Thread: Fascia angle

09172005, 07:56 AM #1Veteran Contributor
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Fascia angle
I posted in this forum because of the recent threads about roofing angles...it might be better suited in the Exterior Details forum and if it gets moved because of this my apologies...
I'm doing a lot of cedar exterior trim. On a couple different models we have dormers with square cut tails. Obviously a dormer with a plumb cut tail just gets a level cut where it meets the main roof...with the square cut tails it gets a level bevel.
I'm not sure how to calculate this compound angle. I've been doing it using the cave man method of holding a fascia board up and scribing the angle/trial and error. This works but isn't really the way I like to do things. The cedar soffit ply also runs back to the main roof on a compound angle.
Would anyone be willing to help me through the calculations to figure this angle out? I use a Construction Master IV...but I'd welcome any methods. The diagram I'll post is of a 10/12 dormer on a 7/12 main. There are a few different combinations but I should be able to figure the others out once I get this one.
Thanks.
Tom
(I'm trying to upload a diagram (bmp sized to 640x480  Irfanview) but I get a message saying "document contains no data".....any quick help or suggestions to help me upload a diagram?)

09172005, 09:35 AM #2Regular Contributor
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Re: Fascia angle
If I read right? 10/12 is 40 degrees and 7/12 is 30 1/4 degrees. I think that's all you need.
You have to be lazy to be efficient.

09172005, 09:43 AM #3Veteran Contributor
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Re: Fascia angle
Originally Posted by doodabug
I was finally able to upload the drawing as a jpeg, after failing with a bit map....

09172005, 10:14 AM #4Veteran Contributor
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Re: Fascia angle
Stayfair: Try this Square Tail Fascia Calculator. It works for any pitch, any pitch, any plan angle (for example, an octogonal gazebo with irregular pitches and an angle between the eaves = 135°). Change the default values displayed to Main Pitch = 10/12, Adjacent Pitch = 7/12 and Total Deck Angle = 90° and click on the 'Main Side" button.
There are links to sketches images and explanations of the math. (Sorry if it seems complicated. I keep detailed notes for myself for future use). These are the Formulas for the Square Cut Fascia but someone else with experience on a CM will have to tell you which keys to press.Last edited by Joe Bartok; 09172005 at 11:27 AM.

09172005, 11:57 AM #5Veteran Contributor
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Re: Fascia angle
Stayfair, have fun with that calculator I posted but don't use it for this situation. I was in a hurry and didn't read your post carefully or study the image. (At times like this I recall the words of the eminent scholar and famous mathematician Homer Simpson: D'oH!).
What you're describing is something I haven't really considered but I'll take a shot at it. What we need is the intercept of a 12/10 (the reciprocal pitch for 10/12) with a 7/12 pitch at 90°. For this combination the miter angle = 52.78951° (37.21049° angle on the fascia) and the blade bevel = 43.70247° ... but I may be interpreting the miter and the angle on the fascia bassackwards.
I'll be offline soon but will think about this over the weekend. Try making a test cut on a scrap of lumber and let me know how you make out.Last edited by Joe Bartok; 09172005 at 12:05 PM.

09172005, 12:40 PM #6Veteran Contributor
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Re: Fascia angle
Thanks Joe, I'll definately enjoy playing around with those calculators...this is such a excellent forum.
If my CAD skills were a little better it'd make things much easier;)
It's been months since I had a dormer like this to trim, but I know there are many coming up fairly soon. One thing I recall doing that got me relatively close on the angle was with the frieze board under the soffit.
The frieze obviously lays flat on the dormer wall so it gets a 7 level to sit on the main roof. It also gets a 40° bevel so it fits nicely under the cedar soffit. If I take my speed square and hold it on the long points of the level bevel and read it across the thickness of the board, that seemed to get me pretty close to the angle needed. Cave man methods, but workable for the time being.

09172005, 01:12 PM #7Veteran Contributor
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Re: Fascia angle
Stayfair: I thought about this problem a bit more. I was worried about the tilt of the reciprocal 12/10 pitch influencing the value of the miter angle, but I don't think it does. Here's the logic behind the numbers I posted. I treated this problem as a Hip roof with the following givens:
12/10 pitch meets 7/12 pitch at 90° angle between Eaves
Angle on the Fascia = 12/10 Side Sheathing Angle = 37.21049°
Saw Miter Angle = 12/10 Side Jack Rafter Side Cut Angle = 52.78951°
Saw Blade Bevel Angle = 12/10 Side Backing Angle = 43.70247°
Yes, this is an excellent forum. Sorry about that incorrect first post. I didn't bother deleting it; perhaps someone will find that calculator handy another time.
EDIT: I'm going to experiment with this over the weekend. The Backing angle of the 7/12 side doesn't influence the bevel angle, in which case we need a calculation as per a sleeper:
Blade Bevel = 90°  (Sum of Backing Angles) = 90°  (43.70247° + 12.72599°) = 33.57154°.Last edited by Joe Bartok; 09172005 at 03:56 PM.

09192005, 10:00 AM #8Veteran Contributor
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Re: Fascia angle
Folks, I thought about this fascia question over the weekend. The solution of the miter/bevel angles for Square Cut Fascia intercepts Main Roof and a Valley sleeper are related. I developed the angles and made a model out of bristol board as per the first sketch. A similar jig or model could easily constructed from plywood to test a proposed cut.
Three solutions are given. The first employs the online calculator. The second utilizes ratios and the Pythagorean Theorem; trigonometry is applied only to calc the angles. The method is easy to remember and the math can be done in the field with any scientific calculator. If a calculator isn’t available there’s solution number three: development of the angles. The development can be constructed with sufficient accuracy using a compass and straightedge or a framing square and chalk line. I constructed the angles with a compass and straightedge and measured 56.5° for the dihedral angle on the fascia (= 33.5° blade bevel) and a hair over 52.5° for the saw miter angle … that’s close enough!

09192005, 03:59 PM #9Veteran Contributor
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Re: Fascia angle
Originally Posted by Joe Bartok
I appreciate the time you spent on the question Joe, I'll spend a good amount of time going over your solutions and trying to apply them to a Construction Master IV (no trig)...thanks again.
I read these forums far more than I post as I, as a rule, feel that most of these discussions are a little beyond me...I'm glad that after finally deciding to post a question it didn't have an overly (embarrassingly) simplistic answer.
Thanks again,
Tom
....and close enough! is right, our level cut on the fascia was always a sliver over 50°, again cave man style of measuring...I'll get the math down this time though if it kills me;)Last edited by Stayfair; 09192005 at 04:02 PM.

09192005, 08:54 PM #10Veteran Contributor
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Re: Fascia angle
I used my CAD program to draw a squarecut fascia tracking a 10/12 dormer roof and dying onto a 7/12 main roof.
I got a miter angle of 52.7895 and a bevel angle of 33.5715. I am pretty sure of the miter angle, but the bevel might in fact be 90 degrees minus my number, or 56+ degrees.

09202005, 10:29 AM #11Veteran Contributor
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Re: Fascia angle
Bob: I think you're good with the 33.5715° as the saw blade bevel. I made a 3D model as shown in the sketch in my web page and measured a dihedral angle of 56.5°. To obtain this angle on the stick we would have to set the blade bevel to the complementary value of the dihedral angle, about 33.5°.
Stayfair: Hang in there with the math! It looks more complicated than it actually is because I keep fairly detailed notes for future reference. But once you work through it you'll find there's really nothing new here. It's the same process as calculating the sleeper for a Valley except we use the reciprocal 12/10 pitch of the fascia.
I reread my explanation of the development of the Backing angles posted yesterday and it was as clear as mud; that's been edited. I've also added a link to this Slideshow: Development of the Backing Angle. A picture is worth a thousand words.

09212005, 10:27 AM #12Veteran Contributor
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Re: Fascia angle
Another way of viewing the Square Fascia Bevel angle:
Hip Rafter Model of Square Cut Fascia intercepts Main Roof
Nothing really different here. The model and calculation are as per as a Hip rafter. This is how I developed the 3D model. Solutions for the Saw Blade Bevel are given using both the Pythagorean Theorem and by working directly from the 7/12 Main Roof and 10/12 Dormer pitch angles.Last edited by Joe Bartok; 09212005 at 10:31 AM.

09222005, 09:47 AM #13Veteran Contributor
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Re: Fascia angle
The content of Hip Rafter Model of Square Cut Fascia intercepts Main Roof has been edited. The math is the same but hopefully I’ve clarified why it works.
The graphics still show the ratio method of solving the blade bevel. But there’s an easier way and for the folks who just want the formulas and don’t care about the math, the formulas, expressed directly in terms of the initial pitch angles, are:
Saw Blade Bevel Angle = arcsin (sin Dormer Pitch Angle × cos Main Roof Pitch Angle)
Saw Miter Angle = arctan (cos Dormer Pitch Angle ÷ tan Main Roof Pitch Angle)
Angle on the Fascia = arctan (tan Main Roof Pitch Angle ÷ cos Dormer Pitch Angle)
Specifically for the fascia on the 10/12 Dormer intercepting the plane of the 7/12 Main Roof as per Stayfair’s drawing:
Saw Blade Bevel Angle = arcsin (sin 39.80557° × cos 30.25644°) = 33.57154°
Saw Miter Angle = arctan (cos 39.80557° ÷ tan 30.25644°) = 52.78951°
Angle on the Fascia = arctan (tan 30.25644° ÷ cos 39.80557°) = 37.21049°
NOTE: The formulas given in this post assume an intercept of 90° for the ridges. If we have an irregular octagon (or whatever) in plan with dormers framed on large main spans, the dormer ridgehip eave intercept would not be at right angles when viewed in plan. In such cases the fascia angles would have to be determined by the more general methods (i.e. "sleeper") given in previous posts.Last edited by Joe Bartok; 09222005 at 01:57 PM.