Calculating Buildable Area in an Attic Space

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In many of the designs I have done the second floor was built primarily within the attic space. This tutorial discusses how to figure out the buildable space within the attic.

IRC 2009 R.305.1 specifies that:

For rooms with sloped ceilings, at least 50 percent of the required floor area of the room must have a ceiling height of at least 7 feet (2134 mm) and no portion of the required floor area may have a ceiling height of less than 5 feet (1524 mm).

This specifies that the walls must be at least 5' tall so I need to know exactly where the 5' ceiling line is within the attic. I also want to know where the 8' ceiling line is (actually 8'-1.125").  This will let me know where I can place walls to create rooms that will have no sloped ceiling. This will give me a starting place for the design.

Once you have the first floor designed place a Reference Circle reference point at a location on an exterior wall. If you are not familiar with Reference Points see the Proper Use of Reference Points tutorial to see how they are used.

Use File-->Save-->Save As to save a copy of the first floor and name it something like Second Floor. Then reduce this down to just the exterior walls. I do this by using Erase-->Type Erase. Check All Items then uncheck Walls (Bearing). Then I use Edit-->Change Wall to convert all of the exterior walls to Plates, making sure to select Outside of Current Wall's Outermost Bearing Material and Outside of New Wall's Outermost Bearing Material. For this to work correctly the Plates must be defined as Bearing so if your Plates are not bearing you will need to define them as Bearing.

Next edit one of the plates and change the Height to 1.5", change the Offset to 1.5" and check Offset Down, and then on the Common tab uncheck Cleanup. Unchecking Cleanup locks the plate so it won't automatically adjust up or down when you do a cleanup. If you use SoftList then click on the SoftList tab and uncheck "Include in SoftList". Select Ok to save your changes then use Edit-->Repeat Edit (Box) to make the same changes to all of the rest of the plates.

2nd Floor Plate with 0 Offset

So let me explain what I just did. By saving the First Floor to a new file I am creating an exact copy of the First Floor. But all I want is the exterior walls to use as a starting point for the Second Floor. So I erased everything else. I want these walls to be plates so I can place my roof on the plates. My plates are defined as bearing. If your plates are not defined as bearing you will need to edit the wall definition for the plate and change it to bearing. I adjusted the height and offset of the plates so that they will actually be in the same location as the top plates for the First Floor plan. Because I gave the plates a height if 1.5" I needed to offset the plates down 1.5". If we had not offset the plate down the bottom of the plate would have been at 0" elevation, relative to the second floor, and would have effectively created another plate on top of the first floor walls. See the illustration on the left.

2nd Floor Plate with 1.5" Offset Down

Once you have the second floor plates set up it is time to put on the roof. With the second floor plan active switch to Roof Mode and draw in your roof. Make any changes necessary such as pitch changes, gables, overhangs, etc. to complete the roof.

Once you have the roof drawn the next step is to draw the locations of all hips, valleys, and ridges. You could do this by saving a Birdseye view of the roof or by tracing over the roof using the method described in the Double Hip Roofs tutorial. I typically draw in the roof. If you create a Birdseye of the roof you can then merge the saved Birdseye into the floor plan using File-->Merge-->Merge Drawing.

Click to See Full Size Image

So now we have our wall plates, our roof, and lines indicating what the roof looks like. For this example I'm using a roof with 12/12 side pitches and 8/12 front and back pitches to illustrate that this method works no matter what your roof configuration.

Click to See Full Size Image

Next we need to calculate the 5' and 8' ceiling lines. I'll do this on the right side of the house which is a 12/12 pitch. We also need to allow for the floor system in our calculations. So draw up a simple cross section showing the outside wall, a rafter set at the correct pitch, and a floor system. For this example I'm using 11 7/8" TGI joists with a 1 1/8" Subfloor. This gives us a total floor system height of 1'-1".

Offset the top of the floor system line 5' and 8'-1 1/8" up and then extend these lines to the bottom of the rafter. This will give us the 5' and 8' ceiling locations.

Take measurements from the outside of the wall to the points where the ceiling lines meet the bottom of the rafter.

Click to See Full Size Image

Back on our second floor plan draw two lines and dimension them off the right side plate the same distance as the 5' and 8' ceiling lines. Extend the lines to meet any hips or valleys along that roof plane.

Click to See Full Size Image

Turn on snaps (F11) and start drawing lines beginning at the end points of the 5' and 8' ceiling lines and terminating at any hips or valleys you encounter on each roof plane. If you have done everything correctly you should end up back at the end points of your original 5' and 8' ceiling lines. You now know the buildable area within the attic space and also where you can place rooms so that they will not have any sloped ceilings.

Bill is the owner and maintainer of

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