Creating an Attic Truss

Reference Number: KB-00933
Last Modified: March 14, 2024

The information in this article applies to:

Chief Architect PremierVertical SeparatorHome Designer Professional


How do I create an attic truss?

Attic truss detail


An attic truss is a type of roof truss with an open space at its bottom center to accommodate a living space, with webbing above and on either side of this living space.

In order to create an attic truss, certain conditions must be met in the plan:

  • The structure must be wide enough and the roof steep enough to allow space for a loft area with attic rooms on either side of it. The loft area must have Attic rooms on both sides and must be separated from these Attic rooms by Knee Walls.

  • The loft area cannot be on Floor 0 or Floor 1 - it must have a living space below it that has a flat ceiling.

  • The roof must bear on the walls defining the floor below the loft area. It cannot bear on knee walls.

  • Each side of the roof must be composed of a single roof plane forming a continuous slope with a single pitch from baseline to ridge.

  • The loft area must have a flat ceiling.

To create the structure and build the roof

Because attic trusses require specific conditions to be met before they can be created, we'll begin by drawing a simple structure in a new file that meets those requirements.

  1. Select Build> Wall> Straight Exterior Wall Straight Exterior Wall from the menu, then click and drag to draw a rectangular structure.

    In this example, a structure measuring 30' x 40' is used.

    Floor plan measuring 40' x 30'

  2. Using the Select Objects Select Objects tool, click on the vertical wall on the left to select it, then click the Open Object Open Object edit button to open the Wall Specification dialog.

    Wall Specification dialog opened to the Roof panel with the Full Gable Wall option selected.

    • On the Roof panel, select Full Gable Wall, then click OK.

    • Repeat this process with the vertical wall on the right, as well.

    Note: You can also use the Change to Gable Wall(s) edit tool to convert hip walls to gable walls without having to open the Wall Specification dialog.

  3. Select Build> Floor> Build New Floor Build New Floor from the menu, and build a second floor derived from the 1st floor plan.

  4. Select Build> Roof> Build Roof Build Roof from the menu, and on the Roof panel of the Build Roof dialog:

    • Check the box beside Build Roof Planes or Auto Rebuild Roofs.

    • Select the Trusses Framing Method, then specify an appropriate Heel Height.

      In this example, a Heel Height of 12" is specified.

      In X14, Home Designer Pro 2023, and prior versions, check the Trusses (no Birdsmouth) box instead, uncheck the Automatic Birdsmouth Cut box, then specify your desired values.

    • Specify the Pitch (in 12) of the roof to be built.

      In this example, a 12" in 12" pitch is used.

    • Check the Ignore Top (2nd) Floor box.

    • Specify any additional settings to your liking, then click OK to close the dialog and build the roof as specified.

    Notice the dashed lines that now display in the 2nd floor room area in a floor plan view. These are ceiling break lines, which represent the division between the flat, full height ceiling in the center of the room and the low, sloped ceilings on either side. The area with the flat, full height ceiling is loft area that will be accommodated by the open space at the center of your attic trusses.

    Floor Plan View of the second floor with ceiling break lines and dimensions displayed.

  5. Select Build> Wall> Straight Interior Wall Straight Interior Wall from the menu, then click and drag two interior walls along the ceiling break lines.

    • It's easiest to draw them near the ceiling lines, then move them into position after they are drawn.

    • If you want, you can create a custom wall type with sheetrock on only one side if the rooms with the low ceilings are to be unfinished.

  6. Select the two interior walls and click the Open Object Open Object edit button to open the Wall Specification dialog. On the Roof panel, select Knee Wall to specify these walls as knee walls, then click OK.

  7. Next, using the Select Objects Select Objects tool, click on one of the rooms with a low, sloped ceiling and click the Open Object Open Object edit button to open the Room Specification dialog.

    • On the General panel, select Attic from the Room Type drop-down list.

    • Repeat this process with the room on the other side of the loft area.

    Building with interior attic walls drawn in

The conditions necessary to produce attic trusses are now in place.

To create attic trusses

  1. Select Build> Framing> Roof Truss Roof Truss from the menu, then click and drag a line that is perpendicular to the ceiling break lines and runs from exterior wall to exterior wall.

    The resulting truss will automatically extend out to the eaves.

    Floor plan view with the first truss in place

    Note: A Message may appear asking "The layer "Framing, Roof Trusses" is not displayed. Do you want to turn on the display of this layer in the current view?" Click Yes to display the Roof Trusses layer in this view.

  2. Select the roof truss and click the Open Object Open Object edit button. On the General panel of the Roof Truss Specification dialog:

    Roof Truss Specification dialog open to the General panel with specifications for the bottom chord depth and the options for Attic Truss and Force Truss Rebuild enabled.

    • Change the Bottom Chord depth to match your floor structure depth for floor 2.

      In this example, 11 7/8" is used.

    • Check the Attic Truss box.

    • Check the Force Truss Rebuild box.

    • Click OK to close the dialog and apply your change.

      At this stage, you could also check the "Lock Truss Envelope And Webbing" box as well to prevent the truss detail from changing if your roof is complete.

  3. Make sure Temporary Dimensions Toggle Temporary Dimensions are toggled on, enable the Select Objects Select Objects tool, then click on the attic truss to select it.

    Click on a temporary dimension that shows how far the truss is from either end wall, and type the distance that the first attic truss should be from the wall.

    In this example, the truss is placed 2' from the outside of the wall's framing.

    Positioning the truss using the temporary dimensions

  4. With the truss still selected, click the Multiple Copy Multiple Copy edit button.

    Note: Multiple Copy is not available in Home Designer Pro. Instead, use the Transform/Replicate edit tool. As an example, if you wanted to create 6 copies all separated a certain distance from each other, you would check the Copy box, set the Number of Copies to 6, then check the option for Move and set the X Delta to be 24". Negative values can also be set for each of the deltas if needed.

  5. Click the Multiple Copy Interval Multiple Copy Interval secondary edit button to open the Multiple Copy dialog, specify the desired Primary Offset for All Trusses, then click OK.

    The Primary Offset is the interval at which trusses will be replicated when Multiple Copy is used. In this example, the default value of 24" is used.

    Multiple Copy settings dialog showing the offset distance for trusses.

  6. Hover over the main Move edit handle on the truss until you see the Multiple Copy  cursor display, then click and drag across the structure, in the direction of the opposite end wall.

    • As you drag, the preview outlines of new trusses display at 24" intervals.

    • When you release the mouse button, the new trusses are created.

    Floor plan view showing the attic trusses evenly distributed.

  7. Create a Framing Overview Framing Over View to see the results.

    3D view of the finished, attic trusses

Now, additional framing components, such as lookouts and fascia boards, can be generated automatically using the Build Framing dialog , if needed.