Ask Katie: Thoughts on roofs and roof/wall-height relationships

Q: My husband and I are building a new house and are confused by roof pitches and wall heights. Does the roof pitch have a relationship to wall height?

Cathy from Hammond, LA

For having her question selected for this post, Cathy is receiving a complimentary set of KHS photo note cards.

A: Often it does. Roof slopes and the relationship of the roof to wall height (relative to the floor and ground) are critical to a successful design. Often the roof and its disposition are the most defining characteristics of a home.

roof slope 101
For the sake of discussion, let’s say there are three roof-slope categories. I consider a lower slope to be 7/12 and below. A steeper slope is 10/12 and above. A mid-range slope is everything in between. Think of roof slope in terms of rise and run. A 4/12 slope means that for every 12-inch horizontal run, the roof slope rises four inches.

Roof slope is partly a function of climate, available roofing materials, and structural considerations. The steeper the roof slope is, the easier for accumulated snow or water to slide or run off. So, wetter, snowier climates tend to have steeper roofs. Materials that perform well on a steeply sloped roof may be problematic on a roof slope below 4/12. Don’t forget that the primary role of the roof is to provide overhead protection from the elements. Roof slope can also have structural implications, with flatter roofs having different requirements than more steeply sloped roofs. Some roof/wall-height relationships can trigger other structural considerations too.

Familiar building types or styles are often characterized by certain roof-slope ranges. Lower-slope roofs are common on porches and on Ranch- and Prairie-style homes. Spanish influenced, Italianate, Craftsman, and classically inspired homes also often have lower-slope roofs. Mid-range slopes can be seen on some Georgian, Federal, and folk homes. Steeper roof slopes characterize many Colonial and Victorian era homes.

Plus, we tend to associate different roof slopes with different feelings. To me, a steeper roof can seem more sheltering and protective. It’s probably the mid-range slope that appears in most childhood drawings of home. Perhaps, it depends where you grew up. For many, there is simply something comforting and home-like about a nicely sloped roof.

roof/wall-height relationship
On a one-story home it can be pretty straight forward. If you're looking for a flat ceiling, set the wall height based on preferred ceiling height, often between eight and nine feet. The taller the wall is, the more expensive, in terms of material and labor. Then choose a roof slope based on the many factors discussed above in “roof slope 101”. If you would prefer a cathedral ceiling, consider a comfortable window-head height and a ridge height that isn't overwhelming. Among other considerations, experiment with roof slopes that satisfy your minimum and maximum parameters.

On a one and one-half story home, the roof/wall height relationship can get a little more complicated on the second floor. In such homes, the roofs are largely occupied. Depending on the geometry of the home and the desired amount of second-floor space featuring sufficient headroom, the roof rafters may be received at the second-floor level or on a knee wall which is in the plane of the exterior wall below. A knee wall is as it suggests. It may be knee height above the floor or taller, but generally it’s well below seven feet above the floor. To increase headroom and access to daylight, most one and one-half story houses include dormers. The second-floor knee wall may be shortened or eliminated altogether if the size and disposition of the dormers create sufficient usable space.

Of course, in one story, and one and one-half story homes, the roof eaves, and thus overall wall heights, are lower than in a two-story home. Lower eaves help to ground a building and keep it a relatable scale. Houses two stories and taller can risk looking boxier and more imposing.

The more stories, the more likely that zoning height restrictions will also play a role in the design of the roof/wall-height relationship. Full two-story buildings have full-height walls on the second floor with attic or cathedral space above. Two and one-half story homes involve many of the same roof/wall-height relationship factors as one and one-half story homes. If a steeper slope roof is preferred, coupling it with a shorter, third-floor knee wall may satisfy a height limit. Or, alternatively, if achieving a certain size habitable third-floor area is a priority, then a mid-range roof slope may bring the building height to a level under the limit. Throw dormers in the mix to gain additional habitable space, and the third-floor, exterior, wall height may be reduced further.

As with most design, determining the best roof/wall-height relationship is a matter of balancing function, context, aesthetics, and regulations. Exploring these types of relationships is very much the heart of residential design.

by Katie Hutchison for House Enthusiast

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