A gambrel or "barn" style roof is simply a gable roof with a change in slope partway
up the roof.
Besides its unique appearance a gambrel roof also serves to maximize the usable
floor space in the attic.
The preview drawing displays an sectional view of the roof
that updates as the dimensions are changed.
Easy Rafters groups gambrel roofs into two categories, regular gambrels and
A regular gambrel is one that fits inside a circumscribed semi-circle as shown below
(the shape of the roof is essentially one half of a regular octagon). The slopes
for a regular gambrel roof are fixed at 28 31/32 over 12 for the lower rafters and
4 31/32 over 12 for the upper rafters (these slopes are rounded to 29/12 and 5/12
for display) and the length of each side or face will always be equal. Whenever
the lower span dimension is changed the other dimensions are automatically recalculated
to maintain the same regular proportions.
Custom gambrels on the other hand allow complete flexibilty of design without the
constraints of the regular gambrel option.
A regular gambrel fits inside a circumscribed semi-circle.
The custom gambrel option allows complete flexibility of
You can even calculate something like this with the custom
Easy Rafters provides for three different methods of joining the upper and lower
roofs. The purlin method calculates top and bottom cuts that bisect the angles at
the roof joints with ridge and purlin boards between them (the ridge thickness can
be set to zero if no ridge/purlins are used). The wall supported (flush) method
supports the upper rafters on walls with the lower rafters butted against the wall
studs. The wall supported (raised) method is a variation of the wall supported (flush)
method that raises the upper rafter to create a square cut fascia line at the break
in the roof.
For clarity, the ridge/purlin boards are not shown in the
preview drawing. They are displayed in the upper and lower rafter drawings however.
Wall Supported (Flush)
The wall supported (flush) method may not provide enough
bearing for the upper rafters where they rest on the wall plates. You may need to
modify the design to provide more bearing for the upper rafters. This might be accomplished
by adding ceiling joists, using wider bearing walls, and/or by by modifying the
One way to provide more bearing for the upper rafters is
to move the walls in and modify the top cut of the lower rafter as shown here.
Wall Supported Raised (Custom Gambrel only)
To Calculate a Gambrel Roof
Select Gambrel Roof from the Roof Type dropdown list or click on the appropriate
toolbar button to calculate a gambrel roof. The preview window will display an elevation
view of the gambrel roof that reflects the actual dimensions entered. The drawing
pages display dimensioned side views of the bottom and top rafters respectively.
Regular or Custom
Select the type of gambrel before entering any of the other dimensions.
note: The lower span should be the first dimension entered
after selecting the gambrel type.
Enter the slope of the upper rafter or select from the dropdown list.
note: The upper slope is locked for regular gambrels.
Enter the slope of the lower gambrel rafter or select from the dropdown list.
note: The lower slope is locked for regular gambrels.
Enter the span of the upper part of the gambrel roof. Changes to the lower slope
or lower height will affect the upper span dimension.
note: The upper span is calculated automatically for regular
Enter the lower span of the gambrel roof. This should be the first dimension entered
after selecting regular or custom.
Once set the lower span is not affected by changes to any of the other dimensions.
The upper height is the distance from the peak of the lower rafter to the peak of
the upper rafter.
note: The upper height is calculated automatically for
The lower height is the distance from the floor to the peak of the lower rafter.
note: The lower height is calculated automatically for
Enter the horizontal overhang of the lower gambrel rafter. Do not include the thickness
of the fascia board in the overhang dimension.
To calculate a gambrel like the one pictured below, set the overhang and heel height
to zero (be sure to lock the heel height).
Heel Height/Seat Cut
Accept the default heel height and seat cut dimensions calculated by Easy Rafters
or override the defaults by locking one or the other and entering the desired measurement.
note: The default heel for gambrel is 1/2 of the lower
rafter plumb cut height.
Enter the lumber sizes for the rafters, ridge/purlins, and fascia. If no ridge board
or purlins are used, enter zero for the ridge thickness.
Select the type of fascia cut desired by clicking on the appropriate toolbar button
or by selecting the appropriate item on the Settings menu or the popup menu. Gambrel
roofs fascia option include square cut, plumb cut, and level cut.
Upper Fascia (Wall Supported (raised) only)
The Upper Fascia field appears on the toolbar for Custom Gambrels when the Wall
Supported (Raised) option is selected. Enter the height of the upper fascia in the
field or select from the dropdown list of standard lumber widths. The upper fascia
will always be square cut regardless of the lower fascia type.