Rafter length and rafter cuts, a practical, no calculations way.

Here is a rafter length question, from Raymond A who lives in SWANSEA United Kingdom.

Raymond, you have asked a question that I should have already done when I first wrote this roofing section, here is my take on it:-)

Many times when doing a roof it is just as easy to get your angles in a practical trial and error way.  No calculations involved.  There is absolutely nothing wrong with setting up the ridge with a couple of props and temporary braces and then measuring the lengths.

  • You may be extending on from an existing roof.
  • You may want to set up the ridge temporarily, with maybe a couple of rafters tacked in position, so that you can step back and eyeball it from the street or get an impression of the inside space if you are high sloping ceilings.
  • You may not be too confident with your calculations.

For whatever reason sometimes it is just easier to measure and mark in place.  I used to always check my hip lengths with a tape measure before cutting them.  It is easy for a 30mm difference to happen.

wrong length for a rafter
Rafter length - Don't do it like this.

OK, if you just whack a nail into the edge of the ridge and measure to the edge of the wall plate or top plate, then you can see that you are getting the wrong size.  What you see above is a fairly exaggerated view, but you can see that the top cut angle would be way out and the length would also be a touch out.

measure the rafter
Rafter length - Getting the right position to hold the rafter temporarily.

To get the correct length and angle you have to somehow allow for the depth of the rafter, less the depth of the birdsmouth if any.  There are a few ways to do this.   The sketch above shows one way and shows the basic principal.

  • Temporarily fix the ridge and brace the wall in the correct positions.
  • On a small roof with a fairly shallow pitch you can physically lay a rafter in a position that will give you the information that you need.
  • Clamp a piece of scrap timber (say an offcut from the ridge) to the wall in what you guess is a good position.  I have shown it on the inside, so that the drawing is simplified, but it could easily be on the outside.  If you like you could also just lay a couple of pieces of timber on top of the wall.  Anything just to lift that rafter up.
  • Work out what your distance at 'A' is.  In my example I am using a 125 x 50 rafter.  Say I want a small birdsmouth (definitely no more than 1/3 of the depth) so I say that I will do a 25 deep birdsmouth which will will leave me 100 of solid timber left in the rafter. 100 is my dimension 'A'  If I didn't want a birdsmouth at all, then the full depth of the rafter, 125, is size 'A'
  • The distance 'A' is measured square off the rafter. Not vertically.
  • By trial and error move the rafter up or down until you have it set right.
  • Take you spirit level and mark your rafter top cut and outside wall line to enable you to mark out your birdsmouth cuts.
  • You now have a sample rafter that you can fully mark out and use as a pattern.

If the roof pitch is too steep, or the rafters long and heavy, or if you don't have an offsider to hold it in position for you while you mark it, then you could try these ways.

string for rafter
Rafter length - Using a string line to get the correct top cut.
  • Use a light batten of timber that you nail to the ridge, try to minimise sag as you mark the lengths and use an adjustable bevel to get the angle.
  • Use a string line to do the same, just make sure that you secure it some how with distance 'A' in mind.
  • When you have the top cut measured with a bevel, saw the top of a sample after and place it in position, with the top of the rafter sticking higher than the ridge by the the depth of the birdsmouth.   That is, using the example above, let the top of the rafter be 25 above the ridge, and then mark your length at the wall plate.
finished rafter position
Rafter length - The finished position

Not found it yet? Try this FAST SITE SEARCH or the whole web

coates hire
Hire Equipment
Furniture Fittings - Architectural Hardware - Electronic Locking Systems - Technical Hardware

BuilderBill sponsorship

Quick Illustrated Roofing Glossary Pages.

Types of roofs
Roof Features
Roof Framing
Roof Trusses
Roof Coverings



Please Note! The information on this site is offered as a guide only!  When we are talking about areas where building regulations or safety regulations could exist,the information here could be wrong for your area.  It could be out of date!  Regulations breed faster than rabbits!
You must check your own local conditions.
Copyright © Bill Bradley 2007-2012. All rights reserved.
Page copy protected against web site content infringement by Copyscape