Roofing roof slope, roof slope, roof slopes

Roofing roof slope, roof slope, roof slopes


Expert: Mark Sundberg — 10/30/2008


How can I determine roof slope on a two story house without getting up on the roof and measuring with a measuring tape?


Aloha Karen,

Well, there’s a couple of ways, some more accurate than others. Fortunately, most roofs are built with a specific roof slope, one of the usual ratios. These are typically, 1/8:12, 1/4:12, 1/2:12, 1:12, 2:12, 3:12, 4:12, 5:12, 6:12, 8:12, 9:12, 10:12 and 12:12. While that’s still quite a lot, you can pretty much eliminate most of them just looking at the roof slope, particularly if it’s a gable roof so you can see the roof nearly on end. Hip roofs are a bit more difficult. So say you are looking at a roof that is a 4:12 (one of the most common roof slopes), you can tell right off it’s not one of the fractional slopes, those less than 1:12. It’s just obvious. And the same goes with the high slopes, greater than 6:12. So you can pretty much limit your guess to between 3:12 and 6:12. Same with low slopes and high slopes, you can eliminate the really obvious.

If it is a gable roof, a roof that has the end showing and doesn’t wrap around the building like a hip roof does, so there’s a high pointed wall on the ends, you can just stand back as far as you can and take a picture. You want to be as close to right on end as possible and as perpendicular to the end as possible. Then print the picture out and measure the picture. Doesn’t matter if it’s to scale, just lay a ruler across so it just barely touches the tops of the eaves on each side and draw a line across. Then measure the length of the line and from the line up to the ridge point. You’ll get something like 3" or 4" for the length of the line, depends on the size you print the picture out at. If the roof is 3:12, then the distance from the line to the ridge point will be 1/4 the length of the line from eave to eave. If its 4:12, it will be 1/3. If its 5:12 it will be a bit harder. Then you can just divide the length into the height and if the fraction you get is about 0.42 (or 5/12) then you have a 5:12 slope. If it’s 6:12, the line will be twice as long as the height.

If it’s a hip roof, you can try to get really far back and if possible, up on the roof of a car or up a ladder about 8′ or so. You want to get as close to being exactly perpendicular to the roof as possible and then take your picture. It’s just not as reliable or accurate because it’s hard to tell what the lines are doing when they are out of plane.

You can also try measuring the eaves, if they aren’t soffitted. Measure the distance from the wall to the inside of the fascia or gutter or to the edge of the roof if there are neither of those two. Then measure from the ground up to the roof at the wall and at the roof edge. Divide the difference in the two heights by the width and you’ll get your slope. Remember, it’s mostly likely going to be on of the typical slopes, so though you might get a fraction of .39 you will know it’s probably really .42 and your measurements were just off some due to not being parallel or perpendicular. It’s more likely it’s a 0.42 or a 5:12 than a .33 or 4:12. You can do the same as measuring the eaves inside if you have open or cathedral ceilings. Just make sure your roof is the same slope as your ceiling.

Roofing roof slope, roof slope, roof slopes

Inside measurements are usually more accurate than outside because your floor is more likely to be level than the ground is. One way to eliminate that error outside is to use a big framing square to establish a level line perpendicular to the wall and measure to that from the high point of the wall and the eave.

Another method is to make some paper cutouts of each roof slope, just draw a line 12" long and another up whatever the slope is and cut them out. Then hold them up at arm’s length while looking at the end of the roof and see which one looks closest.

You can also use a protractor the same way, hold it up and adjust the angle to match the roof and then read the angle. a 3:12 will be 14 degrees. 4:12 will be 18.26 degrees. 5:12 will be 22.61 degrees. 6:12 will be 26.58 degrees. Generally there is enough difference between them you will be able to decide which it is.

You can also use your protractor on the underside of the eaves, measuring the angle of the roof to the wall. Just remember the angle you measure will be the compliment of the roof angle, so subtract your measured angle from 90 degrees.

One other method is to move close and farther away from the wall perpendicular to the roof slope, until you are sighting exactly along the slope of the roof, so you can just see the edge, but none of the roofing itself. Then measure the that angle or if you don’t have a protractor, measure the height of your eyeball and it’s distance from the wall and the distance from the height of your eyeball to the underside of the roof at the eave/wall corner. This may sound really complicated but if you use one of those laser sighting devices that can measure vertical angles or that can spot a line on the wall to measure from to eliminate any ground unevenness, it can be quite accurate.

There are probably a bunch more, this is what I could come up with off the top of my mind.

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