I just spent all day (seemingly) putting up 3 framing members. Here’s the situation…stick built roof, all cathedral ceiling. It has a 8/12 main gable roof with a 10/12 gable dormer set to one side so that the rakes intersect. Of course it was designed to have the fascia’s aline and for the soffits to be of equal width (16″ from frame to fascia face).
To further complicate things, the structure was designed to use a major valley (runs from outside wall to main ridge beam and carries the end of the lower dormer ridge beam) and minor valley.
The major valley is a double 11 7/8 lvl and the dormer ridge and minor valley are double 2 x 12’s.
Some days are hero days…today was a goat day. Is this really as complicated as it felt? The architect gave me precious little to go on, and in fact the roof plan and section were inconsistent with one another as far as certain key dimensions go.
I thought I was pretty smart and efficient as far as framing is concerned until today…so if anyone has any good tips or ideas for resources to make this kind of thing easier, I’m all ears (or eyes as the case may be when reading posts.)
Edited 7/3/2008 11:50 pm ET by averagejoe
Replies
Do you have a Construction Master calculator? If you do I could talk you through the sequence.
But my best suggestion is that you snap out the major framing members on the deck if you have the space. In my experience, it is absolutely the best way to wrap your brain around framing that you can't get handle on. Usually, if I get 'stuck' I'll start popping a few lines and before I'm halfway done with them my brain is whirring away and I've "got it". If snapping it out still doesn't click with you, at least you can use the rise/run relationship (which is right there on the floor in front of you) to figure out your pitch and diagonal lengths. And all your intersecting angles (cheek cuts) will be right there on the deck too.
Edited 7/4/2008 8:00 am ET by dieselpig
I often get roof plans where the guy who drew the plan doesn't have a clue what the roof will actually end up looking like.
Heck, this week I got a "plan" for a 3,000 square foot 2 story house with complicated, swooped roof lines partially sketched on the front elevation. The guy didn't really know what the rest of the roof would look like, so there was a note on there "elevations by truss manufacturer".
I always start out by figuring heel heights. (OR HAP, as some of the stick framers call it.
Then I take the roof planes one at a time, figuring out what will happen with them. Once a few of them are solved, the rest seem to fall into place.
.
I realize that what I do is different than what you do. But I think you can approach it the same way - Break it down into steps. Concentrate on one section at a time, and block the rest of it out of your mind.
No one can visualize an entire roof all at the same time. You'll only drive yourelf nuts trying to do it.
If you have the plans in PDF format, you could always post 'em here and see if you got some suggestions.
Break it down to small tasks. Maybe start with the ridge. go to a reg rafter. then the valley rafter. I know it is easier said than done, but buck up you will get it.
You could hire a consultant framer. For a few dollars one of us could fly out to show you how!! Of course it might be a little expensive.
You ought to contact Frenchy, he knows everything!!!
Edited 7/4/2008 9:33 am ET by frammer52
Joe, you probably are attempting to do the impossible.
I'm going to guess that you are trying to build the inside ceiling with the same rafters that you are building the outside roof. I'm also going to guess that you are trying to align the inside hip with the corner of the room.
That won't work unless you create a false ceiling on one plane or use different height rafters.
Bob's next test date: 12/10/07
When I was a framer, we had a crazy roof to do and my boss was good at visualizing, but even he finally had me take a board up and hold it while he looked from the ground so he could visualize better how it all went together. I still had no clue until we were about half way through!
I have at times put up temporary mock-ups out of 1x's screwed together--sure beats nailing together the real thing to find it isn't working!
Some people find construction calculators or scientific calculators (for using trigonometry) helpful but they aren't necessary. You can use a basic calculator and geometry to find everything you need. Even the geometry is optional.
Here's how I'd "build" a solution to your problem. It's not the fastest possible way to approach the problem but it is very tangible.
The first thing I do is calculate the ratio of the run to the rise for the two pitches. To do this for the 8-in-12 pitch, divide 12 by eight: 12/8 = 1.5. This means that for every 1-in. of rise there is 1.5-in. of run. Repeat the process for the 10-in-12 pitch: 12/10 = 1.2. In this pitch, there is 1.2-in. of run for every inch of rise.
Now, find the ratio of the runs to each other by dividing the longer ratio by the smaller: 1.5/1.2 = 1.25. This tells you that, at any given rise, the run of the 8-in-12 pitch is 1.25 longer than the run on the 10-in-12 pitch.
On a regular roof, where the pitch is the same, the runs for both roofs remain the same at any given pitch. In plan view, the runs form a square, with the valley slashing at a 45-degree angle across it.
On this irregular roof, where the pitches are different, the runs are different at any given rise. In plan view, they form a rectangle and the valley slashes diagonally across it at an angle other than 45-degrees.
Using the ratios of run, draw a rectangle that is proportional to the rectangle for the two pitches. Let's use 20-in. for the run of the 10-in-12 side. Multiply that dimension by 1.25 to determine the ratio of the run for the 8-in-12 side: 1.25 X 20 = 25. Draw a rectangle with sides of 20-in. and 25-in. on a sheet of plywood.
Now draw a diagonal across the rectangle. This line represents the run of the valley in plan view. You can determine the length of the line mathematically by plugging the numbers 20 and 25 into the Pythagorean Theorem or you can simply carefully measure it. It is 32-in.
Now you can use this drawing to measure the bevel angles for the plumb cuts of the valley rafter. You can use a Speed square or protractor for this or simple take the angle with a bevel square and transfer it to your saw. The angles should be about 39 and 51 degrees.
You can also use the 32-in run dimension to find the pitch of the valley rafter. First, determine the ratio of the rise for either side. First let's use the 8-in-12 side 8/12 = .666. This means that for every 1-in. of run there is .666 in of rise. Using that ratio, find the rise of the 25-in side: .666 X 25 = 16.65. Now, let's check that against the 10-in-12 side: 10/12 = .833. And: .833 X 20 = 16.66.
The rise of both sides, of course, is the same. It is also the rise of the valley. So, the valley rises 16.66-in over the aforementioned 32-in. run. Divide 16.66 by 32 to find the ratio of the rise to the run: 16.66/32 = .521. Multiply this number by 12 to find the in-12 pitch: .521 X 12 = 6.249. The pitch of the valley, then is 6 1/4-in-12.
You can use this pitch to lay out the level and plumb cuts of the valley rafters.
You can also use the drawing to find the backing angles for the valleys. Lay out a 6 1/4-in-12 triangle on a scrap of 2 X 12 and cut out the triangle. Set the base of the triangle on the "valley" (diagonal) line on the side you want to find the backing angle. Put the point of the triangle even with the corner of the retangle. Mark the side opposite the edge that's along the line where it intersects with the side of the side. From that point to the top edge of the triangle (i.e., the hypotenuse) is the amount you need to bevel the valley. Use a combination square to record that distance, mark a line on the side of a scrap of 2 X that has been cut off square. After marking a line in the cross section of the scrap the goes from the mark on the face to the opposite corner, measure the backing angle.
The lengths of the two common rafters can be determined many ways. I like to use ratios. On your rafter square, under the number 8, you'll find the number 14.42. This is the hypotenuse of a right triangle of with a rise of 8 and a run of 12. Dividing 14.42 by 12, gives you the needed ratio: 14.42/12 = 1.202. For every 1-in. of run, therefore, there's 1.202-in. of length along the slope of the rafter. The same procedure works for the length of the 10-in-12 rafter. Under the number 10, you'll find 15.62. So, dividing 15.62 by 12 gives you the needed ratio: 15.62/12 = 1.302.
There is no entry on a rafter square for a 6.25-in-12. You can simply measure the length in place. I'd use geometry. I would plug these numbers into the Pythagorean Theorem (on my $3 calculator) and find the "length per foot run" (13.53) for a 6 1/4-in-12 triangle. The I'd divide by 12 to find the ratio: 13.53/12 = 1.13. For every 1-in. along the run of the valley, therefore, there is 1.13-in of length.
The run for the valley, by the way, can be calculated using the 20-in. x 25-in. rectangle. The valley run is 32/25 (1.28) times longer than the 8-in-12 side. After you determine the run for the 8-in-12 rafter, multiply it times 1.28 to find the valley run.
Edited 7/4/2008 1:57 pm ET by Mudslinger
you need to look at will holliday's book the roof cutters secrets. in the book he carefully details the steps to do a full scale layout as other posters have suggested, and it will help you answer other questions as well.
it is complicated, but it is not hard, you can do it if you break it down and organize it.
the other thing that may help a lot is if you get gene to draw you a rendering in sketchup, if you can do that as well then you can play with the pieces, and figure dimensions and see how it is going to look, kind of like how another poster described holding a piece up and looking at it, except much easier on the computer screen.
The lengths and angles in any "complex" Hip or Valley roof can be broken down into right triangles, and combinations of right triangles forming tetrahedra. All of the angles may be constructed with nothing more than a compass and straightedge.
Geometry of Hip and Valley Roof Framing and Joinery Angles
Timber Framers Guild ... Hawkindale Angles
SBE Builders Online Tools by Sim Ayers
While it is vital to understand the underlying geometry, and carpenters have managed to get along with geometry alone for centuries, calculators and computers are inexpensive and I find it more efficient to use trigonometry to solve the roof dimensions and angles.