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Would like to know how you cut the valley rafter when you have two different pitches on intersecting roofs.
I have done it before using the old
tried and true string method but there
must be another way. The problem has
always been the height the rafter sits
off the top plate to line up with both
roof planes. How does the 17/12 idea
fit in here?
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Phil,
Joseph Fusco
View Image
"Whenever, therefore, people are deceived and form opinions wide of the truth, it is clear that the error has slid into their minds through the medium of certain resemblance's to that truth."
Socrates
*Phil,Joe was right. Here I am, unable to resist a question regarding "split pitch" roof framing, even tho I really want to shut this thing down and go get some breakfast tacos.Actually, Phil, valley rafters are easy compared to hip rafters in the same situation. This is because the common rafters and the valley jacks plane into the CENTER LINE of the valley rafter. So, there is no need to cut backing bevels on it.As you nail the jack rafters to the valley rafter, you hold them up a little above the edge of the valley rafter so they plane in to the centerline. On the steeper side of the roof , you would hold them up higher to get them to plane in than on the less steep side, and just how much this measurement is depends on the roof pitches involved.Your concern about how high the rafter sits off the top plate so that both roofs plane in properly is a valid concern, but easy to understand.The starting point is to look at the birdsmouth that you cut on all of the common rafters as well as the jack rafters. After you cut out this small triangular piece of wood for the birdsmouth, determine the PLUMB measurement that remains above the birdsmouth at the outside plate line. It's impotant to keep this measurement the same for all common and jack rafters for both roof pitches. This measurement is sometimes referred to as the HAP, (height above plate), a term introduced to many of us by Marshall Gross in his book, "ROOF FRAMING"If the "seat cut" (the level cut) of the birdsmouth is 3 1/2" you'll find that for 2x6 rafters that the HAP will be around 4 5/16", plus or minus, and for 2x8 rafters about 6 3/8"Let's assume that your rafters are all 2x6's and that the HAP is 4 5/16" for all common and jack rafters in the roof. This is the same measurement that the CENTER of the valley rafter should have as it passes over the outside plate line (the building line). It's that simple. If you double the valley rafter, just make sure that at the location that the valley rafter crosses the plate line, that the plumb measurement of the valley rafter at its center point is 4 5/16" Hip rafters are a bit more involved, but not much.
* Phil,
Joseph Fusco View Image
*Phil,Still there?I see that I didn't answer a second question that you had. Namely, "How does the 17/12 idea fit in here?"It doesn't.The number 17 has no special significance in a split pitch situation, although you could use it on the body of the framing square as long as you use the correct number on the tongue.If you're still following this thread, post the two roof pitches that are involved and I'll show you how to get the correct plumb and level cuts on the hip/valleys.
*Ken, thanks for the info but I think it would bebetter if I took you up on your offer to show meexactly how its done. Lets say we have a 14/12intersecting a 10/12 roof. Its seems to me thatthe HAP for 10/12 common rafters is not the sameas for the 14/12 rafters so what HAP do you usefor the valley rafter? Some days I don't thinkas fast as I used to so be patient OK Thanks
*Phil,Let's assume that the rafters are 2x8's, which typically are 7 1/4" wide.If you draw a plumb line on a rafter for a 14/12 pitch, you'll find that it measures something very close to 11 1/8". If you now lay out a birdsmouth with a 3 1/2" seat cut (level cut), the "heel cut" on the birdsmouth will be 4 1/16", leaving you with a HAP of 7 1/16"Now, repeat this process for the 10/12 pitch.This time the plumb line will measure 9 7/16", and a 3 1/2" seat cut will result in a 2 15/16" heel cut, leaving you with a HAP of 6 1/2"Why not pick a number in between the two, say 6 3/4", and use it for the HAP for BOTH pitches, as well as for the valley rafters? This will make the seat cut on the 14/12 rafters a little more than 3 1/2" and the seat cut on the 10/12 rafters a little less than 3 1/2", but it's not etched in stone anywhere that the seat cut on a rafter always has to be exactly 3 1/2".Anything else I can help you with?
*Phil,
Joseph FuscoView Image"Whenever, therefore, people are deceived and form opinions wide of the truth, it is clear that the error has slid into their minds through the medium of certain resemblance's to that truth." Socrates
*Phil,You could take Joe's advice and go with the HAP of the lesser pitched roof if you like (6 1/2" on the 10/12 rafters). It would only amount to a difference of 1/4" from the HAP measurement that I suggested of 6 3/4". No big deal.If your rafter stock is 2x6 however, you may want rethink this process.The 10/12 plumb cut on a 2x6 rafter is 7 3/16". If you make the seat cut 3 1/2" as on the 2x8 rafters, you would find the HAP to be very close to 4 1/4". This means that you would have to remove about 41% of the width of the rafter stock when cutting the birdsmouth. If you then use 4 1/4" as the HAP on the 14/12 rafters, you would be removing very close to 50% of the width of the stock. If you hold the seat cut to a given measurement, say 3 1/2", you will find that as the pitch increases, that you will be removing a greater percentage of the stock to make the birdsmouth. If the pitch were 24/12, for example, you would remove about 57% of the width on a 2x6 rafter.In his book "Roof Framing" pages 35-36, Marshall Gross suggests that as a general rule, don't remove more than a third of the stock. This is a general guideline and as Joe suggests, with steeper pitched roofs, it may not be be so critical, but certainly is something to be aware of. You might want to lessen the seat cut slightly, to allow more of the stock to remain in some cases, but not to the extent that you lose good bearing and nailing.
* Phil,
Joseph Fusco View Image
*Joe,You make some good points. Following the 2/3rds rule that Marshall Gross suggests is a good idea, but one that I don't strictly adhere to, especially in split pitch roof framing.Let's suppose that Phil's rafters are 2x6. The 10/12 plumb cut on a 2x6 is 7 3/16". Two-thirds of that is 4 3/4".In an earlier post in this thread, you mentioned that you would use the HAP of the less steep roof pitch on all the rafters. The 14/12 plumb cut on a 2x6 is 8 7/16". If we use 4 3/4" as our HAP on these rafters, we would be removing about 44% of the rafter stock, more than the 2/3rds rule permits.So let's try it another way. Let's start with the 14/12 rafters and determine the HAP using the 2/3rds rule.2/3 X 8 7/16" = 5 5/8" HAPIf we use 5 5/8" as the HAP on all the rafters, we would find that the seat (level) cut on the 14/12 rafters would be 2 7/16" (not ideal, but can live with) and that the seat cut on the 10/12 rafters would be 1 7/8". I personally wouldn't cut a seat cut this small (1 7/8'). It doesn't give you enough bearing or enough area to nail properly.So what's the solution?One option would be to add a piece of 3/4" material to all of the rafter plates that the 10/12 rafters fall on ( but not under the valley rafters). This would allow you to cut an extra 3/4" away from the birdsmouth and get you back to a decent seat cut of 2 3/4". ( using the 5 5'8" HAP)Another option would be to make different HAP's 0n the 10/12 rafters than on the 14/12 rafters, and leave the plates alone. This however, would force you to offset the valley rafters from their intended location. This creates additional problems with adjusting ridgeboard lengths, jack rafter lengths, etc., that many framers would not appreciate.What would I do in this situation? I would use an HAP of about 4 3/4" on all of the rafters even though it violates the 2/3rds rule on the 14/12 rafters. This would result in a seat cut on the 10/12 rafters of 2 15/16", and a seat cut of 3 3/16" on the 14/12 rafters. I would use these measurements for either 2X4 or 2X6 walls.Would appreciate feedback on this from you Joe, and others also.
*Ken,
Joseph FuscoView Image"Whenever, therefore, people are deceived and form opinions wide of the truth, it is clear that the error has slid into their minds through the medium of certain resemblance's to that truth." Socrates
*Joe,Most houses down here in San Antonio, Texas, where I have worked and lived for the past 5 years, have their roofs framed with 2x6 rafters, usually 16 O.C. Purlin bracing is then added at places in the roof where the rafters are over spanned. It doesn't snow down here. 2x8 and 2x10 are also common to a lesser extent, usually being used for houses that have ceramic tile roof coverings.As far as the numbers that you put up in your last post, I don't see where we have a difference of opinion on what you said. I think I made it quite clear that if the 10/12 plumb cut on a 2x6 rafter is 7 3/16" and you take 2/3rds of this number, you would end up with an HAP = 4 3/4", just as you said in your post. Obviously, this is the amount remaining, 2 7/16" or 1/3rd being removed to cut the birdsmouth, resulting in a seat cut of 2 15/16"The 14/12 plumb cut on a 2x6 rafter is of course, as you say, larger, about 8 7/16". If you make the HAP on these rafters also equal to 4 3/4", you would have to remove 3 11/16", or about 44% of the stock. That violates the 2/3rds rule that we have been discussing, but I would cut them this way regardless. This would result in a seat cut on the 14/12 rafters of 3 3/16". Since all of the seat cut would bear on the rafter plate (whether it were 2x4 or 2x6), I don't see how the strength of the rafters that run from the plate, up to a ridgeboard, or hip rafter, would be comprimised at all. In my opinion, it's only when the seat cut is longer than the plate width, that you begin to compromise its strength, as some of the seat cut has no wall to bear on.I can only assume then, when Marshall Gross talks about the strength of the rafter after the birdsmouth is removed, he is referring to the strength of the rafter TAIL. Personally, I can live with less than the 2/3rds rule when I need to. If the rafter birdsmouth is pulled in tight to the plates before it is nailed, it will be plenty strong, even though 44% of the stock was removed, especially after the fascia, sheathing, blocking and soffit are added.Joe, I know that you and many others own a Construction Master Calculator, so here's my roof framing tip of the day, which you may already be aware of.To find the length of the plumb cut on a rafter, enter the pitch, then the width of the rafter stock as the RUN. Press diagonal for the answer.Have a nice day.
*Ken,
Joseph FuscoView Image"Whenever, therefore, people are deceived and form opinions wide of the truth, it is clear that the error has slid into their minds through the medium of certain resemblance's to that truth." Socrates
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Joseph FuscoView Image"Whenever, therefore, people are deceived and form opinions wide of the truth, it is clear that the error has slid into their minds through the medium of certain resemblance's to that truth." Socrates
*Phil,Still there?I see that I didn't answer a second question that you had. Namely, "How does the 17/12 idea fit in here?"It doesn't.The number 17 has no special significance in a split pitch situation, although you could use it on the body of the framing square as long as you use the correct number on the tongue.If you're still following this thread, post the two roof pitches that are involved and I'll show you how to get the correct plumb and level cuts on the hip/valleys.