Roof Framing Estimator

Purpose and model boundary

This calculator takes off the framing lumber and roof sheathing for a stick-framed roof you draw, and estimates how much of the attic floor is livable under a rule you choose. You draw the roof as seen from above, at the roof edge (the drip line): its outline, its ridges, hips and valleys, and each facet's pitch. It returns every member type to buy (common rafters, hip and valley rafters, hip, valley and cripple jacks, the ridge board, fly rafters and lookouts, sub-fascia, ceiling joists or rafter ties, collar ties, gable-end and knee-wall studs and plates, and the trimmers and headers around roof openings), the stock pieces by size and length, the board feet (cubic metres in Metric), the sheathing sheets, an optional cost, and the attic floor area by headroom.

It is not a structural design, a framing plan, a code approval or a cost estimate. The member sizes are your choice: no span, load, deflection or connection is checked, and no size is recommended. Trusses are not modelled. Roof framing must be designed or approved by a licensed professional engineer or architect where your jurisdiction requires it. The livable area is an estimate under the rule you choose, not a determination under any code.

Inputs and units

The drawing is the first input. The drawing tool works out the roof's geometry and sends it as tables: each facet's plan and true area, the length of every kind of roof line, the wall line inside the overhangs, every framing member with its place and length, a summary of the members by type and size, the settings it framed with, and the floor area at or above each clear height, for the whole attic and for each attic room. The spreadsheet never recomputes that geometry; it checks it against the drawing and the form, and then applies its own rules.

The inputs beside the drawing are, in each system:

• Rafter spacing: 12, 16, 19.2 or 24 in (US), or 30, 40 or 60 cm (Metric), measured along each eave run.
• Member sizes, one for each member class: rafters (with the jacks, fly rafters, trimmers and headers), the ridge board, hip and valley rafters, ceiling joists or rafter ties, collar ties, gable-end and knee-wall studs and plates, lookouts and sub-fascia. US sizes are nominal (2x4 to 2x12); Metric sizes are actual, 47 mm thick and 100 to 250 mm deep.
• Eave overhang and rake overhang: how far the roof runs past the wall line, measured level, in feet or metres.
• Ceiling framing: ceiling joists, rafter ties, or none where a structural ridge carries the roof.
• Collar ties: off, on every rafter pair or on every second pair, at a fraction of the rise or at a height above the attic floor.
• Knee walls: off or on, with their height.
• Attic floor to the top of the rafters at the wall line, the roof sheathing thickness and the ceiling finish below the rafters: these set the clear height under the roof.
• Sheathing waste and the area of one sheathing sheet.
• The livable-area rule and its heights: the lowest counted height, the full height and the share needed at full height, the weight of the middle band, or a single threshold.
• Prices: lumber per board foot (per cubic metre in Metric) and per sheathing sheet. They start at 0, so every cost reads 0 until a price is entered.

Each system keeps its own figures, such as 16 in and 40 cm, or 2x8 and 47 × 195 mm, which are not conversions of each other. Switching between US and Metric never converts a number you typed. The defaults are the calculator's, listed on the spreadsheet's Data sheet as values to review before use.

Governing relationships

• Buying rule. Each member row buys the shortest stock length at least as long as its cut, compared at six decimals: a 14.000000 ft cut buys 14 ft stock and a 14.000001 ft cut buys 16 ft. Short pieces of one row share a stock piece when they fit (the stock length over the cut, rounded down); nothing is shared between rows, and the saw kerf is ignored.
• Long members. A ridge board, sub-fascia or knee-wall plate longer than the longest stock length is bought as butted pieces: the length over the longest stock, rounded up, pieces of the shortest stock that fits. Any other member that long is listed as special order and left out of the volume and the cost.
• No waste on framing. Members are counted exactly. Waste applies to the sheathing only.
• Sheathing. The true area of every facet, framed or not, times one plus the waste, over the sheet area, rounded up after rounding to six decimals. Exclusions such as skylights and chimneys are framed around with trimmers and headers and are not taken off the sheathing: the roof covers them in this estimate.
• Volume. US: nominal thickness × nominal depth in inches × stock length in feet / 12 per piece, in board feet. Metric: actual thickness × depth in millimetres × stock length in metres / 1,000,000 per piece, in cubic metres.
• Sizes come from the form. The drawing's size column is compared with the form for the rafters, the ridge board and the hips and valleys, which the drawing needs to place them; every other member is sized by the form alone.
• Livable area. Each threshold is read at the nearest step of the drawing's headroom curve, 1 in (US) or 25 mm (Metric), so 5.04 ft reads 5 ft. Under IRC R305.1 sloped ceiling and ANSI Z765 finished area, the counted area is the floor at or above the lowest counted height, and the check passes when the floor at or above full height is at least the share you set of it; a failing check keeps the counted area and says it is not livable as one room under this rule. The banded rule counts the floor at or above full height in full and the floor between the two heights at the middle weight. The single-threshold rule counts the floor at or above one height.

Calculation sequence

1. Drawing and typing stay on the page. Calculate sends the drawing, its framing tables and the form.
2. The page computes the framing tables at Calculate from the drawing and the form, so they always match the inputs sent with them.
3. The spreadsheet checks the inputs, then that the tables are present and were made for the form's settings, then that they agree with the drawing.
4. It applies the buying rule to every member row, adds up the stock pieces, the volume and the special-order members, and counts the sheathing sheets.
5. It reads the headroom curve at the rule's thresholds and works out the livable area, the bands and each attic room.
6. It multiplies by the prices, when entered.

The 3D framing view and the headroom bands on the drawing are views: they change no figure and show no number.

Outputs and interpretation

The first card gives what to buy: the stock pieces, the lumber volume, the sheathing sheets and the members listed as special order. The member cards count each member type. The What to buy table lists the stock pieces by member, size and stock length, with the longest cut in each group; Each member type sums them by type. The headroom card gives the livable area under the chosen rule, the floor at full height, the rule's check and the share at full height; the headroom bands and each attic room's figures follow. The chart shows the floor area at or above each clear height, with the rule's thresholds as red dashed lines. The roof cards and tables give the plan and true roof area, the sheathing area with waste, the wall line, each facet and each kind of roof line.

Validation and status logic

The status reads OK, CHECK or NOT VALID.

• NOT VALID means the drawing, its tables or the inputs do not describe a roof this calculator can take off, and the reason says what to fix. Framing, sheathing, cost and livable-area results read 0 and their result tables are blank; the drawn outline's gross, net and exclusion areas still show.
• The drawing's own rows are checked again, because a request need not come from the page: "NOT VALID: outline and exclusion points must fill consecutive rows, with no blank row between them"; "NOT VALID: every outline point needs both coordinates"; then, for each exclusion in turn, "NOT VALID: every point of exclusion 1 needs both coordinates", "NOT VALID: every point of exclusion 2 needs both coordinates", "NOT VALID: every point of exclusion 3 needs both coordinates", "NOT VALID: every point of exclusion 4 needs both coordinates" and "NOT VALID: every point of exclusion 5 needs both coordinates". On the page none should appear, because the drawing always writes whole points in consecutive rows.
• The two framing-table items: "NOT VALID: the roof's framing tables are missing or were made for other inputs; draw the roof on the page and press Calculate", and "NOT VALID: the roof's framing tables do not agree with the drawing; draw the roof on the page and press Calculate". On the page neither should appear, because the page recomputes the tables at Calculate.
• A roof with a facet pitched below 3 in 12 or above 24 in 12, a roof that needs more framing rows than the calculator holds, overhangs too large for the outline, an attic too tall for its headroom to be charted, and more than 10 attic rooms are each refused with their own sentence.
• CHECK means the answer computed but deserves attention: a facet above a pitch change or in a dormer that is not framed (its sheathing is counted), members longer than the longest stock length listed as special order, attic rooms that no longer fit inside the wall line, a livable-area check that fails, or exclusions drawn, which are not taken off the sheathing.

Assumptions and limitations

• The roof's geometry is the drawing's. The spreadsheet checks it with sanity bounds that are not a proof.
• Ceiling joists that span more than the longest stock length read CHECK and special order on many ordinary houses; splicing them over a bearing wall is common practice and is not modelled.
• Facets above a pitch change or inside a dormer are not framed in this version; their sheathing is counted and their headroom is in the curve.
• Fasteners, blocking, ridge beams sized as beams, attic partitions and labour are not taken off.
• Low-slope roofs under 3 in 12 and roofs steeper than 24 in 12 are not framed.

Restrictions and non-computing states

This calculator is available on its web page and through its /api/calc endpoint. It is excluded from MCP discovery, schema and calculation tools. The page produces the framing tables from the drawing at Calculate; an API caller must supply consistent drawing, form and canvas-produced tables. The workbook's bounds are sanity checks, not a proof that submitted tables describe the drawing. Under NOT VALID the framing, sheathing, cost and livable-area results read 0; the drawn outline's gross, net and exclusion areas remain available.

Errors and warnings

Fix a NOT VALID by doing what its sentence says, usually by redrawing the roof or changing an input, then press Calculate. Check the quantities against your plans before ordering.

Sources and further reading

• International Code Council, 2021 International Residential Code, section R305.1, Ceiling height. Cited by name and clause for the shape of the sloped-ceiling rule: a lowest counted height, a full height, and the share of the counted floor that must have full height. No code text is reproduced.
• ANSI Z765, Single-Family Residential Buildings: Square Footage, Method for Calculating. Cited by name for the shape of the finished-area rule's ceiling-height thresholds. No text of the standard is reproduced.
• National Institute of Standards and Technology, American Softwood Lumber Standard, Voluntary Product Standard PS 20-20 (Revision 1, October 2021), Table 3. nist.gov. The dressed sizes of the US lumber list, and the board foot.
• National Institute of Standards and Technology, Guide for the Use of the International System of Units (SI), NIST Special Publication 811, Appendix B.8. nist.gov. The exact foot, 0.3048 m.
• Eric W. Weisstein, "Polygon Area," MathWorld, Wolfram Research. mathworld.wolfram.com. The shoelace formula behind the outline, wall line and attic areas.

The banded rule has the shape of section 4 of the German living-area ordinance (WoFlV); it is named as a shape only, and no compliance with it is claimed. The metric sizes are confirmed against the published size table named on the spreadsheet's References sheet.

Links were current at build time. If one moves, search the document's title rather than reporting a broken link as a defect.

The spacings, stock lengths, sizes, overhangs and heights are the calculator's defaults, listed on the spreadsheet's Data sheet as values to review before use, not values from these sources or from any code. These sources describe the rules' shapes, the lumber sizes, the units and the geometry; they certify no particular roof or result.