RoofJacker – Roofing Ready Reckoner

Roofing Ready Reckoner

Web app for calculating roofing dimensions
Skip to the app – CLICK HERE

Roofing Ready Reckoner

While teaching at my local technical college, I realised that the popular book of Roofing Ready Reckoner look-up tables had been in use for decades, I felt that it was surely time for a change.

Working together with my son in March 2011, the result was a spreadsheet that he soon coded into HTML, this in-turn became a single page web app, which allows you to enter just 3 measurements and be presented with all the dimensions you need to construct a roof.

This digital Roofing Ready Reckoner app is a simple piece of trigonometrical calculation, which is designed to replace the unwieldy hardcopy look-up tables that are also known as the Roofing Carpenter’s Ready Reckoners. These are used to calculate the various angles and measurements required when building a roof.

With just three measurements you will be able to use this Roofing Ready Reckoner and calculate the following roofing dimensions: Rise per metre Run, Rafter Length, Hip Length, Jacks Diminish, Common Rafter Seat Bevel, Common Rafter Ridge Bevel, Hip or Valley Seat, Hip or Valley Ridge, Jack Rafter Edge and Purlin Edge.

The Roofing Ready Reckoner (v1.484) provided on this page is designed to be responsive and will work on desktop, tablet and mobile devices be they iPhone or Android, so it’s suitable for use when you’re in the office, at college or on site.

Simply input your measurements for Pitch, Run and Centres, then click “Calculate” and let RoofJacker do the hard work and ready reckoning for you!

Roofing Ready Reckoner App

Enter your measurements:

Pitch: Run: Centres:


  • Rise per metre Run:
  • Rafter Length:
  • Hip Length:
  • Jacks Diminish:
  • Common Rafter Seat Bevel:
  • Common Rafter Ridge Bevel:
  • Hip or Valley Seat:
  • Hip or Valley Ridge:
  • Jack Rafter Edge:
  • Purlin Edge:

To download as PDF click this icon once, it may take a few seconds:

RoofJacker – Roofing Ready Reckoner v0.484 © 2011 MC2
RoofJacker – Roofing Ready Reckoner v1.484 © 2018 Neon Saw
Comments welcome at
Disclaimer: RoofJacker is currently provided for free, as is.
Neon Saw accepts no responsibility for errors, however caused.