Creating an Attic Truss

Reference Number: KB-00933
Last Modified: July 14, 2021

The information in this article applies to:

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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

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

  1. Select File> New Plan New Plan from the menu to open a new, blank plan.

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

    Floor plan measuring 40' x 30'

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

  3. Click the Select Objects Select Objects button, then click on the vertical wall on the left to select it then click the Open Object Open Object edit button.

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

    • On the Roof panel of the Wall Specification dialog, select Full Gable Wall and click OK.

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

  4. Select Build> Floor> Build New Floor Build New Floor from the menu, and build a second floor based on the first-floor plan.

  5. 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.

    • Specify the Pitch of the roof to be built. In this example, a 12" in 12" pitch is used.

    • Check the box beside Trusses (no Birdsmouth).

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

    • Click OK to close the dialog and build the roof as specified. It will bear on the top plates of the first floor walls.

    Notice the dashed lines that now display in the 2nd-floor room area on floor plan view.

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

    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.

  6. 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.

  7. Select each of these two interior walls and click the Open Object Open Object edit button. On the Roof panel of the Wall Specification dialog, select Knee Wall to specify these walls as knee walls.

  8. Next, click the Select Objects Select Objects button, then click on one of the rooms with a low, sloped ceiling and click the Open Object Open Object edit button.

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

    • Repeat this step 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 an attic truss

  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.

    Floor plan view with the first truss in place

    The resulting truss will automatically extend out to the eaves.

  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 case, 11 7/8" will be used.

    • Check the box beside Attic Truss.

    • Check the box beside Force Truss Rebuild.

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

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

  3. Create a Framing Overview An orthographic overview of an attic truss. to see the results so far.

    Elevation view of an attic truss.

  4. Select File> Close View to return to floor plan view.

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

  6. 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.

    Positioning the truss using the temporary dimensions

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

  7. 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.

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

    Multiple Copy settings dialog showing the offset distance for trusses.

    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.

  9. Click and drag the selected truss's Move edit handle 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.

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

    3D view of the finished, attic trusses