Sunday, July 12, 2015

Curved Hip Rafters

I came across this method of drawing out the curves of hip rafters in a Carpentry & Building magazine published in 1880 and thought it was pretty easy way to establish the curve of any hip rafter using this method. It can be used for the curved hip rafters on flared radius eave rafters, curved hip rafters on bay windows or a hip rafters with any eave angle.

To use this method draw the elevation view of the common rafter and hip rafter above the plan view of the radius rafters. Draw the radius/ Arc of the common rafter. Then draw equal spaced horizontal lines that intersect with the hip rafter slope line in elevation view. Drop some perpendicular line where the horizontal lines intersect the hip rafter slope line and the common rafter slope line . Then draw rectangular boxes to establish the cure of the hip rafter based off the curve of the common radius rafter.

Geometric drawing of the curve of a standard hip rafter and Octagon hip rafter  with a common radius rafter. 

Drawing for the curve of the bay window hip rafter and the curve of the hip rafter against the wall of a bay window.

Section of a Bay window in Plan View

How to draw out the arc of the common radius rafter on a bay window.

Geometric drawing of the curve of bay window hip rafter and the curve of the hip rafter against the wall of a bay window using the rectangular boxes to establish the curves of the rafters.


  1. Sim,

    A shortcut based on the geometry you present would be to divide the three different rafter lengths by the same number of spaces (used to find the plot points at the Common Rafters Drops, from the RL Line), transfer these plumb dimension drops to the two Hips at the corresponding divisons and connect the dots.

    An easy method to divide the RLs by 8 is to cut three pieces of heavy paper strips, like the strips found binding the ends of two 4' x 8' sheets of drywall, one cut to each of the three rafter lengths (RLs) and fold each strip in half three times (fold lines are equally spaced). Now use these strips unfolded and placed along the top shoulders of the rafters (starting from the HAP line, (or fascia?), working up as to account for the Ridge boards, also the Ridge's Effective Length and Height (minor) adjustments should be revealed, if needed) and transfer the plumb dimensions to plot the curves. (For the 24" Run as shown in your illustrations, (8 x 3" = 24") 3" spacing along the Common seems adequate. For greater Common Runs, a greater number of divisons may be more beneficial to plotting the longer hip curve lines.

    One last thing that helps make this Curved Bay Roof simpler to frame is to layout the rafter spacing as to not have commons placed at the main ridge ends, This will eliminate the compound Diamond Head plumb cuts at the First Hip. (No Three-way rafter connections. I find layouts starting from the center of the Face wall will often eliminate the issue and make cutting the sheathing even quicker by making bookend matches for the 4 pieces cut. The sheathing cuts at the hips can also be plotted along the common rafter plots, Use a narrow stip of ply bent down over the common to transfer the sheathing plot lines, Dimensions can then be taken fro Plan-View and applied.)

    1. Good advice Richard. I'll have to give it a try.

      Thanks, Sim

    2. Sim,

      Very interesting method, much so different from they several methods I know.
      Thanks for that post