How to drop a hip rafter.

Submitted by Ray Thornburg on Sat, 12/08/2012 - 19:04

 

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    How to Drop a Hip Rafter

 

 

 

This discussion will explain how much to drop a hip rafter. Let me explain..... a hip rafter rises from the corner of a building at a 45 degree angle and if the carpenters used the same HAP on that rafter as he did the commons the corner of the rafter would rise up above the plane of the roof causing a hump. To stop this the carpenters can either drop the rafter by the proper amount (which is why they call it "dropping the hip") or chamfer the edges to match the incline of the hip. Study the diagram below. Click image to enlarge. But first: Consider buying my book The Roof Framers Field Manual. It helps keep this site up. 

 

 

How to drop a hip rafter picture.  Take the rise of the main roof and multiply it by 1/2 the thickness of the hip. Then divide the answer by 16.97. (framers use 17) multiply that answer by 16 to get sixteenths of an inch.  

For example main roof is an 8 in 12, 1/2 rafter thickness is 3/4". 8 x .75 = 6; 6 divided by 16.97 =.3535; .3535 x 16 = 5.65 or 5/16". Now simply measure up from the seat line of the birds mouth 5/16" for the correct drop.

 

You may wonder why we use 16.97. This is because 16.97 is the diagonal of a 12" square and 12" is the run of the common rafter. Thusly 16.97 (use 17) is the run of the hip.

 

 

There is a way to do this without all that math. (in case you left your calculator at home)

The amount of drop is determined by laying a level line anywhere on a hip rafter and measuring 1/2 the thickness of the rafter stock on the level line and then measuring the plumb distance at that point. The seat line is raised this amount so the rafter drops accordingly. Remember on a hip rafter we use 17 to mark the level cut. For example 8/12 on the common rafter but 8/17 on the hip and valleys. Marking the 8 will give you a plumb line; 17 a level line.

Occasionally a hip rafter cannot be dropped (because it is very thick) or part of it needs to be chamfered. In this case measure the distance of the calculated drop at the top edge of the rafter along a plumb line of course. Set your saw to the incline of the hip and rip the beveled chamfer. The incline of the hip is calculated as follows rise divided by 16.97 = TAN; Inverse function of tan on the calculator gives us the incline of the roof.  For example 8/16.97 = .471420; press inv tan .471420 and the answer is 25.2401 degrees. Set your saw to 25 degrees and rip the chamfer.

See This Page to learn how to figure the odd unit when laying out the tail of a hip rafter.

 

There is more discussion on hip rafters at our How to lay out a Hip Rafter page.

 

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Submitted by Ray Thornburg on Sat, 12/08/2012 - 19:04

Comments

Chamfering the Hip
Anonymous | Tue, 02/14/2023 - 07:01

Hi there Ray, I’d say you are sharp as a tack, and I’m an old School guy from across the Atlantic. I couldn’t help but notice the Hip chamfer calculation that you did above and I get 23 degrees old steel square method unit length of Hip on the body and unit rise on the tongue 8/18.76 inverse tan = 23 degrees.

18.76 is the calculated
Anonymous | Thu, 03/09/2023 - 16:16

18.76 is the calculated length per foot of run given on the framing square. Multiplying this number by the run(half span) will give you virtual length from center of ridge board to outside edge of wall. To find 18.76 the Pythagorean theorem is applied- the square root of 8 squared times 16.97 squared. I'm not trying to sound like a know it all here, just sharing an explanation to hopefully help clear it up for you. It turns out that 25.2 degrees is in fact the correct saw setting for 8/17 hip/valley rafter face angle. Happy Building!

chamfering the hip
Ray Thornburg | Wed, 02/15/2023 - 14:10

Thank you....IDK about your measurement system....we use inches and feet here. So on a 8 in12 roof,  the incline of the hip would be 8 in16.97. So the chamfer would be the same as the incline of the hip;  which would be 8/16.97 = .471420 which would be the tangent. Now you got to press inv tan .471420 to get 25.2401 degrees or 25 degrees, 14 minutes, 24 seconds. For practical purposes measure up the same as the drop and set your saw to 25 degrees to chamfer the hip.

However if you used a different measurement system I imagine the math would be the same. You would simply substitute when needed. Hope this helps.....Ray

Hip and Valley Rafters.
Anonymous | Mon, 03/11/2019 - 17:23

Excellent. Clear and concise.

Is a valley rafter dropped
Anonymous | Wed, 03/07/2018 - 02:32

Actually it is not dropped..Look at where the valley sits on the intersecting plates . The valley cannot move to its proper place because the edges hit the two plates as the rafter slides towards the ridge.So you continue the level line at the seat for 1/2 the thickness of the valley rafter..and cut out that wood... This allows the valley to move forward towards the ridge but when it does that the two sides also slide forward and lay on the plates in the case of a 1 1/2" thick valley 3/4" in from the edge of the building at that point the edge actually sits lower because it is forward of the edge of the building

Is a Valley Rafter dropped
Anonymous | Thu, 05/05/2022 - 07:15

Yes a very good point! Over the years I’ve noticed that there is a lot of misunderstanding to a Valley Rafter being dropped, in reaity the Valley really just needs to sit in the right place in reference to the HAP, of the Common Rafters for example if the Valley is Backed with an inverted vee the HAP measurement on the side of the Valley Rafter will be equal to the HAP of the Common Rafters at the plate lines and if it’s an unBacked Valley the HAP measurement will be at the Center of the Valley where the 2 plate lines intersect and this of course is equal to 1/2 the thickness of the Valley Rafter moving forward.

I’ve got a question! Ok we
Anonymous | Mon, 11/16/2020 - 07:34

I’ve got a question! Ok we know that the top edge of the valley Jacks are heading down in a plane to the centre line of the Valley and let’s say we are using a 2” square edge Valley Rafter. My question is do you think it feasible to keep the Valley Jack rafters flush with the corner top edge of the Valley Rafter. For example in a case where the HAP is kept the same where it intersects with the outside edge of the plate. In turn then the under side corners of the valley boards are bevelled to the reverse angle of the backing bevel. ? as opposed to using a vee backed valley or a square edged valley with bevelled fillet pieces planted on to the top of the Valley Rafter ?

Re: I've got a question
Ray Thornburg | Fri, 11/27/2020 - 21:43

"My question is do you think it feasible to keep the Valley Jack rafters flush with the corner top edge of the Valley Rafter?"

If I understand your question the answer would be no. The valley jacks have to plane to the center of the valley rafter so they are raised up the same amount as the hip drop. For 2" thick lumber there is no real practical reason to make it any more complicated than that. Now if the valley was as wide as 6 inches for instance then it might be worth calculating and ripping a wedge piece to fill in but even then I doubt it would be worth your time. However it (the filler piece) would be the same as if you were chamfering the hip if you so desired to get fancy. Like I said  though I'm not completely sure if this was what you were asking but I hope it helps....

Re: Is valley rafter dropped?
Ray Thornburg | Thu, 03/08/2018 - 08:10

The valley rafter is not dropped but any rafters (valley jacks) which are nailed to the valley have to be raised the same amount as the drop so the top of the rafter will plane out with the center of the valley. Hip rafters and Valley rafters are laid out the same way....only hip rafters are dropped....Hope this helps....

Dropoing a hip rafter
Anonymous | Sun, 08/06/2017 - 09:25

Here is the reason you drop a Hip and valley rafter.. If you look at where the hip touches the corner of the building you will see it has to move 1/2 the width of the hip towards the ridge to line up with the common seat cut..So if the Hip is 1 1/2 wide it would have to move 3/4 foward. since it can;t do that it has to be dropped. Say the Heel of the common is 5 inches then move the plumb line of the hip forward 3/4 and mark 5 inches and that is the cut line.SCPB