Know Your House: Components of a Roof

by Bud Dietrich

After you've installed your foundation, put down your floor structure and erected yourwalls, it's time to build your roof — one of the most important architectural elements. In fact, from the colonial home with its gable roof to a Prairie-style home with its hip roof, from a modern home with its low-sloping roof to an elegant mansard on an urban townhouse, we can't imagine a house style that doesn't have its associated roof configuration. Aligning the roof shape and configuration with the overall aesthetic you're after is essential to getting the look you want.

A roof also has an impact on the interior. Simply put, if all of the interior rooms have a flat ceiling of the same height, you can save time and money and have the roof built with manufactured trusses. If you're looking for some ceiling height variety and a vaulted ceiling in some rooms, you'll likely go with a stick-built roof that allows for this kind of flexibility. Or you can combine these two approaches to save time and money where possible while getting those special spaces you want.

Here are the different parts of a simple roof structure, how they come together and how they impact the interior spaces.

by Bud Dietrich, AIA »

A simple, stick-built triangular roof structure has three main components.

First, there's a ridge board, which is a horizontal wood element at the peak of the roof, establishing the apex of the roof's triangle.

Next are the rafters,which are fastened to the ridge board and slope downward to the exterior walls. The rafters do the heavy lifting for a roof structure. While resisting the downward force of gravity, the sloping rafters also provide the means by which the house sheds water, keeping the interiors dry and habitable.

Last are the ceiling joists, which also act as ties. While the rafters resist the downward force of gravity, the ceiling joists will resist any outward thrust. Just think of it like this: The rafters, having stood up to gravity, want to take a rest and lie down. But if they were allowed to, they would push the exterior walls outward — not a good thing. The ceiling joists won't let this happen; they just keep pulling the walls back in.

by Bud Dietrich, AIA »

An important design consideration is where to locate the ceiling joists, or rafter ties. These don't have to be set at the top of the exterior wall; they can be set higher up, allowing for a taller ceiling.

What's important is that the rafter ties be placed in the lower third of the overall roof structure height. This makes sense, as most of the outward thrusting action these ties are designed to resist is located lower in the roof structure, closer to where the rafters meet the exterior walls.

Setting the ceiling joists higher like this is, with the addition of some framing at the room ends, also the method used to create a tray ceiling.

by Bud Dietrich, AIA »

Let's say you want a really tall vaulted ceiling for a large great room. If this is the case, you'll want to get rid of the rafter ties altogether and replace the thin and light ridge board with a heavier and stronger ridge beam.

The rafters will get securely fastened to this ridge beam so that the whole assembly will resist the outward thrust of the rafters. And because these ridge beams can be quite massive, they become a distinctive architectural element in their own right.

by Bud Dietrich, AIA »

Another roof element, though not a common one, is the purlin. This is a board that spans rafters, or trusses, providing a way to fasten the roof sheathing and subsequent roofing materials.

Purlins were traditionally used when the rafters were placed farther apart to save on materials in utilitarian buildings such as barns. But purlins have become popular in residential construction because of their unique architectural look. In fact, using purlins will create the illusion that the actual ceiling floats above the rafters, something that can be architecturally distinctive.

by Bud Dietrich, AIA »

Last but not least aremanufactured trussesmade of 2-by-4 (sometimes 2-by-6) lumber and metal connecting plates. Trusses such as these are used quite often on large tract developments due to the efficiency of their repetitive design.

But these trusses are also used in custom construction too, as they can be a great way to save time and money. Even with a complex roof shape, trusses can be engineered and built on a made-to-order basis. And because of the efficient use of material and less waste generated when they're built in a factory, manufactured trusses can be a more sustainable way to build.

One caveat about a house built with these types of trusses: A roof built with manufactured trusses is more difficult to modify than a comparable stick-built roof. So if your home has ceilings all the same height and a truss-built roof, you'll find it more difficult to open rooms up and get that tall ceiling you want for that new great room. Be sure to get help from an architect or engineer before you modify the roof structure.

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