Roofs Explained

Author’s Note: I am better at explaining building concepts verbally to clients during a home inspection than I am at writing about them (or at writing in general). I also feel that construction concepts can be learned by anybody and understanding them will help homebuyers make more informed decisions when buying their homes. The hope is that these articles will culminate into a book that is approachable and has valuable information to the consumer. I am also publishing these articles on my blog for feedback on how this article can be better. Please comment on my grammar or how I write. You are my editor.

The Conceptual Roof

Let us simplify the concept of roofing. They come in all different types, shapes, and sizes but no matter what kind of roof a building wears on its head, they all work in one of two ways depending on the slope of the roof. Before we explain slope, just remember this concept.

The more the roof slopes, the more like it works like series of umbrellas. The more the roof is flat, the more it works like a pond liner.

If a roof is sloped or has a pitch, it can use shingles. Shingles are like a bunch of umbrellas. They take a raindrop and let it drip to the next shingle until it eventually falls off the edge of the roof. If the roof is flat or has a low slope, the roof material needs to be more waterproof. The low slope roof should be able to hold pooling water as if it was covered by something as waterproof as a pond liner. The occupant becomes the bunny sleeping dry and cozy in her hutch underneath the pond. The amount of slope is determined by the Rise and Run.

Rise and Run

Pretend one is standing on top and at the edge of the roof (not recommended). That person begins to walk toward the ridge of the roof (the point at the top of the slope). If that person traveled 12 feet across and found herself 4 feet higher in the sky, she was walking on a 4/12 roof.

What is about to be said here may push your math teacher over the edge, but don’t reduce the fraction. We keep the denominator at twelve to keep things simple.

Umbrellas and Pond Liners

There are so many different types of roofs. Now that asphalt prices are skyrocketing (or going through the roof, get it!) several other types of roofs are appearing on the residential market like vinyl or PVC roofing. But we are going to keep it simple by discussing three very common examples of umbrellas and pond liners. First we will explain composition shingles as our umbrella, and second we will explain built-up asphalt gravel surface roofs as our pond liner. We will also discuss modified bitumen as another pond liner.

The anatomy of the modern composition shingle is a section of fiberglass fabric, impregnated with asphalt, and coated with mineral. The mineral is what makes the roof shingle surface feel like course sand paper. The mineral coating is like a strong sun lotion. This keeps the sun from chemically breaking down the asphalt material.

The roof surface must be prepared before installing these shingles. First there must be a flat surface. Plywood is sometimes installed on the roof if the original surface is not flat enough. Multiple layers of composition shingle is allowed by most building jurisdictions. This is not recommended because the composition shingles perform best on the flattest surface. Next, building paper is installed on this surface, and then the shingles.

The pond liner we will discuss is a built-up asphalt gravel surface roof (BUR). This roof is commonly referred to as a tar and gravel roof. This term is technically incorrect because these are built using asphalt, which is derivative of oil where tar is a derivative of coal.

A BUR roof is much like building one giant composition shingle over the entire building. First, there is a surface from which to start. Then building paper is applied and then asphalt is slathered on top of the building paper. This is done about three times (three layers or more) until it is time to apply the gravel. Just like the mineral on the composition shingle, the gravel is there to protect the asphalt from sun damage. Other sun blocks besides gravel are used such as roll composition, which is a mineral coated asphalt sheet applied over the built-up roof (referred to as a “glass cap”), or special paint designed to protect against sun damage.

There is one more pond liner worth mentioning because is it frequently uses as an alternative to a BUR. This material is modified bitumen. It is a roofing material that comes in a roll. The material is about 40 inches wide, rolled over the roof surface, and the seams are either torched together or bonded with a listed adhesive. That’s right torched. A gas torch softens the edges of this material and melts the seams together. Because of this method, this roof is often referred to as a “Torch Down” roof. Several jurisdictions are no longer allowing the torch method as it poses a fire hazard. The surface either is black (and should be painted), painted, or has mineral embedded into the surface very much like a composition shingles.

Flashing

Oh so vital! Flashing is how parts of the roof that are vulnerable to leaks are sealed or protected. Often, brand new roofs are installed but still leak because the flashing is not sufficient or correctly installed. Vulnerable areas include where things poke through the roof surface like chimneys, plumbing vents, gas appliance vents, roof edges, and changes in plane like roof-to-wall joints, roof valleys.

The discussion of flashing is focused on the shingled or umbrella type roof. This is because the flashing on a built-up roof are not generally seen. BUR flashing is integrated into the asphalt material.

We will over simplify roof flashing into two types, gooey flashing that comes from a can or caulking tube like mastic, and flashing fabricated from a solid and flat material like sheet metal. Mastic or other gooey flashings tend to be asphalt based. These are often used on repairs or and sometimes on new roof applications. Often, folks would rather send their children to college then to pay for better solid flashing. This stuff dries out every few years and needs to be re-applied. Sheet metal flashing is more reliable. Think of a piece of sheet metal flashing as a specially designed shingle (or umbrella) that both fits around a pipe protrusion and is properly placed within the shingles (or other umbrellas) to let the rain drop drip (or flash) over the other shingles.

Flashing comes in different sizes and shapes depending on what it is protecting. For example, flashing at a roof-to-wall joint or a skylight both utilize step flashing. Step flashing is just sheets of metal bent at a right angle and slipped between the counter flashing of the skylight or wall and also slipped under the shingles in a way that rain will flash off.

Edge flashing is just that. It consists of long pieces of sheet metal bent at an angle placed at the edge of the roof so rainwater does not damage the edges of the roof surface. On the drip edge of the roof this is placed under the building paper like an umbrella placed beneath another umbrella.

Roof Issues

Roofs leak and when they do, it’s an issue. The three main causes for roof leaks are; deteriorated roof material due to age, insufficient flashing, and improper installation. The latter is discussed because many are surprised when their new roofs leak. In most cases, the aforementioned roof types last approximately 20 years. Different types of shingle weights and products do last longer but 20 years is a rule of thumb.

Age

Composition roof deterioration is visible in a variety of ways. In some cases it is obvious. The shingles are crumbly (their asphalt substrate has dried out), they are cracked, or simply falling off. A more subtle form of deterioration is mineral wear. The sand like coating washes off, exposing the asphalt material. The sun does the rest of the damage until the roof needs replacement. Worn mineral means the roof is nearing the end of its life. Worn mineral and exposed fiberglass fibers mean it should be getting its affairs in order.

Built-up roofs, whether they are gravel surface, painted surface, or protected by a glass cap eventually wear. Deterioration is apparent at black spots where the sun protection has worn off (or maybe wasn’t originally applied) and the sun chemically broke down the asphalt-based material. Wear is also apparent when the roof surface has several cracks on the surface resembling alligator skin. This is where the term alligatoring comes from. These roofs are also nearing the end of their lives and it is up to the roofer to decide if they are willing to make repairs or only willing to replace the roof. Many roofers, understandably, do not want to be the last person on record to work on a roof that may leak. Roofers have been successfully sued when roofs leak, even if the leak occurred far from the area they repaired. Don’t let anybody say a roof only needs maintenance unless those words passed through a willing roofer’s lips.

Insufficient Flashing

Insufficient flashing can also fall into the category of “Improper Installation”. This topic gets its own section because it is a common cause of roof leaks. Improper flashing is similar to an umbrella being out of place. For example, a flashing component is placed over a plumbing vent but the up slope portion of the flashing is not tucked beneath a shingle. This is like having one of the umbrellas out of place and water flashed beneath this component and into the building. This incorrect application often occurs when new plumbing or a gas appliance is installed requiring a new vent. The installer will often incorrectly flash this vent.

Using mastic instead of solid flashing can result in leaks. This “flashing in a can” dries out every few years. The material cracks and each crack is an avenue for water to intrude into the building.

Missing counter flashing is another cause of leaks. Suppose a roof connects to an exterior wall. There is flashing under the shingles and this flashing sticks out and is placed against the siding. If that flashing is not tuck under the bottom of siding or a another piece of flashing that is seal against siding, then rain will drip down the exterior wall and seep behind the shingle’s flashing and seep into the building. This is like having a missing umbrella.

Edge flashing is often missing. This is a bent piece of sheet metal placed underneath the shingles along the edge of the roof. This flashing is necessary to prevent rain water from damaging the eaves through capillary action (capillary action is that stubborn rain water that doesn’t drip off the last umbrella (shingle) but works its way beneath it). Sometimes the edge flashing is installed incorrectly. The edge flashing at the drip edge of the roof belongs under the building paper. The edge flashing at the rake of the roof belongs over the building paper but under the shingles. Often the rake flashing is installed over the shingles, which is incorrect and drives rain water into the roof fascia causing decay.

Improper Installations

Remember the first thing to keep in mind?

The more the roof slopes, the more like it works like umbrellas. The more the roof is flat, the more it works like a pond liner.

Improper installation often is a condition that misses this fundamental idea. A common example is a shingled roof installed on a low-sloped surface. An umbrella is installed where a pond liner is needed.

A low-sloped shingled roof sometimes works for a while until something happens that causes water to pond on the roof. Leaves and other debris collect on roofs. On a low-sloped shingle system, a debris dam causes rain water to pool on the roof and this water seeps beneath these umbrellas.

Roof deck terrain also need to be considered when installing a pond liner like built-up roofing. If the roof surface has sags, which is not unusual on an older building, drainage considerations need to be made. Either a drainage scupper needs to be installed in the deep area or the roofing deck needs repair. Even though built-up roofs are waterproof, they are not designed to hold water for extended periods of time. On roofs with parapet walls (the exterior walls are higher than the roof surface) a secondary scupper should be installed incase the main drainage scupper is clogged. We don’t want water stored on the roof because it is very heavy and can cause structural damage.

In Conclusion

A qualified professional should inspect the roof for defects that are subtle but consequential. You don’t have to be a roofer to spot obvious defects. If you have a pair of binoculars, pitched roofs can quickly reveal defects. Even if the roof appears satisfactory or even new, get an inspection. Improper flashing details, incorrect use of materials, and incomplete work all lead to leaks in what appear to be pristine new roofs.

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Tags: Roofs