Skillion Roof Rafter Calc

The ultimate skillion and lean-to roof framing calculator designed for carpenters, builders, roofers, and DIY enthusiasts. Get precise rafter measurements, birdsmouth cuts, batten layouts, and professional diagrams — all from your iPhone or iPad.

WHY THIS CALCULATOR?

Skillion and lean-to roofs are the most common roof style for sheds, carports, patios, pergolas, awnings, and home extensions. Yet most roofing apps focus on hip and gable roofs. This app is purpose-built for single-slope roof framing, giving you everything you need to frame with confidence on the job site.

COMPLETE RAFTER CALCULATIONS • Rafter top length and total length • Roof fall, pitch, and angle • Birdsmouth plumb cut and seat cut dimensions • Cut set-back measurements • Perpendicular rafter depth • HAP (Height Above Plate) calculation • Automatic deep birdsmouth warnings

SMART RAFTER SPACING • Equal spacing — distributes rafters evenly across the wall • Exact spacing — maintains your specified spacing with adjusted end bays • Running set-out dimensions for every rafter position • Alternate spacing suggestions (next larger / next smaller) • End rafter set-in support

BATTEN LAYOUT • Automatic batten position calculations • Equal or exact batten spacing options • Running set-out for every batten • Double-top-batten detection when spacing is exceeded • Custom batten overhang support

PROFESSIONAL DIAGRAMS • Plan view — top-down rafter layout with all running dimensions • Rafter detail — side view showing birdsmouth cuts, angles, and key measurements • Profile section — cross-section with wall heights, rafter angle, and overhangs • All diagrams drawn accurately to scale

DUAL UNIT SYSTEMS • Imperial — feet, inches, and fractions down to 1/32 of an inch • Metric — millimeters for international standards • Switch between systems instantly

EXPORT AND SHARE • Export a complete PDF report with input data, calculated results, and all three diagrams — perfect for printing on the job site • Export results as plain text for quick sharing • Share via AirDrop, Messages, Mail, or any app on your device

BUILT FOR THE JOB SITE • Works completely offline — no internet needed • Clean, intuitive interface optimized for iPhone and iPad • Fast, accurate calculations based on proven trigonometric formulas • Upper wall and lower wall height calculations • Overhang support for both upper and lower sides • Wall thickness integration for precise framing dimensions • Upper wall toggle for freestanding lean-to configurations

Whether you are a professional framer laying out rafters on a new build, a contractor estimating materials for a client, or a weekend DIYer building a backyard shed or carport, this calculator saves you time, reduces costly errors, and ensures every cut is accurate.

Stop doing complex roof math by hand. Download now and take the guesswork out of skillion roof framing.