Gambrel Framing

ClemS

anyone have extensive experience doing gambrels?

I did maybe three total, they aren't all too common around here. what is a good pitch combo, or ratio?

Also, do you set your purlins vertical or half&half on the converging pitches?
I've done half&half in all three scenarios.

wallmaxx

Do you mean like a barn-type roof system?

I have done only one. The architect told me that there is some kind of "magic" combination of pitches to make the thing "look right". I will scour my files to get you that info............(since I am doing nothing else right now in the dust ridden Columbia Basin, living 5 days a week in a trailer, framing a project 250 miles from home).......now I have something to do 'til its lights out.

wallmaxx

http://www.secrets-of-shed-building....brel-shed.html

From the above page

Gambrel roof design

If are going to build your own gambrel shed then you need to be able to determine the setting out of the roof slopes. The gambrel roof pitches can vary considerably and many ways of setting out can be can be given. In the sketches below are two methods which give a good result.
In the first method the perimeter of the semicircle is divided into 4 equal parts and each point is then joined to form the roof profile – known as a demi-octagon.

For method two the perimeter of the semicircle is divided into 5 equal parts, the two lower intersections and the top of the semicircle are joined to form the roof profile.


The below methods were set out by George Collings in The Lockwood’s Trade Manual – Roof Carpentry (Originally published in 1926 and went through at least 7 printings!)

Attached Thumbnails

 

wallmaxx

We built the side walls plumb on the inside then put rafters on a 20 something and 12 from the top of the wall back down to the slab. Then it was just conventional rafter framing for the upper rafters to a structural ridge.

shed builder

I primarily build storage barns for a living but I guess the ratio would be the same for a house. If you want all sides of the roof to be equal in length cut each end of the truss 22 1/2 degrees. For an 8' wide gambrel roof the length for each piece of the truss would be 36". Now using that as your starting point, add 9" to to each "piece" per 2' of building width. 10' wide truss piece would be 45", 12' would be 54" and so on. I guess you could use this formula up to about a 24' wide before the truss would be too tall. This method makes the roof look very pleasing to the eye because each side is the same length.

To get more height on a narrow building like a 16' wide you can use the 20,25,20 method. Which is 20 degrees at the base 25 degrees at the humps and 20 at the ridge. This will make the roof taller and have a more elongated look. Alot of this stuff I just scale it down and lay it out on the floor or my shop table to see what it looks like before I commit to it.

Vester

Kgmz

A classic Gambrel is like WallMaxx showed there. You can vary the pitches a little, but to be a classic Gambrel all points much touch the semi-circle, rafter tail end and the 3 points on the roof.

And the base of the half circle is where the rafter tails end, not the floor up there.

festerized

Gambrel roof
12 pitch lower section
8 pitch higher section
Building span 30’
Run 15’
Lower ridge “letter c in table one” 8’ using a micr lamb ridge 1 ¾” x 11 7/8”
12 pitch
8’ run
8’ rise
11’ 3 ¾” dig.
Upper 8 pitch
Now take 8’ run from each side away from span,
30’ -16’=14’
14’ now becomes 8 pitch span
Now subtract 1 ¾” ridges from span
1 ¾” x 2 ridges = 3 ½”
Subtract 3 ½” from 14’ span=13’8 ½”
13’8 ½” is now your “working” run
8 pitch
13’8 ½” run
9’ 1 11/16” rise
16’ 5 11/16” dig.
Total rise
12 pitch Rafter with 7” standing height ”birds mouth”=8’ 7” rise
8 pitch rafter 9’ 1 11/16” rise
8’ 7” plus 9’ 1 11/16= 17’ 8 11/16” rise
Now you have to add 8 pitch angle
Book is my truck so I’ll just guess and say its 5/8” offset
So add 5/8” x 2 =1 ¼”
1 ¼” plus 17’ 8 11/16” = 17’ 9 15/16” Total rise from top of pl8 to top of ridge

ClemS

thank you gentlemen (primarily walmaxx). I thought there'd be something to that effect.