Gambrel Roof Shingle Calculator

What makes the classic barn shape more interesting are the gambrel roofs. From far away, it appears basic; a simple box with some flair. But the trickery lie in the geometry. Because there’s a different pitch on each side (the top slope is shallow; the bottom slope is steep), most people has trouble measuring their barn and guessing. They need to consider them individualy. For example, each section require its own amount of square footage and pitch factor. Guess incorrectly and you’ll either end up running short on shingles or buying too much. The steep angle also increases the overall surface area which can be difficult to plan for.

After you input your dimensions, the calculator does all the math. It splits the measurement into an upper run and a lower run, then multiplies by the correct amount. A 5:12 pitch on top only adds a minimal amount of area over the footprint. An 18:12 pitch on the bottom extend it way out there. That’s where a lot of folks mess up. If they have a horizontal run of six feet, they think that means their roof require six feet of shingles. Not true. Each vertical foot of rise may require almost eleven feet of coverage. You can specify the run/over in both directions, independent off one another, so that your bundle count will reflect real surface area (not simply shadow cast).

How to Measure a Gambrel Roof

And then there are the transition lines. Where does the roof slope up? That’s an important detail and it’s not just a line on paper. This exist in the real world, so it has to be sealed with specially designed ridge material or cap shingles for a valley. Most gambrel roofs (and full ones) has two such lines. Ignore those, and water get in. The calculator asks you how many of these lines there are, counts them, and adds the cap bundles required to your estimate. It is a little extra on top of grand total. But it is the difference between a leaky roof and a weathertight one.

The reference table lists different shingle types, and their coverage per bundle. This is for if you wanted asphalt but ordered cedar instead. Another variable is waste. Until you get up there on the roof it sounds abstract. You realize that shingles don’t fit together perfectly in endless sheets but has to overlap, be cut, or be trimmed, especially around rake and eave edges. And it’s particularly bad on a complex gambrel roof compared to a simple gable. The tool allows you to input the waste percentage, typically from 10-18%. If you’re new to roofing I’d err on the high side. Better to over-buy by one bundle and return it, than be short and make a second run back to the lumber yard. Waste take into account damaged shingles or other errors made while installing the roofing.

The rhythm changes depending on material selection. A simple three-tab shingle will give more bang for the buck in terms of coverage per bundle versus architectural shingles which are heavier and nicer looking. Cedar shingles also look good on a barn. However, they require more care during installation and takes up more bundles. The calculator takes all this into account automatically. Simply select the exact product desired and it tell you exactly how many bundles to grab off the rack. You no longer have to do the math in your head while looking at your shopping list.

So what’s my point? A gambrel roof add beauty without losing valuable square footage. But that beauty cost more in materials and complexity. When you break the roof down into its geometric parts you have a clear game plan as opposed to an estimate that makes little sense. You’ll be able to order exactly what you need. And you’ll understand why you’re paying for it. Trust me, the few minutes it takes to measure twice is well worth the hours of clarity this provides. Your budget remains intact and the roof looks awesome. This is the best possible outcome a builder could of hope for.