The Roof Over Your Head
When we speak of the necessities of life, food and water come first. Shelter is next. My father used to say, "Be grateful for the food on the table and the roof over your head".
Virtually every form of human shelter involves a roof of some kind. We are going to explore the subject of roofs as they relate to modern residential homes; the various styles, how they are built, and what we can expect from various roof covering materials. This is not meant to be all inclusive, just a discussion of the most prevalent types of Canadian roofs.
What does a roof do for you? In the simplest terms, it keeps out precipitation and conserves the interior climate of your home. However, the style component has become a very important aspect of roofs. Style can make a home more distinctive and can also respond more effectively to the prevalent climate in your area. Some styles do a better job of shedding snow and rain while others may be more secure against wind damage.
The most common styles are Gable and Hip, but don't forget Domes, Flat, Mansard, Shed, Pyramid, Saltbox, etc... There are also variations and combinations of all these styles. If you are curious, a simple Google search can provide you with numerous images. This article generally refers to pitched roofs.
When lumber was readily available and inexpensive, larger dimensional cuts were in wide use for roof framing. So, the roofs of older homes (up until the 1960s) have rafter type framing. This allowed for useful storage space, with some attics even converted to living space when there was sufficient head room. However, as these larger cuts of lumber became scarcer and more expensive, architects and builders moved to what is commonly known as stick framing. Smaller pieces of wood were tied together in such a way that they could provide practical load carrying ability. These "trusses" used to be built on site, but now they are commonly factory-built to a new home's specifications and then raised onto the home's framing after the exterior walls are built. These are called pre-engineered trusses.
Both types of framing have advantages and disadvantages. Rafter framing is extremely sturdy and has sufficient bearing capacity to allow storage in the attic. Pre-engineered truss type framing is built in controlled conditions in a factory, out of the weather.It is less labour intensive to install and can be erected more quickly.However, with multiple sticks crisscrossing the attic space, and with diminished bearing capacity across the bottom of the trusses, the attic will not have a very useful storage capacity. A CAHPI qualified home inspector can usually identify the type of roof framing and its condition.
Tying the framing together for greater structural rigidity and providing a surface for the installation of the finish roof covering, sheathing has also evolved over time.In very old homes, the roof sheathing consisted of straight edged boards, often milled to various widths. These made for a strong roof (many of which have survived to this day), but as the boards shrank (as wood will do) gaps appeared between the boards. This was not a problem when the original roof covering was nailed to the sheathing.But, when the original roof covering became too worn or damaged to perform well, the gaps made for a less secure nailing of the replacement roof covering. Most roofing contractors will insist on installing sheet type sheathing over the original boards to meet warranty requirements.
After that early era, builders began using tongue and groove boards for roof sheathing. This eliminated the gap problem as these boards fit more snugly together.However, as this type of milled board became more expensive, builders starting using sheet type lumber such as plywood, wafer board, and oriented strand board (OSB). This type of sheathing also made for a less labour-intensive installation.
Plywood is the most durable of the three types named above, but it too has become quite expensive. The initial move to wafer board sheathing did not work out well. The load capacity was generally inferior to plywood and the adhesives used in its production did not hold up well in humid environments. It often had a shorter than desired service life. Science came to the rescue with the development of oriented strand board (OSB). This type of sheathing uses layers of perpendicular oriented wood chips. This gives it load capacity comparable to plywood.Manufacturers also started using improved adhesives, greatly improving its resistance to moisture. At present, this is the predominant type of roof sheathing being used in the construction of new homes. While some builders still prefer plywood, the OSB is so far standing the test of time.
There are many types of roof coverings, but we will discuss only the most popular in use for residential construction in Canada. We will help you identify the characteristics of those coverings and offer some expectations about their service lives.
By far, the most prevalent type of roof covering in North America is asphalt shingles (sometimes called roof tiles). There has been a relatively recent shift in how these shingles are made. For decades roof shingles consisted of a wool felt mat impregnated with tar (asphalt) and then covered with granular stone. These are now called organic shingles. The granular stone finish on the top side of the shingles is present to protect the asphalt in the shingles from the sun's UV rays; those UV rays break down the asphalt over time, leading to cracking and greater fragility. As these shingles age and are exposed to the weather, they will lose some of the granular stone cover and begin to curl and claw, eventually cracking and becoming brittle and progressively vulnerable to wind damage. There are lots of variables at play, but the average service life of this type of shingle is 15-20 years. Minor wind damage can be easily repaired, but greater and more frequent damage can lead to water leaks and significant interior damage if the roof is not re-shingled. A qualified CAHPI home inspector can reliably establish the condition of your roof covering.
Most asphalt shingles today use a non-organic fiberglass mat that is less vulnerable to mechanical damage, and the stone grit used to finish the top of the shingle often includes zinc granules which inhibit the growth of moss and lichens. These more modern asphalt shingles, while their performance history is still unfolding, appear to have a service life in the 25 to 30-year range. This is the best version of asphalt roof shingles.
Wooden roof shingles are usually fashioned from cedar and pre-date the use of asphalt shingles; some people still like the appearance of them.However, in climates with multiple freeze thaw cycles over a typical winter, they do not hold up well.In the right climate, they are very durable. These days, they would prove to be an expensive option.
There is a growing trend toward galvanized metal roof coverings; both as shingles and as sheet metal.Once cost prohibitive for most homeowners, metal has become more popular, with improved availability and more affordable pricing.It has a long-life cycle (30 years +) and comes in a variety of styles and colours.They are usually smooth and slippery, so walking on them can be challenging.Also, depending on the roof pitch, snow and ice once built up, can slide off; sometimes causing injury or property damage.Ice stops should be installed to minimize this possibility.Installation by a qualified contractor is highly recommended.
In Canada, where we can have significant snow and ice accumulation, flat roofs on residential homes are less common.Until the last half of the 20th century, flat roofs were most frequently installed with a built-up tar and gravel covering.These roofs required frequent maintenance and were more prone to leaks.When we now build a flat roof, we typically cover it with a modified bitumen roll roofing material.This material is very durable, and the seams are torch sealed to guard against leaks.Properly installed, they can have a service life of 30 years with little or no maintenance.Rubber membrane coverings can be used as well but are less common.
As a new technology, solar voltaic shingles are now a viable choice. They convert the sun's energy into electricity which can be used to power appliances in your home. At present, they are very expensive, but we can expect that to change as demand pushes production efficiencies.
Other possible covering materials are fiberglass, clay tiles, thatch, and sod. All are in limited use in Canada.
Most modern roofing materials require little maintenance, depending on some variables.Shaded roof coverings will have a greater predisposition to grow moss, algae, and lichens. Over time, these growths can damage the asphalt and shorten the service life of your roof covering. This can be minimized by keeping tree growth away from your roof.If that is difficult, the installation of zinc flashing at the ridge cap can help.During rainfalls, the zinc will wash down the roof covering and discourage organic growth.
Birds love to perch on roofs and will leave droppings that encourage organic growths. There are devices that will discourage bird presence if that becomes an issue.
Many roof coverings will experience occasional wind damage, especially as they begin to age.Ideally, some of the original lot of shingles can be stored away for repairs. Remember though, working on roofs can present a dangerous fall hazard. Repairs should be completed by a properly equipped and experienced roofing contractor.
Entire books have been written about roof design, installation, and maintenance. Any extensive discussion of roofs also addresses attic conditions. That is to be the subject for another day. There are many aspects of roof installation that are not covered in this article.Ice and water shield, flashings, underlay, etc .It is essential that roofs are designed and installed by professionals to ensure safe and consistent performance; so, you can be grateful for the roof over your head!
Author: Brian Hutchinson, RHI-Retired