Roof that slopes in 2 directions

[jedbjorn]

I am new to Revit and I got it specifically to do a project I think my current program, Chief Architect X8, is not well suited for. I need a tool by tool explanation of how to do this, and it would be greatly appreciated.

We have a "butterfly roof" which is kind of like an inverted gable. It slopes down from the outside at a 4:12, and the resulting valley, which will have to be heat matted, will slope at a perpendicular towards the building edge at a 1:12 in order to drain. I have the butterfly shape, as seen in the attached screen-shot, but I need an accurate downward slope of 1:12 and I don't know how to do that.  I am not familiar enough with families to design this without a little help in the tool department. Also, if it is relevant, I am using Revit LT. Any help you guys can give would be great, thanks. 

[samuelsanf]

Hello again @jedbjorn, @ToanDN and @Alfredo_Medina

Here's a few things:

• You can make only one roof, because you can draw more than one Slope Arrow in your Sketch.
  
  
• In this video I put the math into Revit, trying that these can apply to any case in general.
  
  
• Notice that omitting repeating decimals may result in inaccuracy; so I use the multiple by 3 to avoid repeating decimals and so that the dimensions do not look very small in the video. If you are working with imperial units in fractions, then you do not need to multiply your values.
  
  
• The (-1)* that I put in the formula is to get negative values, so that the arrowhead points down. To make the slope lower toward the center of the roof.
  
  
  

Here the formula (in the video I do it geometrically, so you don't need to use more calculator than Revit).
I have these values:

Slope in X -------------------> 4/12   ..... could be whatever you want.

Slope in Y -------------------> 1/12   ..... could be whatever you want.

Multiplication factor ------>     3    .... It's a least common multiple, you can omit it if you don't have repeating decimals or if you are working in fractions.

Multiplication by -1 ------->    -1    .... Only if you want the arrowhead points down. You can omit it by turning any of the above values to negative.



= { -1 } * { [ (3 * 1 / 12 ) * ( 1 / 12 ) ] + [ ( 3 * 4 / 12 ) * ( 4 /12 ) ] }

= { -1 } * { [ 0.25 * ( 1 / 12 ) ] + [ 1.00 * ( 4 /12 ) ] }    <----- This is the expression I typed into the Slope Arrow properties.

= { -1 } * { [ 0.02083 ] + [ 0.33333 ] }

= { -1 } * { 0.35416 }

=  - 0.35416   or   - 17 / 48


I don't know if this could work in general but I can obtain 17 / 48 with:

1  +  4  +  12   =      17
.                             ------
1  x  4   x  12   =      48

In order to see the formula well, you have to hide the command panel of the video. I'm sorry that the mouse got into the formula.

[samuelsanf]

I got this, but I don't know why Revit put slope with 4⅛" / 12" instead of 4¼" / 12" 
Everything else is fine. The values in the slope arrow (When edit the sketch) and the rounding of the slope units.

[jedbjorn]

Nice. Thanks a lot. This is really great. I'm thinking there should be a YouTube video of this method. Have you considered making one? You tube was my first research, and it was only nominally helpful.

[samuelsanf]

This the sum the previous method shows here. For now the screencast shared in the third comment above this one must be enough 😛

[samuelsanf]

Ok, Revit was right. Here is the final and simpler expression: Pythagorean theorem

Y   =   1 / 12

X   =   4 / 12

H²   =     Y²  +  X² 

H    =    √  ( Y²  +  X² )

H   =    √  [ (1/12)²  +  (4/12)² ]

H   =    √  ( 1/144 + 16/144 )

H   =    √ ( 17 / 144 )

H   =   4.1231056256 / 12

H   ≈   4 ⅛ / 12   ◄──── this is the real slope 


From previous comments   4¼ / 12   or  0.35416   or   17 / 48    is    Δh   the difference in height between the arrow head and arrow tail.
The Slope is   ≈ 4 ⅛ / 12

[samuelsanf]

Here is a summary of the general rules


We have:

---

Length in X axis   =   XL
Length in Y axis   =   YL

Slope in X axis      =   XS
Slope in Y axis     =   YS

---

*Lenghts of the Roof; Slopes of the Roof; Axes relative to the Roof.

To obtain the diagonal length of the Slope Arrow ( HL ), we must scale the length of the side with less slope to be proportional to the length of the greater slope side times the ratio of the smaller slope to the greater slope:

We will call the Length of the greater slope axis   LA  (Long Length of the right triangle).
Then we must obtain a new Length in the lower slope axis called   LB   (Short Length of the right triangle).
Let's call the greater slope SA ; and the lower slope SB

•  If    YS   >  XS :

---

LA  =  YL 
LB  =  YL  ×  ( XS ÷ YS )
SA  =  YS
SB  =  XS

---

• If    XS   >  YS :
  

---

LA  =  XL
LB  =  XL  ×  ( YS ÷ XS )
SA  =  XS
SB  =  YS

---

In Revit you don't need to calculate the length of the Slope Arrow ( HL ) nor the slope of the Slope Arrow ( HS ), because revit computes it automatically. You just draw the line that connects it's cathetus   LA and LB   then specify the difference in height of the Slope Arrow ( ΔH )

---

But if you want to do the math:

HL   =   √ ( LA²  ×  LB² )

HS   =   √ ( SA²  ×  SB² )

---

The difference in height of the Slope Arrow ( ΔH ), or the difference in height of the hypotenuse vertex's, is equal to the vertical distance from the highest point of LA to the lowest point of LB    or    equal to the sum of vertical distance of LA plus the vertical distance of LB 

To obtain the difference in height of LA we have to multiply it by SA
To obtain the difference in height of LB we have to multiply it by SB

---

ΔH   =   ( LA × SA ) + ( LB × SB )

---

If we want the Slope Arrow to point down, we have to multiply the expression by -1

---

ΔH   =  -1 ×  [ ( LA × SA ) + ( LB × SB ) ]

---

[samuelsanf · Accepted solution]

@jedbjorn, @ToanDN

Actually Revit let you to draw Slope Arrows with the desired slope as an input. I didn't know that until few days ago.

Now reading again @Alfredo_Medina's comment, I think that what he wrote.

So, the math formula was simpler:

[chrisplyler]

I can't tell you how many times I heard classmates grumbling about "When will I ever use this in the real world?" thirty years ago when I was in school.

I used to answer, "Thirty years in the future when you're trying to slope a roof in Revit."

And they would all look at me funny. BUT I GOT THE LAST LAUGH!

[jedbjorn]

Ha! I am just a draftsman, and I didn't go to school. But you can tell
those kids I have actually seen this on plans and in the real world,
especially in more contemporary projects. We didnt go this way on my
current project, so the discussion became theoretical, but I did see a
skewed slope roof recently on another project. It does happen. Thanks for
the support!

[jedbjorn]

Thanks for the thought you put into this! really helpful. Our project ended
up going with crickets instead, but this is good stuff to know. I am taking
the math and saving it for the future.

[jedbjorn]

Thanks for the thought you put into this! really helpful. Our project ended
up going with crickets instead, but this is good stuff to know. I am taking
the math and saving it for the future.

[npsteinr]

How can I add beams to a slope in two different dirrections? Once I have a roof that is "twisted" it doesnt allow for me to create a reference plane. Thank you!