Saturday, August 4, 2012

Apprentice Carpentry Roof Framing Geometry Part 1

Tony McGartland,  who teaches carpentry at a college in N.Ireland and who is also responsible for training the better apprentices for carpentry competitions in the UK, sent me a couple of Google SketchUp models the other day. From past carpentry competitions in the UK. These Google SketchUp roof framing models made me realize how poor the American Union Apprentice program is compared to the apprentice programs in Europe. France?,Switzerland, Austria and Germany have specialized training for their apprentices for these apprentice competitions and are light years ahead of the United States of America apprentice program in roof framing construction and geometry. If we sent American apprentices to these carpentry compensation, not only would they not medal they would most likely embarrass themselves. By the time I completed my 4 years of the Union Carpentry apprenticeship program I had already cut and stacked (framed) about 100+ track home roofs, but I didn't know anything about roof framing geometry. Yes, I used a framing square to mark and layout the rafters, but if anyone had asked me to draw out the roof framing geometry of the roofs I was cutting, I would have said no I can't draw out the roof framing geometry to the roofs that I've cut.

This is part 1 in Apprentice Carpentry Roof Framing Geometry for the Americans and maybe the Canadians. 

Roof framing geometry is empirical-type knowledge.
Information gained by means of observation, experience, or experiment.

It will take more than just reading to understand roof framing geometry. You'll need to draw out the roof framing geometry. Preferably on 4x8 sheets of plywood or mdf  to gain the experience you will need to compete in the apprentice carpentry competition. You'll need a compass, straight edge and framing square. Yes, you can use your calculator in the apprentice carpentry competition, but you'll also have to draw out the roof framing geometry as well. Just like they did 100+ years ago.

I suggest getting a 6" and 16" compass like these compasses from Lee Valley.

While your at Lee Valley you might as well order a Chappell Framing Square

First up, The Basic Roof Framing Kernel

This is a plan view of a roof framing kernel with the section view drawn above the plan view.The plan view is also referenced as the ground plan. The real roof surface angles and lengths are located by drawing an arc (radius=common rafter length) at the apex of the common rafter run (point D) in plan view. The common rafter is also called the principal rafter.

F-E = Common Rafter Plan View Run
A-E = Hip Rafter Plan View Run
P-R = Jack Rafter Run
E-H = Common Rafter Real Surface Rise 
F-H = Common Rafter Real Surface Length
A-G = Hip Rafter Real Surface Length
R-S = Jack Rafter Length

Angle FAB = Eave Angle
Angle EAB = Plan Angle
Angle EFH = Common Rafter Slope Angle
Angle GAE = Hip Rafter Slope Angle
Angle NCD = Roof Sheathing Angle
Angle DNC = Jack Rafter Side Cut Angle

The next 2 drawings are  the basic roof framing kernel drawn in Google SketchUp. The basic roof framing kernel is drawn unfolded and folded up into 3D roof surfaces of the roof framing kernel. This is also a good example of swing an arc to establish the points of the real roof surfaces.

Here's a link to the Google SketchUp file Basic Roof Framing Kernel.
In part two of Apprentice Carpentry Roof Framing Geometry we'll add the hip rafter backing angle to the basic roof framing kernel.


  1. For some contractors, framing a roof is one of the most difficult projects to undertake in construction. However it is also one of the most fulfilling. The two most common roof framing construction methods are stick roof framing and truss roofing.

    -Eugene Head

  2. Great explanation and it is obvious that it would take someone who is very confident to attempt to be in the roof framing business. It also will take a person who is unafraid of heights. Why is it that I have seen many roofs going up in the winter time? Is there a reason why contractors would not wait for summer weather? Thanks to whom ever can answer.
    Handrail Brackets

    1. Lumber is easier to work with during winter. In summer, any undried lumber (which is most) will dry too quickly, causing it to bow or crown too much and make it nearly impossible to work with. Also, we can cut pieces a bit short if needed, making for easier installs because we know it will expand in the warmer months.

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