Roof Basics

Roof construction is a varied subject and even in residential roofing work, which is what I will be talking about here, it has changed so much over the last few years.

A basic human need is for shelter, which for most of us consists of a few walls and a roof over our heads. roofs come in many shapes and sizes and this page is by way of an introduction to the subject with links at the right to article pages that go into greater depth.

Angles, Slope or Pitch

One of the main reasons for having a roof is to keep out rainwater and snow. The way that we do this to provide a slope to the surface of the roof.

If you are starting from scratch, you must decide the pitch. In Australia and Europe, we define the pitch in degrees. So, a 5-degree pitch is flat, and a 45-degree pitch is fairly steep.

In North America I believe you refer to the slope of the roof as the ratio of the rise over the run. Usually in inches with the run being 12″ So an American drawing will note the slope as 4 in 12, that is 4 inches in 12 inches.

If I am designing a roof, the pitch determines what material I can and cannot use for the sheeting. For example, if I want to use corrugated Iron sheeting, there is a minimum pitch specified by the manufacture, it may be 5deg. Similarly, with roof tiles it could be 15degrees. Check out the material specs before you go ahead.

If you live in a high rainfall area like me, you need to be wary of almost flat roofs. If you are subject to snowfall you will have to make your roof steep enough to shed snow easily.

A Few Common Roof Types

Here is a standard hip roof on a rectangular plan. The four faces of the roof are almost always at the same pitch, which makes them symmetrical about the centre line.

hip roof with overhanging eaves

Roof basics – A hip roof with overhanging eaves.

Hip roofs usually have a consistent level fascia, meaning that a gutter can be fitted all around. The overhang of the roof at the wall is called the eaves. So, we could typically say “the eaves are 900”, meaning that the overhang of the roof at the eaves is 900.

If you are going to line your eaves, (cover them in, not leave the rafters showing) it is important to get the fascia parallel to the wall and the rafters cut off to the right size for whatever eaves lining material you use.

The eaves of a roof perform valuable functions apart from the looks. They protect the walls from the weather. From the rain and hence less maintenance, and importantly for us In Australia, from the sun. Wide eaves help to keep the walls shaded and cool, reducing power use. We have a building code that tells us we must shade our windows in new construction, under certain conditions, to comply with energy efficiency needs. Eaves often do just that.

hip roof overhanging eaves

Roof basics – Hip roof, the extra faces produce valleys

Next is another hip rood, this time set on set on a different plan shape. Because the walls are set at different positions, there are several extra roof faces. The level section at the top of the roof (marked “r”) is called the ridge.

Each ridge is central over the set of parallel walls below it. The triangular faces of the roof are called the hip ends, and they are bounded by the hips (h) themselves. The hips sit on an external corner of the building and rise to the ridge. Where the building has an internal corner a valley (v) makes the join between the sloping surfaces.

With this type of roof, if there is ever going to be a problem with leaks, all other things being equal, it will be in the valley. A build-up of leaves can cause the valley to overflow. The same leaves hold moisture for long periods and so accelerate corrosion in the valley flashing, or valley gutter. People are always told to keep their gutters clean, and rightly so, but a clean valley is far more important.

gable roof with Dutch gable

Roof basics – Gable roof with also a Dutch Gable

Here is the same wall plan again, with a different roof. On the two opposite ends there are gables, and on the other one is a Dutch gable, which is half gable end and half hip end. The gable end as far as the roofer goes is the simplest form to build. Offset that with the cost of the extra wall and it would be hard to say which type is cheaper to build.