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Rafter Calculator

Quickly estimate rafter length, roof rise and plumb cut angle for a simple gable roof.

Inputs
Assumes a symmetrical gable roof. Run is calculated as half of the span.
Horizontal distance from outside wall to fascia.
Common residential roofs are often between 20° and 45°.
For example, a 4 in 12 roof has a rise of 4 for every 12 of run (4:12 pitch).

Results

Roof run (half span) –
Roof rise at ridge (above wall) –
Rafter length (wall plate to ridge) –
Rafter length including overhang –
Plumb cut angle at ridge –
Square set angle for top plumb cut.
These values are for a simple, symmetrical gable roof and do not include allowances for birdsmouth cuts, ridge thickness or material wastage.
Roof profile visualisation
To-scale angle based on your inputs (simplified side view).
Run Rise Angle Rafter

Table of Contents

🔹 What the Rafter Calculator Does

The rafter calculator helps you work out the key dimensions of a simple gable roof in seconds. By entering the building span, optional overhang and roof pitch, it returns the roof run, rise, rafter length (with and without overhang) and the plumb cut angle at the ridge.

Instead of sketching triangles and running trigonometry by hand, you get an instant, repeatable calculation that you can use on site, in the workshop or while preparing material lists.

🔹 Key Inputs You Need

  • Total span of the building – distance from outside wall to outside wall.
  • Rafter overhang (optional) – how far the rafter extends past the wall horizontally.
  • Roof pitch – either entered directly in degrees or as a rise/run ratio (for example 4:12).
  • Units – choose between metric (metres) or US/Imperial (feet and inches).

The calculator assumes a symmetrical gable roof, so the roof run is automatically taken as half of the total span. Overhang, if provided, is added along the rafter line, not just horizontally.

🔹 Quick Start: How to Use the Calculator

To get a fast, accurate result, follow these steps:

  • Select your preferred units (Metric or US / Imperial).
  • Enter the total span of the building between the outside faces of the bearing walls.
  • Type in the desired overhang length, or leave it as zero if there is no overhang.
  • Choose whether you want to enter pitch in degrees or as a rise/run ratio, then fill in the pitch values.
  • Click the “Calculate rafter size” button to generate all roof dimensions and the plumb cut angle.

Once calculated, the visualisation updates to match your chosen pitch, making it easier to picture the roof triangle and double-check that the numbers match your design intent.

🔹 Formulas Used in the Rafter Calculator

The rafter calculator is built around a right-angled triangle formed by the roof run (horizontal), roof rise (vertical) and rafter length (hypotenuse). It uses standard trigonometry to keep results accurate in both metric and imperial units.

🔹 Basic Roof Triangle Geometry

For a simple symmetrical gable roof:

  • Span – total distance from outside of one bearing wall to the outside of the opposite wall.
  • Run – half of the span (one side of the roof triangle).
  • Rise – vertical height from the top of the wall plate to the top of the ridge.
  • Rafter length – distance along the rafter from the wall plate to the ridge.
Quantity Formula (concept) Description
Roof run run = span ÷ 2 Half the total building span, assuming a symmetrical gable roof.
Roof rise rise = run × slope Vertical height based on the run and the roof slope (pitch).
Rafter length rafter = √(run² + rise²) Hypotenuse of the right triangle formed by run and rise.

🔹 Roof Pitch: Degrees vs Rise/Run

Roof pitch can be entered in two different ways:

  • Degrees – angle between the rafter and the horizontal (e.g. 30°).
  • Rise / Run – commonly used in carpentry, especially in imperial (e.g. 4:12 pitch).

Internally, the calculator converts both methods into the same quantity: the slope.

Input type Internal conversion What it means
Pitch in degrees slope = tan(angle) Uses the tangent function of the angle to get rise per unit of run.
Pitch as rise / run slope = rise ÷ run Directly uses the ratio, for example 4:12 gives a slope of 4 ÷ 12.

🔹 Overhang and Plumb Cut Angle

When you add a horizontal overhang, the calculator extends the rafter length along the slope:

  • Extra rafter length from overhang: extra = overhang × √(1 + slope²)
  • Rafter length including overhang: raftertotal = rafter + extra

The plumb cut angle at the ridge is simply the roof pitch angle itself. This is the angle you set on your saw or square when cutting the top of each rafter to fit against the ridge board.

In the visualisation beside the calculator, the roof triangle is dynamically redrawn based on this angle, so the on-screen rafter line matches the numerical pitch you entered.

🔹 Worked Example: Metric Gable Roof

This example shows how the rafter calculator works step by step for a typical small building in metric units.

Example scenario:

  • Total building span (outside wall to outside wall): 6.0 m
  • Desired roof pitch: 30°
  • Rafter overhang beyond wall: 0.45 m
  • Roof type: simple, symmetrical gable
Step Input / Formula Result (approximate)
1. Find roof run run = span ÷ 2 = 6.0 ÷ 2 Run = 3.0 m
2. Convert pitch to slope slope = tan(30°) Slope ≈ 0.577 (0.577 m of rise for every 1 m of run)
3. Calculate roof rise rise = run × slope = 3.0 × 0.577 Rise ≈ 1.73 m
4. Rafter length (no overhang) rafter = √(run² + rise²)
= √(3.0² + 1.73²)		 Rafter ≈ 3.46 m (from wall plate to ridge)
5. Extra rafter for overhang extra = overhang × √(1 + slope²)
≈ 0.45 × √(1 + 0.577²) 		Extra ≈ 0.52 m of rafter along the slope
6. Total rafter length raftertotal = 3.46 + 0.52 Total rafter ≈ 3.98 m (including overhang)
7. Plumb cut angle Same as roof pitch angle Plumb cut angle = 30°

In the calculator, these numbers are produced automatically as soon as you enter the span, pitch and overhang. The visual roof triangle updates to match the 30° pitch, so you can see the shape of the roof you are building.

Once you have your rafter length and spacing, you can move on to estimating material coverage with a dedicated Roofing Calculator to work out shingles or metal sheet quantities for the same roof.

🔹 Practical Uses for the Rafter Calculator

A rafter calculator is useful for much more than just first-time roof layouts. It can support a wide range of framing and renovation tasks where you need reliable geometry without having to solve triangles on paper every time.

  • New builds – sizing rafters for small houses, garden rooms, sheds and garages.
  • Extensions – matching the pitch and height of an existing roof on an added room.
  • Porches and canopies – quickly checking length and angle for small overhanging roofs.
  • Loft conversions – understanding how roof rise and rafter length affect headroom.
  • Deck or pergola covers – planning lightweight roof structures over outdoor areas.

Because the calculator works in both metric and imperial units, you can use it comfortably on international projects or when converting older drawings that are in a different measurement system.

🔹 Common Roof Pitches and When They Are Used

Different roof pitches are used for visual style, weather protection and local climate. Steeper roofs shed rain and snow more easily, while lower pitches use less material and can be quicker to build.

Typical pitch Approx. angle Common uses
Low slope (2:12 – 4:12) ≈ 9.5° – 18.4° Modern designs, porch roofs, some warm climates where snow load is low.
Moderate (5:12 – 7:12) ≈ 22.6° – 30.3° Typical residential homes, a balance between appearance, drainage and material use.
Steep (8:12 – 12:12) ≈ 33.7° – 45.0° Traditional houses in colder regions, good snow shedding, more attic space.
Very steep (> 12:12) > 45° Architectural features, towers or buildings with strong visual impact.

When you adjust the pitch value in the rafter calculator, the numerical plumb cut angle and the on-screen triangle both update, so you can experiment with different roof slopes and immediately see the effect on rafter length and ridge height.

🔹 Measuring Tips for Accurate Input

For the calculator to return realistic rafter sizes, the measurements you enter need to be consistent with how the building will actually be framed.

  • Measure the span between outside faces of the load-bearing walls, not including fascia or trims.
  • Enter overhang as a horizontal distance from the outside wall to the fascia line, not the sloping length of the rafter tail.
  • If you work in imperial, keep your rise/run pitch in the same units (for example inches over inches).
  • Remember that final cut lengths may need adjusting for ridge board thickness and birdsmouth cuts.

Once your main rafter geometry is defined, you can combine it with span and spacing data to plan loads and deflection using local structural design guidance or a dedicated span table from your supplier.

🔹 From Rafter Length to Material Planning

Knowing the rafter length is the first step. On a real project, you also need to decide rafter spacing, count how many pieces to cut and estimate how much timber to order. The rafter calculator gives you the geometry so you can confidently build up the rest of your material list.

Once you have the rafter length and roof span, you can combine that information with your chosen spacing (for example 400 mm centres or 16 in centres) to work out the approximate number of rafters along the wall.

🔹 Typical Rafter Spacing Values

Actual spacing depends on local codes, timber size, span and load conditions, but these values are commonly seen:

Region / system Typical spacing Notes
Metric framing 400 mm or 600 mm centres Often used with sheet materials sized to 1200 mm widths for easy layout.
Imperial framing 16 in or 24 in centres Aligned with common 4 × 8 ft sheathing sizes and insulation modules.
Light roofs / canopies May use wider spacing Only where permitted by structural design and manufacturer guidance.

To estimate the number of rafters on one side of a gable roof, divide the wall length by your spacing and add one extra rafter for the end. For example, a 6 m wall with rafters at 0.4 m centres will need roughly 6 ÷ 0.4 + 1 = 16 rafters per side.

🔹 Sheathing and Roof Area Planning

After you know how many rafters you need and the rafter length from the calculator, you can sketch out the layout of roof sheathing or roofing boards. This helps you minimise offcuts and plan joints over rafters.

If you want to translate the roof dimensions into a total area for roofing materials, underlay or insulation, you can combine the calculated rafter length with the building length and use a dedicated Square Footage Calculator to convert the roof surface into square metres or square feet for ordering.

Always round your timber and sheet counts up slightly to allow for waste, defects and on-site adjustments. Structural safety and local building regulations should guide your final material and spacing choices.

🔹 Assumptions Behind the Rafter Calculator

The rafter calculator is designed as a fast geometry tool, not as a full structural design package. It works under a set of simplifying assumptions that match the most common small roof layouts.

  • The roof is a simple, symmetrical gable with two equal slopes.
  • The rafter runs in a straight line from the top of the wall plate to the ridge.
  • Overhang is entered as a horizontal projection, and then converted into rafter length.
  • The effect of ridge board thickness, birdsmouth seat cuts and heel height is not included.
  • No allowance is made for deflection limits, timber grading or connection design.

These assumptions make the output easy to interpret on site, but you should always combine the results with local codes, span tables and supplier recommendations before committing to a final design.

🔹 What the Rafter Calculator Does and Does Not Do

Area What it does What it does not do
Geometry Calculates run, rise, rafter length (with and without overhang) and plumb cut angle from a few inputs. Does not handle hips, valleys, intersecting roofs or irregular roof shapes.
Units Works in both metric and imperial units, converting smoothly between metres and feet/inches. Does not automatically convert existing drawings or CAD files – you still need to input the key dimensions.
Structure Provides lengths and angles to cut rafters correctly for a typical small roof. Does not check structural capacity, required timber size or fixings for snow, wind or live loads.
Detailing Helps you position the birdsmouth conceptually by showing rise and run on the triangle. Does not calculate bearing lengths, notch depths or code-compliant birdsmouth proportions.

🔹 Safety and Building Code Considerations

Always treat the rafter calculator as a pre-design planning tool. Before cutting timber or starting construction, verify your layout and member sizes using:

  • Local building codes and regulations.
  • Manufacturer span tables for the actual timber size, grade and spacing you plan to use.
  • Professional advice from an engineer, architect or experienced builder for larger spans or unusual roofs.

For complex roofs with valleys, dormers or irregular shapes, you can still use this rafter calculator to understand basic triangles and pitches, then combine that with project-specific drawings or engineering.

🔹 Using the Rafter Calculator in Metric and Imperial

Roof framing projects often mix plans, supplier documents and on-site measurements in different unit systems. The rafter calculator lets you choose between metric (metres) and US/Imperial (feet and inches) so you can work in whichever format matches your drawings or local practice.

Internally, all calculations follow the same trigonometry. Only the way results are displayed changes, so you can switch units without worrying about accuracy.

🔹 How Results Are Displayed

Unit mode Primary output Secondary output
Metric Metres to three decimal places (e.g. 3.460 m) Millimetres rounded to the nearest whole number (e.g. 3460 mm)
US / Imperial Feet to two decimal places (e.g. 11.36 ft) Feet and inches, rounded to the nearest inch (e.g. 11 ft 4 in)
Plumb cut angle Always in degrees Works the same for both unit systems

On site, most carpenters will use the feet-and-inches or millimetre values when marking out rafters, while the decimal outputs are more useful for drawing, estimating and checking against CAD or engineering software.

🔹 Conversion Tips When Mixing Drawings and On-Site Work

It’s common to receive drawings in one unit system and have tools or stock lists in another. A few practical tips:

  • Keep the original design pitch (angle or rise/run) unchanged when converting – only convert lengths.
  • When converting imperial rafters to metric, round to the nearest 5 mm or 10 mm for easier marking and cutting.
  • When converting metric rafters to imperial, round to the nearest 1/4 inch unless your local practice is different.
  • Check that your converted ridge height still lines up with existing roofs, gutters or window heads.

If you need a quick standalone converter for other parts of your project, you can use a dedicated Length Conversion Calculator to switch between millimetres, centimetres, metres, inches and feet while you are planning the roof.

Whichever system you choose, stay consistent throughout one project: avoid mixing metric and imperial measurements on the same cut list to prevent mistakes and mis-cuts on site.

🔹 How to Mark and Cut Rafters from the Calculator Results

Once you have your rafter length and plumb cut angle from the calculator, the next step is marking and cutting the actual timber. The goal is to transfer the calculated triangle (run, rise and rafter line) accurately onto your lumber using a framing square, combination square or digital angle finder.

The general workflow is:

  • Cut and test a template rafter first before cutting the rest.
  • Use the calculator’s plumb cut angle to set your saw or square.
  • Mark the plumb cut at the ridge, then measure back along the rafter line for the total length.
  • Lay out the birdsmouth (seat and heel cuts) where the rafter bears on the wall plate.
  • Mark and cut the rafter tail and fascia overhang last, once the birdsmouth position is confirmed.

🔹 Step-by-Step Layout Example

This example assumes you already used the calculator to get the total rafter length (including overhang) and the plumb cut angle.

Step Action on the timber What to use
1. Mark ridge plumb cut At one end of the board, mark the plumb cut angle given by the calculator and draw a clear cut line. Framing square with angle marks, speed square, bevel gauge or mitre saw set to the plumb angle.
2. Measure rafter length From the ridge cut line, measure along the edge of the board the total rafter length from the calculator (to the outside of the wall), then mark the birdsmouth position. Tape measure and pencil, using the rafter length output in millimetres or feet/inches.
3. Lay out birdsmouth Mark the plumb cut for the inside face of the wall, then mark the horizontal seat cut on top of the wall plate. Keep the notch depth within code limits. Framing square and pencil; local code guidance for maximum notch depth and bearing length.
4. Add rafter tail and overhang From the birdsmouth, measure the horizontal overhang distance you entered in the calculator and mark the tail cut and fascia line. Tape measure, square and the calculator’s overhang value (converted into the units you are using on site).
5. Cut and test Cut the ridge, birdsmouth and tail on one rafter, then test-fit it on the building before copying its layout to the rest of the rafters. Circular saw or mitre saw, hand saw for final trimming, and the test fit on the actual walls and ridge line.

After the template fits correctly at the ridge, on the wall plate and at the fascia line, you can trace it onto the remaining rafters. This approach uses the calculator for accurate geometry, while the test fit ensures small on-site variations in wall thickness, ridge height or wall straightness are taken into account.

🔹 Key Roof Framing Terms Explained

Roof framing terminology can vary slightly between regions, but the same core ideas appear in most drawings and on site. This quick glossary explains the words used throughout the rafter calculator and helps you read plans and span tables with more confidence.

Term Short definition How it relates to the calculator
Span The full distance from the outside face of one load-bearing wall to the outside face of the opposite wall. You enter this as the total span. The calculator automatically halves it to get the run.
Run Horizontal distance from the centre line of the ridge to the outside of the supporting wall. Calculated as span ÷ 2. Used together with pitch to find the rise and rafter length.
Rise Vertical height from the top of the wall plate to the top of the ridge at the same point. The calculator reports roof rise at ridge, based on your chosen pitch.
Rafter Sloping structural member that runs from the wall plate to the ridge and supports the roof covering. The main output is the rafter length, with and without overhang.
Overhang The portion of the roof that projects beyond the wall, usually to protect the facade from rain. You enter overhang as a horizontal distance; the tool converts it into extra rafter length along the slope.
Plumb cut A cut made at 90° to the horizontal, so it is vertical in position (plumb). Often used at the ridge and tail. The calculator gives the plumb cut angle you should set on your saw or square.
Seat cut The horizontal part of a birdsmouth notch that sits flat on the wall plate. Not sized by the calculator, but its position is guided by the run and wall location in your layout.
Birdsmouth Combined plumb and seat cuts that allow the rafter to bear on the wall plate without twisting. The geometry (rise and run) from the calculator helps you place the birdsmouth accurately on your template rafter.
Ridge board Horizontal member at the top of a pitched roof where rafters meet on each side. The plumb cut angle ensures the rafter fits correctly against the ridge; allow for the board thickness in your layout.
Pitch Steepness of the roof, expressed either in degrees or as a rise over run (e.g. 4:12). This is one of the main inputs. It controls the angle in the visualisation and the plumb cut angle output.

Understanding these terms makes it easier to go from calculator results to real-world framing. When you move on to more complex layouts such as hip roofs or dormers, these same ideas (run, rise, rafter and pitch) form the basis of all the additional calculations.

🔹 Example Scenarios: Sheds, Extensions and Porches

The same rafter geometry appears in many day-to-day projects. The main differences are the spans, pitches and overhangs you choose. Here are a few typical scenarios where the rafter calculator can save time and help you avoid mistakes.

Project type Typical inputs How the calculator helps
Garden shed or small outbuilding Span: 2.4–3.6 m (8–12 ft)
Pitch: 25–35° or 4:12–7:12
Overhang: 200–300 mm (8–12 in) 		Quickly finds rafter length for common stock sizes, plus a consistent plumb cut angle so each rafter sits neatly on the ridge and wall plate.
Single-storey rear extension Span: 3.0–5.0 m (10–16 ft)
Pitch: matched to existing house roof
Overhang: tuned to gutter and fascia detail 		Lets you match the existing roof pitch numerically and check that the new ridge height aligns with existing eaves, gutters and window heads before you order timber.
Front porch or canopy Span: 1.2–2.4 m (4–8 ft)
Pitch: often steeper for visual impact
Overhang: generous to keep doorway dry 		Provides rafter lengths for short, steep roofs where small changes in pitch make a big visual difference. The live visualisation helps you pick a slope that looks right from the street.
Carport or open-sided shelter Span: 3.0–6.0 m (10–20 ft)
Pitch: low to moderate, depending on roof finish
Overhang: sized to protect vehicle edges 		Supports early feasibility checks: you can trial different spans and pitches to see how they affect rafter length and ridge height before committing to posts, beams and foundations.

In all of these cases, the workflow is the same: decide the span and pitch, use the rafter calculator to get lengths and angles, then combine those results with structural guidance, span tables and local building rules to finalise timber sizes and spacing.

🔹 Quick Checklist Before You Cut Rafters

Use this short checklist to make sure the numbers coming from the rafter calculator match the reality on site. Taking a minute to confirm these points can save a lot of time and timber once you start cutting.

Check What to verify Why it matters
Unit system Confirm whether the project is being measured in metric or imperial and match the calculator’s unit toggle. Mixing metres with feet and inches is a common cause of wrong rafter lengths and ridge heights.
Span measurement Re-measure the distance between the outside faces of the bearing walls, not the inside faces. An incorrect span flows through to the run, rise and every other result, throwing off the whole roof.
Pitch value Make sure the pitch in degrees or the rise/run ratio matches the drawings or the existing roof you are copying. A small pitch error can move the ridge line by many centimetres over a long span.
Overhang distance Decide the horizontal overhang (to fascia or gutter front) and enter that value, not the sloping rafter tail. Overhang affects both the rafter tail length and the visual proportions of the eaves and fascia.
Ridge and birdsmouth allowances Note the ridge board thickness and any code limits for birdsmouth depth before finalising cut lines. The calculator gives geometric length; real cut length may need adjusting for these details.
Template rafter Plan to cut one test rafter first using the calculated length and plumb cut angle, then test-fit it on site. A successful template confirms your inputs and helps you copy accurate marks to all remaining rafters.

After this checklist, you can confidently use the rafter calculator as your reference for cuts, while accounting for real-world factors such as existing building tolerances, ridge board size and local framing rules.

🔹 Common Mistakes When Sizing Rafters

Even experienced builders can run into issues when converting drawings into real rafter lengths. Many problems come from small misunderstandings about measurements, pitch or where the rafter actually bears on the wall. The rafter calculator helps reduce errors, but it is still important to understand where mistakes usually appear.

Mistake What happens How the calculator helps
Using inside span instead of outside span Rafters end up too short, ridge sits lower than expected and the fascia line may not align with existing roofs or drawings. The calculator expects the outside-to-outside span. Double-check your tape measurement against the value you type into the span field.
Mixing unit systems Combining metres from drawings with feet and inches on site leads to rafter lengths that are out by several centimetres or inches. The unit toggle keeps everything in one system. Change to metric or imperial first, then enter all span, overhang and pitch values consistently.
Confusing roof pitch angle and saw angle The mitre saw or bevel is set incorrectly, so the plumb cuts do not match the ridge or fascia. The calculator shows a clear plumb cut angle in degrees. This is the angle you set on the saw or use with a speed square for the rafter ends.
Entering overhang along the slope The tail sticks out too far or not far enough, because the value was taken along the rafter instead of horizontally from the wall. The input asks for horizontal overhang. The tool then converts that into extra rafter length using the chosen slope.
Ignoring ridge thickness and birdsmouth depth Rafters fit the triangle but need trimming on site, or the ridge finishes higher/lower than planned. Use the calculator for the base geometry, then adjust the final cut layout on your template rafter to include ridge thickness and code-compliant birdsmouth dimensions.

When in doubt, run the same roof through the rafter calculator twice with carefully checked inputs, cut one trial rafter and test it in place. Once the template fits correctly, you can safely copy it to the remaining rafters for a consistent, accurate roof frame.

🔹 Using the Rafter Calculator in a Full Roof Project

The rafter calculator works best when it is part of a simple, repeatable workflow for planning and building a roof. Instead of treating it as a one-off tool, you can use it at several points during a project, from early design through to final cutting and installation.

🔹 Step-by-Step Workflow

Stage What you do How the calculator is used
1. Concept and layout Decide on the basic roof form (simple gable), approximate span and a target pitch that suits the building style and climate. Try different spans and pitches to see how much the ridge height and rafter length change, and to check that the roof will sit comfortably on the existing walls.
2. Preliminary sizing Rough out wall positions, window heads and door openings, and consider how much eaves overhang you want for weather protection and appearance. Enter your chosen overhang and confirm the total rafter length needed, making sure that the fascia line and ridge height are where you want them.
3. Structural checks Consult span tables, supplier data or an engineer to choose timber sizes and spacing that meet local code requirements for snow, wind and live loads. Use the calculator’s run and span values as the basis for those checks, and verify that your proposed rafter section is suitable for the calculated span and pitch.
4. Material ordering Prepare a cut list for rafters, ridge, wall plates and bracing. Add a waste factor for offcuts, defects and on-site adjustments. Take the rafter length (with overhang) and multiply by the number of rafters per side, based on your chosen spacing, to estimate the total linear length of rafter stock required.
5. Marking and cutting Mark a template rafter, cut it, then test-fit on site before copying the layout lines to the rest of the rafters. Use the plumb cut angle and rafter length directly from the calculator output to lay out the ridge cut, birdsmouth location and tail on your template piece.
6. Installation and adjustments Install rafters, checking ridge straightness, spacing and alignment with walls and fascia. Trim small discrepancies on site where needed. If site conditions differ from the original measurements, re-run the calculator with updated span or pitch values and adjust only the affected rafters or bays.

By following the same workflow on each job, the rafter calculator becomes a reliable reference point. It gives you a consistent set of dimensions and angles, while your drawings, span tables and on-site checks provide the structural and code-compliance side of the design.

🔹 How to Read the Rafter Visualisation

The live diagram beside the rafter calculator gives you a fast visual check that the numbers make sense. It redraws the roof triangle using your current pitch, so the angle of the rafter line matches the plumb cut angle in the results table.

Think of the visualisation as a simplified side view of one half of your gable roof. It is not a full structural drawing, but it clearly shows how run, rise, rafter and overhang relate to each other.

🔹 Key Elements in the Diagram

Element What it represents How it reacts to your inputs
Red rafter line The main rafter from the top of the wall plate up to the ridge. This is the piece you will be cutting. Rotates to match your roof pitch. As you increase the angle, the rafter becomes steeper and shorter for the same span.
Horizontal “Run” arrow The horizontal distance from the wall to the point under the ridge. It is half of the total span. Lengthens as you increase the building span. The label updates with the same numeric run you see in the results.
Vertical “Rise” arrow The vertical height from the wall top up to the ridge line. Grows when you increase pitch or span. Higher pitches and longer runs mean a larger rise.
Overhang line The extension of the rafter beyond the wall towards the fascia and gutter. Gets longer as you increase the horizontal overhang input. At zero overhang, it almost disappears.
Angle arc at the wall A small arc showing the roof pitch angle between the horizontal and the rafter. Opens or closes as you change the pitch. Its label shows the same angle as the plumb cut value.

🔹 Practical Ways to Use the Visualisation

The dynamic sketch is most useful when you are still deciding on a final roof pitch or want to communicate the idea of the roof to a client or team member without showing full drawings.

  • Experiment with different pitches to see how much the ridge height rises relative to the wall.
  • Adjust the overhang until the eaves proportion looks right, then lock in that number for cutting.
  • Use the angle label as a quick reminder of the plumb cut setting for your saw or framing square.
  • Take a screenshot or printout of the diagram to include with your job notes or client proposal.

Having both the figures and the matching visual triangle on the same page makes it much easier to spot input errors. If the roof suddenly looks extremely steep or shallow, that is a prompt to double-check your span, pitch or units before you cut any timber.

🔹 Frequently Asked Questions about the Rafter Calculator

🔹 References and Sources

The rafter calculator and supporting explanations on this page are based on standard right-angle trigonometry, common carpentry practice and publicly available building guidance. Below is a non-exhaustive list of useful references and further reading.

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Type Title / Description Publisher / Author Link / Details
Online calculator Roof Rafter Length Calculator – common rafter geometry, span/run/rise relationship and roof pitch examples. Blocklayer https://www.blocklayer.com/roof/rafter
Online guide Roof Pitch Calculator – explanation of roof pitch in degrees vs rise/run and basic framing layout. MyCarpentry.com https://www.mycarpentry.com/roof-pitch-calculator.html
Technical manual Wood Frame Construction Manual (WFCM) – span, load and connection guidance for light-frame roofs. American Wood Council https://www.awc.org/codes-standards/publications/wfcm-2018
Building code International Residential Code (IRC) – roof framing provisions, rafter spans and notching rules. International Code Council (ICC) https://codes.iccsafe.org/
Book Roof Framing – step-by-step methods for laying out and cutting common, hip and valley rafters. Marshall Gross ISBN (example edition): 978-0910460402 – available via major technical book retailers.
Article Practical carpentry articles on roof framing, rafter layout and birdsmouth details. Fine Homebuilding Magazine https://www.finehomebuilding.com/
Math reference Right triangle trigonometry – definitions of sine, cosine, tangent and Pythagorean theorem. Any standard geometry textbook or online math reference For example: https://mathworld.wolfram.com/RightTriangle.html

Always verify final rafter sizes, spans and connection details against your local building regulations and manufacturer span tables, as requirements differ between countries and climate zones.