A Roof Over Your Head

Understanding your roof and what it needs could save you money and headaches.

The Big Umbrella

Every home has one but few people understand how it works and what it needs. It keeps you cool in the summer, warm in the winter, and separates the hostile elements from the comfort inside. It is one of the most expensive systems to replace in a home, yet you can add years to its life by caring for it. You can also save on expensive repairs by knowing when it needs to be replaced. This wonder umbrella is your roof.

As an inspector, I find that roofs are the most problematic home system, mainly because most roof systems are incorrectly installed, and partly because many homeowners don't take care of them. Too many roofing contractors cut corners, use inferior materials, and too many of them either don't know what they're doing or just don't care. Homeowners don't think of caring for their roofs, and why should they? Roofs don't come with owner manuals, or beep when maintenance is required. The homeowner must know this and take the time to keep the roof well-maintained. Sadly, time is at a premium these days for most of us. Many homeowners only realize that their roofs are in distress when it's too late.

Anatomy of a Roof


All roof systems have common parts. They may look different, depending on the design and type, but these parts all perform basically the same function. The fundamental element of any roof is the structure. It is like your body's skeletal system. It provides shape and strength to your roof system. These days, most roof structures are engineered trusses, built in a factory and assembled on-site. Residential roof structures are almost always wood, which is a great building material, but it has its own problems.

Wood can get wet as long as it drys out quickly. However, when wood gets wet and stays wet, that's when decay starts occurring. If your roof is leaking, the ventilation in the attic is not working, or a duct is discharging into the attic, your wood could stay damp and start deteriorating. Eventually, your roof will start sagging from decaying timbers and eventually will fail. In the meantime, your attic could be a fantastic breeding ground for mold.

Roof Deck (Sheathing)

The roof deck consists of sheets of plywood or (today) OSB (oriented strand board) fastened to the rafters. The sheathing provides a surface to attach the shingles and underlayment to, as well as a solid surface for roofing specialists to traverse. Roof decking should be at least 1/2” thick. If the roof surface feels springy all over, the sheathing is probably too thin.

The preferred material for sheathing is 1/2” or (better) 5/8” exterior plywood. Plywood, while more expensive, is more durable in damp environments. When plywood gets wet consistently, it will tend to de-laminate, still retaining around 75-80% of its strength. OSB on the other hand, tends to disintegrate when it is continually wet. This is because the binder softens and no longer holds the strands together. Your foot could go through a deteriorated patch of OSB.

If the roof cover (shingles) is a wood shake or shingle type, the sheathing will be replaced by wood battens running along the length of the roof. This is to allow air to keep the shingles dry from the underside, preventing decay from underneath.


The underlayment is most often black, felt paper, which is stapled directly to the sheathing. It acts as a moisture barrier between the shingles and the sheathing, helping to keep the sheathing dry. The preferred underlayment is a good quality 30 lb. type, although I have found 15 lb. and inferior grade 30 lb. on roof installations. ASTM 30 is the thickest type of felt paper underlayment and will provide superior protection, especially in areas subject to ice and snow.

Other synthetic materials are used for roof sheathing, but the most common type in this area is felt paper. It is air-permeable and water-resistant, allowing your roof system to “breathe” and stay dry.

The Roof Cover

A roof cover generically, is any material fastened to a roof surface to deflect water. This could be composition, wood, concrete composite, or tile shingles; steel panels; roll roofing; membrane roofing; as well as other types. The most common are composite shingles, with laminate shingles being the most common type on new homes today.

Every cover type has a “by the book” life expectancy. This figure will vary according to how the roof is cared for, the quality of the cover, the quality of installation, wind, temperature, moisture, and many other factors. While you cannot control the weather, you can control the shingle quality on a roof cover replacement, the workmanship quality on the installation, and how that roof is cared for.

Penetrations and Flashing

A roof penetration is anything that passes through a roof surface. These are usually chimneys and flues, skylights, plumbing vents, electrical service masts, exhaust vents, roof vents, safety anchors, satellite antennas, and more. Any element that passes through a roof breaks the shingle structure and creates a potential point for leaks. In fact, the average roof is full of holes because of these penetrations.

Water could get into the attic through two entry points at a penetration: the hole itself and the junction between the “pipe” and the roof. Water is kept from going down the hole by a rain cap. Rain caps are covers to keep the rain out of the holes in the roof. The one penetration where it doesn't matter if water gets down the hole, is a plumbing vent. These vents allow air to enter the plumbing drain system, preventing siphoning of P-traps. Because they connect to the plumbing drain system any water entering the plumbing vent just enters the drain. . .no harm done.

The second path into the attic is blocked by flashing. Flashing is any transitional component or system to make the junction between the penetration and roof cover water-resistant. Flashing will most often be metal, but can be neoprene or a combination neoprene and metal piece. Sometimes the flashing and rain cap are one piece, such as with roof and exhaust vent covers. The correct flashing installation is to have the uphill shingles over the flashing, and the flashing over the downhill shingles with no nails exposed on the flashing: a common mistake.

Properly installed flashing will ensure a water-resistant junction of the penetration and the roof. Improperly installed flashing will be problematic and will eventually leak.


Skylights present additional challenges, since the area above the roof is closed and there is a seam between the skylight pane and the frame. Additionally, the heat at the top of a skylight can often create condensation on the pane interior and the shaft walls. The shaft walls must be insulated in the attic to prevent cold air from allowing condensation to form.

The seam between the pane and the frame usually incorporates a gasket to prevents leaks in this area. This gasket can eventually fail and must be replaced. Many homeowners and roofers just caulk this seam, which works for a few years but the caulk will fail and the seam will leak again. Remember that caulk is a temporary fix only.

Skylight flashing is much like chimney flashing in design. If done properly, it will perform reliably for many years.


Ventilation is probably one of the most important factors in the health of a roof and the home's occupants. An attic is typically an unconditioned space. This means that the attic temperature will change with the outside air. An attic however, should not be a closed space. Air must regularly be replaced in the attic to prevent moisture buildup with resulting mold growth and decay. This is the job of attic vents.

There are two sets of vents for an attic: intake and exhaust. The intake vents will always be at the eaves or soffits and the exhaust vents will always be near the ridge of the roof. This is because hot air rises. Cooler air enters the soffit or eaves vents and exits the upper ridge, roof, or gable vents. This arrangement provides continuous air circulation in the attic through natural convection.

The most common problems I find with attic ventilation are soffit/eaves vents blocked with insulation and clogged ridge vents. Baffles are installed over eaves vents to keep the insulation from cutting off intake airflow. The problem is that the baffles are often flimsy and collapse when insulation is blown in and pushes against them. This drastically reduces intake air and prevents effective attic ventilation.

The other problem is with ridge vents. Often, a filter material is used to keep bugs and debris from entering the attic. Unfortunately, this filter clogs over time, cutting off exhaust air, and reducing effective attic ventilation. Another problem I find is that the channel in the sheathing is cut too narrow to allow enough air to escape for adequate ventilation.

In fact, ridge exhaust vents are generally the most problematic and should be avoided. Believe it or not, the best exhaust system is gable vents, followed by distributed roof vents. Gable vents create more of a wind tunnel air flow, allowing better scavenging of attic air. Additionally, they are not as susceptible to leaks as roof vents are.

Where the Leaks Are

Usually, roofs will start leaking in predictable areas: first, around skylights and chimneys, around roof discontinuities such as valleys and roof-wall junctions, then finally around roof penetrations through rain caps and flashings. The times when leaks will occur outside these areas are when the shingles have failed or are damaged, or roofing nails are exposed.

Many homeowners believe that if water is not coming through the ceiling, the roof is not leaking. This is not necessarily true. A roof could be leaking for a year or more before it finally shows on the ceiling. Before a leak shows on the ceiling, it must pass through roughly 8-12” inches of loose fill insulation, which is absorbent, and will dry out with dry weather and good attic ventilation.

Leaks are best detected in the attic, by looking for telltale water stains around roof penetrations, roof discontinuities such as valleys, and roofing nail shanks. This is not something that the homeowner should attempt. The attic is a dangerous area. One slip and your foot could go through the ceiling below, as well as injuring yourself if you fall. Leave traversing the attic to the professionals, such as roofing contractors and home inspectors.

It is a good idea to schedule a roof condition inspection every 3 to 5 years by an independent home inspector. Even if your roof cover is new, it is still a good idea to schedule an inspection. I have seen so much new work done incorrectly and leaking withing a couple months of installation. In fact it is wise to schedule a work-in-progress inspection when having a new roof installed. This can be done by a licensed home inspector or by a city inspector if you take out a permit for roof installation. This way, workmanship errors and poor material quality can be caught before you write the final check. If your roofing contractor voices strong objections to having his work independently inspected, send him packing. You probably will not get your money's worth from him.

In the Gutter

Another important but often overlooked part of the roof system are the gutters and downspouts. The gutters are designed to channel water to the downspouts where it can be discharged either onto the ground away from the foundation, or into an underground (weeping tile) drain system.

The most common problems are clogged gutters and downspouts. If water can't get through the downspout, it will simply overflow over the edge of the gutter, and possibly cause moisture intrusion into your basement or crawlspace.

This is why regular gutter cleaning is an important part of roof maintenance. The gutters and downspouts should be cleaned on a regular basis. Conical (not flat) downspout screens will help prevent downspout clogging.

The “Care and Feeding” of your Roof System

So, what can you as a homeowner do to prolong the life of your roof? The short answer is: keep it clean. At least once each year, the roof, along with the gutters, should be cleaned of moss and debris. This is not something you should do yourself, unless you are safety-aware, and have a good sense of balance. The roof is a dangerous place, better left to the professionals.

The act of cleaning a roof can itself, cause damage if done incorrectly. Hard-soled shoes or shoes with spikes should never be worn on a composite shingle roof. The spikes will damage the shingles and create areas for potential leaks. Another no-no is the use of a pressure washer to clean the roof. The force of the water will dislodge granules, making the shingles deteriorate faster. The preferred method is a soft-bristled broom and a leaf blower.

Also check the flashings from the ground. Are any of them rusted, bent, or otherwise look funny? If so, they may need repair or replacement.

At the same time, make sure your attic ventilation is functioning adequately. This can be done (carefully) by the homeowner by placing a ladder into the attic entry hatch. The minute you lift the attic entry hatch, you should feel air movement past your head. This tells you that your attic exhaust ventilation is working properly. Climb the ladder so your head is inside the attic. What do you smell? Does the attic smell musty and stuffy? If so, your ventilation may not be adequate. Finally, turn off all lights in the attic. Do you see daylight around the eaves? If so, your eaves/soffit vents are probably clear. Shine a flashlight around the eaves and look at the baffles. Are they straight or collapsed? If anything doesn't look right, feel right, or smell right in your attic, it is best to have it investigated by a reputable, licensed roofing contractor and corrected.

Wrapping It All Up

Your roof system is more complex and far-reaching in its effects than you might have expected. There's a lot going on over your head that you may not give a second thought to. Your roof system is also more delicate than you may have expected. It is tough and rugged when deflecting the elements, but “wimps out” under the grinding of spiked shoes.

Still, it is a wonderful system made up of simple parts that, when put together correctly, keep you warm and dry and your house safe from water damage for many years. Take care of it, and it will take care of you.

Tags:  roof  shingles  structure  attic  ventilation  sheathing  care  cleaning

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