to the face of the next (excluding nosing). Not to be confused with tread width, which includes the nosing. The unit run determines the total horizontal distance the stairway covers. IRC requires a minimum unit run of 10 inches. The classic 7-11 rule recommends pairing a 7-inch rise with an 11-inch run for comfortable stairs. Use our Stair Calculator →

V

Valley Rafter: A rafter that runs diagonally from the ridge to the wall plate at the internal intersection of two roof planes, forming a valley. Valley rafters are structurally similar to hip rafters — they use the same length multiplier and require similar compound angle cuts. The key difference is that valley rafters create a concave channel rather than a convex ridge, so water flows along them rather than away from them. Use our Rafter Calculator →

W

Waste Factor: A percentage added to material estimates to account for cutting waste, breakage, defects, and errors. Common waste factors: 5-10% for concrete (simple pours), 10-15% for lumber (framing), 10-15% for drywall, 5-10% for paint. The waste factor is applied as a multiplier: adjusted quantity = calculated quantity x (1 + waste percentage). Higher waste factors are appropriate for complex shapes, inexperienced installers, or materials with high defect rates. Use our Concrete Calculator →

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