The gambrel roof is also known as the good old barn roof (see some excellent pictures of barns here). Unlike the typical gable roof it has two slopes on each side. The top slope is usually gentle while the bottom slope is steeper, sometimes almost vertical.
The Purpose of the Gambrel Roof
Why would you want to have a gambrel roof at all? The most probable reason is that you want to use the space under the roof. The gambrel (called also gambril) construction provides a lot more usable space than a gable roof or a shed roof.
This roof type is also relatively simple to build with its vertical gable ends. But still it's more complicated to build than the basic gable shed roof.
It's also really good looking especially if you like the deisng of the old rural barns. Why do barns have such roofs by the way? Again, because of the extra storage space and more enclosed air they provide.
How To Build It
The top part of it is similar to a less sloped gable with a ceiling joist at the bottom. Then you need two rafters that are almost vertical supported by vertical ashlarings and attached to the bottom beam.
A really good guide on building such roof with lots of good graphics is available here.
You may of course prefer to find ready plans with exact measures. Here are few places to look for plans:
- A bunch of free plans, albeit unorganized, are available here
- Good but expensive barn plans with gambril roofs are available here
- You can download another free plan (PDF) from this link
- It might be a good idea to buy these 50 house and barn plans as a package for only $37.
Gambrel Roof Trusses
This is a popular style of gambrel roof that deserves its own attention. Here is a good article describing more about it.
Calculating the Gambrel Roof
Calculating the roof slopes is important because the right pitch defines the roof sustainability and how snow and water will drop out of the roof. We already have a pitch calculator for gable roof on the site. You can use it for gambrel roof to some extent. The whole gambrel calculation is a bit more complicated:
- You have four angles to calculate
- Consider the roof to be roughly twice wide than high
- Consider low roof pitches
- This article here makes some effort to suggest a calculation method
I couldn't find any ready calculators so you'll have to do the math yourself. At least until we build such calcualtor :)