Adding a room onto an existing home is probably the most cost effective way to increase a building’s usable interior space. In this article, an addition means what a colleague calls a "three sided" addition. This phrase intends to avoid confusion with other sorts of home additions such as raising a building to create a new ground level space, or raising the roof to create a story between a ground level space and a roof area. The three sided addition means that the new and existing building will share an interior wall.

The first consideration when planning an addition is headroom: the height of a ceiling relative to human proportions. Most building codes stipulate minimum ceiling heights, but, as most people prefer ceilings that are at least eight feet (2.5 m) high, a well-designed space will probably meet or exceed these. Ensuring adequate headroom is probably the most challenging aspect of addition design, and is the main reason to start planning an addition from the roof down.

Begin your design thinking by trying to envision what you consider an ideal ceiling height for your addition when finished. As mentioned, most prefer a minimum eight feet, but a few inches less than this will still work in a pinch. It is important to start here, because your new ceiling will likely be hanging from the roof framing that will, in turn, attach to the existing building. If this framing attaches to an existing building too low, your ceiling will be too low. Let’s look at a couple standard roof frame techniques to help clarify.

Gable Dormer: When most kids in the western world draw a house, it will have a gable roof. A gable roof is an upside-down "V." A gable dormer is this same roof shape attached to an existing main building at a right angle. It will have a peak as does the children’s drawing, and where its roof meets the main roof is called a valley. As people have been using gable dormers for centuries, you won’t need to look far for an example. The main advantage to a gable dormer when designing an addition is that the addition’s ceiling height is determined by how high its peak is relative to the main building. Typically, the higher the peak, the greater the available ceiling height.

As with any building project, there is seemingly no end to pro and cons, and compromises need be found. When using a gable dormer frame for an addition, the compromise is that much of its weight will bear on the existing or main roof framing because it overlaps this framing. As the main roof framing was not likely designed to support this extra weight, this main roof frame will need to be strengthened. Of course, there are a few more in and outs to know about putting a lid on your addition using the gable dormer method, but in my opinion, this method is the slickest, and in the long term, will offer better looks than most alternatives. Due to the structural bolstering, and other framing elements required when using a gable dormer, it will likely cost more, as well.

If considering the gable dormer method, one thing to bear in mind is that because a sizable addition’s roof dormer will cover up a substantial portion of the existing roof, hold off on re-roofing until the dormer is in place. This will save burying a lot of new roofing material under the new dormer.

Shed Roof: The shed roof or shed dormer has an unfortunate name, but when artfully built, proves a cost effective roof frame for an addition, as well as an attractive one. Starting again with that inverted "V," the shed-style addition roof is a flat plane say the shape of a floor tile or square cracker that meets one "leg" of the upside-down "V" somewhere. "Somewhere" is the operative word because this versatile addition roof style can, when well supported, be attached anywhere on a building from the main roof to its exterior wall. For now, let’s suppose the shed roof attaches at the base of the inverted "V." Ideally, the roof joists your ceiling is hung from will "land" on the exterior wall plates where the main roof frame rests. This makes for easier framing.

But here’s the tricky part of using the shed-style. Unlike the gable method which has its drainage slopes built into the design, that tile shaped shed roof plane needs to be tilted down, at least a little bit. How much depends on roofing know-how and the materials chosen. Using the so-called 1:12 ratio which I think of as minimum, for every foot the roof extends from the main building, the plane, that tile or cracker, tilts down one inch. The tricky part is that at this ratio, every foot away from the main building is one less inch of headroom. If the addition roof extends 12 feet (4 m) from the main building, an eight-foot-high ceiling becomes seven with the loss of an inch every foot. This means that landing your new addition roof on the existing exterior wall frame may not provide enough headroom, even when using the minimum 1:12 pitch ratio. Try this simple formula using a 2:12 pitch ratio to see why a minimum slope is often used. Losing two inches of headroom per foot results in the loss of two feet (60 cm) of headroom over 12 feet.

With headroom in mind, you’re probably asking, "Can I raise the ceiling to get more headroom?" Yes, but you will simultaneously be determining where your new shed roof plane meets existing work. If that cracker or tile plane lands too far up the inverted "V" of the main roof, it will put weight on existing roof framing not intended to support it. This scenario, as with gable dormers, will necessitate some engineering thinking and doing, but in my opinion, will be worth the trouble. Shed roofs simply look better when they connect to a main roof, as opposed to being hung from an exterior wall under the eave.

Another good way to increase headroom is by lowering the addition’s floor elevation. This is more commonly necessary with single story buildings, but can be a challenge even with a second story addition. The problem is, of course, that by the time that shed roof is extended away from the building and headroom is lost as per the formula, the ceiling is so low as to be impractical. In this event, about the only option available is to "sink" the addition a step or two down to ensure adequate headroom.

A main benefit of the shed roof is its simplicity. It does not demand advanced carpentry skills to execute as far as roof framing goes. Instead shed-style addition roofs are challenging in that they not only require greater thought about drainage and roofing materials, but ask also for consideration of how building loads are transferred to their foundations, as these are often less obvious than with gable-style additions. A last important note about using a minimal or "low-slope" roof is not only that a low-slope roof material must be used, but extra care is needed to ensure the addition’s roof membrane goes well up and under the main building’s roofing material. In general, the lower the slope, the greater this under-flashing.

As always, it’s better when planning a building project to make mistakes on paper instead of on the job. This thinking is particularly true in additions, where certain elements of a plan are pre-determined by an existing structure that may be expensive to alter greatly. Of course, it’s also true that Will usually finds a way, so with a little "top down" thinking about addition roofs and some basic tools, a building’s usable interior space can be substantially increased without mowing a building down and starting from scratch.