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HOW TO: Design the roof

HOW TO: Design the roof

Here’s one way to design the roof…

  1. Decide if you want gables, hips, a combination, or something else
  2. Follow specific rules for drawing the roof:
    • Try to keep all roof surface pitches the same.  Anything else often looks funny.
    • Given this rule, then from above, the roof will look like the following.
    • Each (level) peak will look like a line positioned half way between two parallel exterior walls.
    • For an end gable, the peak line goes all the way to the exterior wall.  In most cases, the peak line reaches the wall at a 90 degree angle (orthogonal or perpendicular).  Since the peak line is half way between two parallel walls, it hits that end wall right in the middle.
    • A hip looks like a triangle from above.
      • One side is the exterior wall that the hip is above.
      • The other two lines reach from the ends of that wall (typically building corners) to a meeting point.  The meeting point, that I’ll call the “hip point”, is normally but not always at a peak line.
      • The angle at which these two lines leaves the corner is controlled by the roof pitch.  If the roof pitch is the same on both sides of the line, then the line divides by two the angles of the walls at the corner.  That is, if the walls come to the corner at 90 degrees to one another, then the hip line leaves the corner at half of that, 45 degrees, splitting the angle in two.  But if the walls come together at 135 degrees, then the hip line leaves the corner at half this angle, or 67.5 degrees.
    • The whole roof can be drawn in terms of peak lines and hip angle lines.  You just need to know the rules above.
    • Some times you’ll simply have a corner in the house, like a 45 degree corner.  In this case, let me clarify and say that you probably actually mean that the walls come together at 135 degrees, as measured from the inside of the house, wall-to-wall.  If you have a sloped roof above each wall, then it’s just like a hip.  There will be a hip angle line leaving the corner, dividing the angle in two.  That 135 degree wall corner leads to a 67.5 degree hip angle line.  This line will stop when it meets a peak line or another hip angle line.
  3. If you’re only using a combination of gables and hips, then try this:
    1. Outside of your walls, draw a “gutter line” that includes the overhang of the roof beyond the walls.  Normally, this gutter line represents a constant level line.
    2. Start with all hips.  You can convert them to gables later.
    3. At every single corner of the house, whether pointing outward or recessing inward, start a partial line reaching inward that splits the angle in half.  These are the beginnings of your hip angle lines.
    4. Now, in your mind’s eye at least, simultaneously extend all the hip angle lines by the same amount.  The first pairs to cross each other will complete those hips.  Some hip angle lines won’t cross.
    5. If hip angle lines cross and you know there’s still more roof there, begin a new peak line emanating from the “collision” of the two hip angle lines, and continuing at an angle that splits those two hip angle lines.
    6. Continue extending, longer and longer, the hip angle lines and peak lines.  Eventually, some hip angle lines will intersect peak lines, at which point they’ll both terminate.  From there, you may find there’s still more roof to go, and a new hip angle line will continue, either up hill or down hill, depending on the situation.
    7. Sometimes you’ll run into situations where must draw construction lines to find where you know the peak must be (between parallel gutter lines).  Sometimes you’ll run into situations where you can only be certain of one hip angle line crossing such peaks.  But then you know the other hip angle line must cross at the same place, and you can add it, running back down hill.
    8. You may find you can’t finish.  You may have to make adjustments to the gutter line positions, or do other funny things.  Sometimes you can leave minor details to be resolved on site during construction.
    9. Now for those walls above which you want an end gable rather than a hip, just continue the peak line to the exterior wall.  Erase the hip angle lines that came from the corners of that same wall.  If the peak line reaches the exterior wall at the middle of it and at 90 degrees (orthogonal, perpendicular), then you’ve succeeded.  Otherwise, you’ve got something funky going on.
    10. Note that if you end up with a roof shedding water down hill and into another object, you’ll have a water leaking problem.  This is often the case with chimney’s.  The standard solution is to add a “cricket”.  This is a small roof shaped like a gable, except that the gable is butted up against the object (chimney).  Thus, the cricket causes water flowing down hill to run parallel to the object rather than straight into it.
  4. After you’ve tried this a few times, you should get the hang of it.  You’ll also begin to realizes how, being able to construct the roof has an impact on the exterior shape of your floor plan.
  5. Good luck!
  6. Now go redesign your floor plan so you can actually build the house…

Below is a sequence of line drawings following the rules above.  The numbers show the order in which lines were drawn.  Blue lines are level peaks.  Yellow lines were “[drawing] construction lines” used to find a level peak in the center.   Not all funky aspects are documented, though, like one more rising edge on the right, near but not adjacent to the other funky edge.  There’s also at least one hip angle line drawn in error.  You find it!  The roof DID get built in real life, so it DID work.  It was also scary working near those funky edges, 25 to 30′ above the ground.  The last drawing, in color, is an early iteration of the roof, shown in 3D.  Note that the 3D drawing doesn’t show the lower shed roof over a portion of the master bathroom.  The 3D drawing also differs very slightly from the line drawings, including a false hip connecting the front porch to the bedroom hip to its right, and some false roofs over the unseen shed roof, and how the garage roof connects to the main roof, and lack of crickets.

 

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