I recently worked with a carpenter who had to frame out a dam for a new skylight in an existing skylight’s hole. His methods were a little questionable and it took him about six hours to get the measurments and cut eight 2×6’s to size.
Basically all we had to do was cut one side to 1/5″ and the other to 5.5″. It was to be a sort of sloping skylight on a flat roof. The distance across the opening was 28.25″ x 60″. Basically it was making a right angle. The problem that took so long was figuring what angle to cut, the issue being a 4″ rise over 28.25″.
What would be the best way to measure this out? Is there a formula to calculate the angles or would it have been best to have marked a 2 x 6 at 28.25″, cut it, then mark 1.5″ on one end, 5.5″ on the other end draw the hypotenuse, cut that line and butt up the studs that would go perpendicular to that piece at each end and trace the edges to find the angle of the first piece and then rip those at an angle on the table saw?
Also if you know any good books or websites with some commonly used carpenter math formulas that would be fabulous.
Thankyou for your time and I hope I explained that well enough!
Meg.
Replies
I would have done it just as you described. I heard some advice about fishing. When you stop catching fish, remember when you 12 with a line, hook and a worm. sometimes we tend to over complicate things. I love to see carpentry done by non-pro's that is well executed because it is accomplished with good old common sense. Certain things though like stairwork I tell my guys ether a two step up from a garage or a PT deck stair figure the hypotenuse mathematically and lay out with it in prep for the time when a true challenge arises. The best way to learn a complicated thing is from experience. In my work I use 3,4,5, the Pythagoran thereon and circumference of acircle every blue moon. I use a simple calculator on my palm pilot. The rest is empirical geometry, trammels and points, On the back of a stanley powerlock tape measure are some useful equations. Its on the powerlock and not on the powerlock II. somteimes the original is hard to find. And also maybe the guy was trying to learn something, nothing worse than trying a new method out with a disinterested coworker looking over my shoulder.
A Construction Master calculator is invaluable for answering rise/run questions. A 4" rise over a 28-1/4" run (or any other combo) can be solved for the diagonal measurement (28.531") or the pitch (8.05 degrees), your choice. I use mine almost every day, and I never do 3-4-5 anymore. Now, I will say this... you have to be able to visualize whatever it is you're building, and you have to be able to determine the math question that you need to solve. Once you do that, reach for the calculator. I think I paid forty bucks for mine? It's the 'IV' model.
That is exactly what I'm looking for! Thankyou!
I love math as much as the next guy, and sometimes I do the math just for the challenge of it. But on those trapezoidal-pyramidal skylight shafts, I generally just frame the opening in the roof, then frame the opening in the ceiling, where I want it (usually the centers line up, even if the two openings are a different size). Then I pull a string line at the corner, and use a sliding t-bevel to get the angles.
BTW, any good introductory trig book will give you probably all the math you'll ever need to figure carpentry angles.
great book to review: MEASURING, MARKING AND LAYOUT, by John Carroll. I refer to it regularly. I discovered it listed in a FHB magazine, but it is available on other websites. good luck.
Excellent! Thank you for the reference!!
you're welcome. good luck.
I would cut the board then set the saw on top of it and adjust the bevel to fit.
Keep it simple and throw the calculator away.
I would call it a curb not a dam.
Edited 6/6/2005 6:02 pm ET by doodabug
Curb! That's what I meant!
What math are you using? Help me what is 1/5" x 5.5". and what is 28.25"x60". I think the 28.25 is degrees but what is 1/5".
I figured it out! Thankyou anyway!