Replies

1. 
   Turnagain | Aug 24, 2002 09:52am | #1

   I got a Construction Master calculator on my last birthday and it is real nice for things like this.  Common rafters aren't hard on a regular calculator but the Construction Master makes it easier. 

   I punched in the numbers you had:

   33'-5 1/2" - 7" (wall thickness) /2= 16'-8 3/4"

   16'-8 3/4" - 1 3/4" (half the thickness of the ridge)= 16'-7"

    17'-5 3/4" is the diagonal of the rafter and the bottom of the ridge should be 5'-6 5/16". 

   A couple of things to remember is that these measurements are measured from the bottom of the rafter to the inside of the wall plate.  Also, if you want the ridge to be flush with the tops of the rafters you'll have to compensate for the length of the 4/12 rafter cut.

   I hope that this is of some use.  Anothe place to look is John Caroll's Working Alone.  It has the simplest explanation I've seen for cutting rafters.  It is something that is easier to show than explain, though.

2. 
   MisterT | Aug 24, 2002 03:03pm | #2

   Sounds to me like you forgot to add the height at the out side of the wall to the total rise.

   Just some thing to check that stood out in my mind.

   happy framing!

   Mr T

   Do not try this at home!

   I am a trained professional!

3. 
   goldhiller | Aug 24, 2002 06:45pm | #3

   Paulg,

   Oh boy…….(accompanied by a sigh.) This would be very simple to show you first-hand, but is difficult to explain. I’ll try nonetheless and hope that I don’t only serve to confuse you more.

   I’m not sure what approach/method that you’re using to arrive at your figures, but more than likely you’re using a formula that concerns the theoretical lengths, heights, and lines of the roof. Your shortening allowances sound appropriate given the dimensions you state and you may only be overlooking one thing. It sounds as if that rafter is trying to tell you, but you’re not listening to it.

   The theoretical line of the rafter is a straight line parallel up the side of the rafter that begins in the 90degree corner of the birdsmouth. The theoretical height of the ridge is the point where that line intersects the ridge. Therefore, you will have x amount of material setting above this line on both the ridge and rafter. Imagine in your mind, two of these opposing rafters with said theoretical lines drawn on them, meeting up there in the air. See all that wood above those lines? If you have set the top of your ridge according to a theoretical formula, it is setting too low by the amount of material that is above said theoretical line on the rafter. This is an easy mistake to make.

   To correct this potential mistake, simply lay out the rafter via your framing square and draw the parallel line, to which I refer, up to the upper end of the rafter. Now measure on what would become the rafter ridge cut, the distance between that parallel line and the top tip of the rafter. This is the amount by which you must raise the ridge in order to construct your true 4/12-pitch roof.

   When we rafter a building, we prop and temp fasten our ridge in place, making certain the it is plumb and centered end to end and side to side on the building. Then every rafter measurement is taken on the basis of “long tip” of the ridge cut to the 90 degree corner of the birdsmouth. Every rafter then fits perfectly. This method will allow for variances in the wall that would otherwise become problematic.

   We use a framing square with clamp on type stair gauges to lay out the rafter. The square is now ready to be used for laying out the angles of the birdsmouth, as well. The seat of the birdsmouth should be as wide as the top plate. It sounds as if you’ve done this with your 3 ½” layout, however, we usually rafter after the ½’ ply sheathing is installed and so this then becomes a 4” seat. When you have a trial rafter made which satisfies the roof, I use a gauge of some sort to help locate the 90 corner of the birdsmouth for all rafters to follow. This can be a mortising gauge or a simple 6” long piece of wood cut to the width that the 90 corner of the birdsmouth is up from the bottom of the rafter. Make your ridge cut first and then hook your tape on that tip and find the intersection of the length measurement and that birdsmouth corner.

   If you don’t have your collar ties in place, your walls are not dead plumb, or your walls wander, you can end up with measurements that vary considerably. Get things as straight as possible and well secured before you begin raftering.

4. 
   goldhiller | Aug 24, 2002 07:21pm | #4

   Paulg,

   A couple more tips………..I wouldn’t advise that you make the bottom end plumb cuts prior to mounting the rafters, but rather let them hang long until you're done. At that time, snap a chalkline down the top of them all from corner of building to corner of building, mark your plumb cuts, and then make them. If you pre-cut the tails of the rafters, any wandering variances of the wall will automatically be mirrored in the tail-ends, and consequently in the fascia, where they will be quite visible for all the world to see so long as the building lives.

   Use a string fastened on top of the upper center of a rafter to its bottom center to aid in straightening it for further measurements and installation of rafters and/or the sheeting. This is particularly necessary if there are any valley or hip rafters involved in your roof design. These must be installed and held straight while you get the measurements for and install any jack rafters.

   If there are hips or valleys involved, don’t forget to change the layout on the square from 12/4 to 17/4 for these layouts.

   
   

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   Edited 8/24/2002 12:26:58 PM ET by GOLDHILLER

5. 
   Joe_Fusco | Aug 24, 2002 08:20pm | #5

   paulg,

   A quick read of your condition says you need to raise the ridge by another 8-5/8" to account for your HAP. This is the amount that is left on the rafter after you cut your birdsmouth that had a 3-1/2" level cut. The 8-5/8" is based on using 2 x10's as rafters. If you didn't use 2x10 then you need to figure what your HAP is.

   Also the 2" reduction you made had to be made square to the plumb cut.

   View Image
   

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   Edited 8/24/2002 1:21:48 PM ET by J Fusco

6. 
   exlrrp | Aug 24, 2002 09:18pm | #6

   A quick tip no one else seemed to say.

   When I'm confused by numbers like this I snap it out on the floor ( or somewhere) in real numbers and work it out there before I cut, this always resolves discrepancies, usually because I'm measuring the wrong place.

   good luck

   James

7. 
   Joe_Fusco | Aug 25, 2002 05:10pm | #7

   Paul,

   Here's a short excerpt on the rafter HAP

   What is meant by the term H.A.P.?

   View ImageThe H.A.P. is a term used to describe the amount of rafter stock that should be left on after the birdsmouth is cut, this amount is then expressed as a value in inches. The H.A.P. can be determined by drawing a line plumb with the proposed  roof slope on the rafter stock and measuring the length of the line, then taking approximately 2/3 of that number. This value will now be the value of the H.A.P..

   View Image

   The H.A.P. value can be easily calculated by using the following formula:

   One other important measurement is the length of the "Building Plumb Line". This is the line that the building intersect the rafter at. You always take 2/3's of this measurement to calculate the H.A.P.

   Building Plumb Line = Framing member width / Sin(90 - (roof pitch angle))

   H.A.P = (Framing member width / Sin(90 - (roof pitch angle))) * .666

   The final aspect of the H.A.P. would be plate bearing. This is to say that if you wished you could "adjust " the H.A.P. in relation to the plate size. This just means that you can cut your birds mouth so it bears fully on the plate.

   H.A.P. (adjusted) = Building Plumb Line - (plate width * tan(roof pitch angle))

   View Image

1. 
   MisterT | Aug 25, 2002 06:47pm | #8

   Joe,

    I believe that the HAP value should be The Length of the plumb line minus the hieght of the birds mouth.

   Not 2/3 of the plumb line, this is a value that is specified in the codes to keep someone frome cutting away too much of the rafter when making a birdsmouth cut.

   The diagram shows the correct dimension.

   Mr TDo not try this at home!

   I am a trained professional!

1. 
   Joe_Fusco | Aug 25, 2002 07:56pm | #9

   Mr. T,

   I don't understand your reply.You draw a plumb line on your rafter and you take approximately 2/3 the length of that line as the HAP.This graphic might help.If the plumb line or building line is 9" the HAP would be 6".

   View Image

1. 
   Joe_Fusco | Aug 25, 2002 09:08pm | #10

   Mr. T,

   After re-reading your post, you are saying the same thing just wording it differently. If the length of the building line is 12" and you set your HAP at 8" (2/3 of 12). This leaves a birdsmouth cut of 4". If you use what you said ("I believe that the HAP value should be The Length of the plumb line minus the hieght of the birds mouth."

   ) the HAP would still be 8". 12-4 =8".

   View Image

1. 
   MisterT | Aug 25, 2002 10:15pm | #11

   Joe,

   I think our mutual confusion. Lies in that we are coming to the answer from different directions.

   I usually make the level cut of my birdsmouth the same as the wall thickness (4" or 6" mostly).

   This is what determines the height of birdsmouth and therefore the HAP that is "leftover"(most times more than 2/3 of the plumb line)

   Capisce?

   Mr TDo not try this at home!

   I am a trained professional!

2. 
   Joe_Fusco | Aug 25, 2002 10:34pm | #12

   Mr. T,

   Well, whatever course you take to cut the birds mouth, what's left is the amount to raise the ridge ;-).

   View Image

3. 
   Paulgern | Aug 26, 2002 06:51am | #13

   to JFusco and T - the crux of the issue is " what's left is the amount to raise the ridge ;-)." it's what i failed to do.

   KEVMCD, i found an article in my old fhb"s by john caroll-i will b e getting his book as suggested.

   GOLDHILLER, no need to sigh. i think i got your explanation just fine.

   thanks again guys.

4. 
   MisterT | Aug 26, 2002 02:05pm | #14

   No Paul the crux is that I said it first!!!!

   I get the milk bones !!!!!!

   They're all mine mine mine MINE!!!!

   GRRRRRRR! Slobber!

   Mr TDo not try this at home!

   I am a trained professional!

5. 
   Paulgern | Aug 27, 2002 08:55am | #15

   Amen.

   all's well that ends well. rafters up and looking good. i love this site.