While a wide range of roof pitches and styles usually work just fine in almost any location, several factors will help ensure it is the right pitch for your particular project. While other site-specific considerations may exist in your exact situation, the primary elements that influence your design will simply be intended purpose for the shed, local climate, and aesthetics. A quick look at all three of these factors will make it easy to find a pitch that will work for your shed.
The Basics of Roof Pitch Geometry
Simply put, pitch is expressed as rise over run, just as the grade is simply this fraction expressed as a percentage. In roofing, it is expressed in inches of vertical rise over inches of horizontal run: A 4:12 pitch being 4 inches of vertical rise over one foot, or 12 inches, of horizontal run. There are advantages and disadvantages to having a steep pitch versus a shallow, or low, pitch_,_ the most obvious of which is the roof's ability to shed or withstand heavy snow, rain or wind. Basically, the steeper the pitch, the better it can shed snow and rain. The flip side to this is that it also becomes more expensive to build, both in materials and logistical difficulty. Knowing local weather patterns and historical records for snow and wind, will help decide what will be best for you. Using a snow load calculator can aid you in this decision.
Different Functions Call For Different Styles
Much depends on what your intended use for the shed actually is. If you are only trying to keep a wood pile dry, or storing a few outdoor tools and light machinery out of the weather, then you might consider a simple, single-pitch roof, since most everything inside will be stored at ground level and pulled out from the outside of the shed. If your intention is to fill the shed to the roof trusses with as much as its capacity will allow, then a gable roof, or twin-span roof with a central ridge and two sloping sides will allow for further loading above ground by utilizing the roof trusses to act as shelving for long items. It also provides more headroom for maneuvering within the confines of the unit. Larger still, and capable of much greater overhead storage and headroom, is the gambrel, or barn-style shed roof.
Aesthetics and Style
Since most locations have weather patterns that will allow a fairly wide range of roof pitches to work for something as small and sturdy as a shed, the biggest consideration is usually whether or not it matches the house. Some builders even choose to match it directly, especially if the shed's roof is attached to the house on the high side, and just one simple span pitch.
Another widely used method is to follow the house's roof pitch on one side, building the ridge off-center, then sloping down more drastically on the shorter side which will be closer to the house. This can be a nice look, even when it is turned with the gable end to the house, and the door is under the short pitch, rather than a gable end.
On country properties or farms, and sometimes even in the city near a garden, the barn style gambrel shed often looks best. While an exact roof match isn't necessary to remain in keeping with the surrounding architecture, this certainly is the easiest way to go to ensure a positive match. This is a personal choice, but know that keeping with the architecture of the home is the easiest way to ensure that all loading aspects have already been factored in.