How to Model Roofs

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    Roofs can be simple or complex, and many designers find them difficult to model in Autodesk Revit. The following tips can help you understand exactly how to achieve the look you are after — no matter how complex a roof you create.

    First, it is important to understand that Revit roofs are “system families,” like walls, ceilings, and floors. Any roof you create exists inside the Revit project and is not a separate component or element like other Revit family (.rfa) files. This means that you cannot save a roof or roof system out to a library like a door or a desk. To import a roof type from one project to another, use the “Transfer Project Standards” command, or just copy and paste.

    Figure 1: Gable roof

    Second, creating a roof in Revit is similar to creating floors or ceilings using a sketch-based method. Roof “types” are also the same as “types” in floors and ceilings. You can change roof types on the fly using the type selector. This is true whether it is a basic, single-element slab roof or a complex roofing system made up of specific material assemblies.

    Out-of-the-box, Revit provides three ways to create roofs:

    • Roof by Footprint
    • Roof by Extrusion
    • Roof by Face

    Roofs by Footprint

    Figure 2: Dutch gable roof

    Using the Roof by Footprint method, create the roof by sketching a 2D outline of the roof's exterior shape in plan view. Define the slopes by picking the lines in the footprint that define the edges of the roof planes. The sketch of the roof must be a closed loop, though you can include other closed loops inside the sketch, which will become openings. The plan level where you start the sketch determines the roof level.

    Flat roof — This simple roof is made using the Roof by Footprint method with no defined slopes on any of the sides.

    Skillion roof — Select overhang length on all sides and define the slope.

    Gable roof — Select overhang length on all sides and define the slope on two sides (see Figure 1).

    Dutch gable roof — A Dutch gable roof is usually a four-sided roof made up of a lower hip roof and an upper gable roof. Create this roof by drawing two roofs and placing them on top of each other using the following steps (see Figure 2):

    Figure 3: Cone glazed roof
    1. Start by creating the lower hip portion of the roof using Roof by Footprint.
    2. Once you have created the hip portion, and you are still in the sketch mode, draw another rectangle inside the roof to create an opening.
    3. Now create the upper gable portion of the roof using Roof by Footprint and the “Pick Line” tool to select the opening in the hip roof as the outline of the gable roof.
    4. When both parts of the roof are ready, make a section view so you can move the upper portion to align with the lower portion of the roof.
    5. Once the two are aligned, use the “Join Geometry” tool to remove the line between the two parts.
    6. To finish, add two walls to fill in the hole in the open gables ends, then attach them to the roof to correct the wall profile.
    Figure 4: Gambrel roof

    Cone glazed roof — Create this roof using the Roof by Footprint method, with the “Sloped Glazing” roof type, using the following steps:

    1. Begin by creating the Roof by Footprint using the Sloped Glaze type.
    2. In the Type Properties for Sloped Glazing, make sure the Layout is set to “Fixed number” for both Grid 1 and Grid 2.
    3. The Grid 1 number will be one half the number of glazed panels (it accounts for one half of the circle of the cone, as Revit creates circles in two segments). In this example, the number is 12, which equals a cone made of 24 segments in each “ring.”
    4. Grid 2 is the number of “rings” the cone is made of. There will be one more row of glazing panels than there are rings. In this example, the number of “rings” is 4, which means 5 rows of panels.
    5. One last thing to ensure that the roof works: It must have a defined slope, even if it's only 0.5 degree. In this example the slope is 30 degrees (Figure 3).
    Roofs by Extrusion
    Figure 5: Saltbox roof

    Create Roofs by Extrusion by sketching a profile of the top of the roof in an elevation, section, or 3D view, then extrude the roof. Set the start and endpoint to determine the extrusion depth. The roof type sets the thickness of the structure. Create the profile using a combination of straight lines and arcs. The location of the profile in the elevation determines the height of the roof.

    Gambrel or barn roof — The easiest way to create this roof is using the Roof by Extrusion method (see Figure 4).

    Saltbox roof — This roof is similar to a gable roof, but the slope and length of the two sides of the roof are not the same (see Figure 5). This results in a steeper, shorter front and a longer, shallower slope for the back. These roofs are typically on saltbox houses that have two stories in the front and one in the back.

    Roofs by Face

    The Roof by Face tool lets you create roofs on any non-vertical faces of a mass. Use this tool when creating a roof from an in-place mass or a mass family you have loaded into your project. These roofs are more complex and integrate into the overall building geometry. Be aware that roofs created using the roof by face tool do not automatically update if you change the mass face.

    Figure 6: Dome roof

    Dome roof — Create dome roofs using roof by face, with an out-of-the-box “Dome” mass family:

    1. Begin by loading the Dome mass family into your project. Choose either the metric or imperial library based on your project needs.
    2. Once the dome mass is in place, make sure it is on the correct level and that the size of the dome is not too “steep” to be converted to a roof. If it is too steep, Revit will let you know with a warning that says, “Can't make roof.”
    3. You can adjust the size of the mass with either the “pulls” in a view, or by selecting the mass and adjusting the radius and/or height in the properties dialog box (see Figure 6).
    4. Now the mass is set up to then start the Roof by Face command to create the roof.
    5. Choose roof type from the type selector, then select both parts of the dome mass and choose “Create Roof” from the contextual tab.
    6. Now that the roof exists, it occupies the same space as the dome mass. Select both using a filter so that you can turn off the mass. You could also delete the mass at this point, but if there is any editing to the roof later, it is much easier to do using the original mass.
    7. Now that the mass is off and the roof visible, you will see a couple of issues. First, the default roof rafter cut is “Plumb Cut,” and if you look at the roof in section you will see that this could be detailed better as the “Two Cut – Square” option. The Rafter Cut options are available in the properties dialog box once you have selected the roof.
    8. The next issue is the line in the middle of the dome, even when you turn on Realistic Mode. Fix this by going into the Graphic Display Options, in the Visual Style area and uncheck the “Show Edges” option.

    Rooflines can make or break projects. They are, after all, a building's crowning glory. Getting roofs right in Revit takes a little time and effort, but the results are worth it.

    Carl Storms is senior applications expert – Building Solutions, IMAGINiT Technologies.

    Heather Alley, R.A., is applications expert, IMAGINiT Technologies.