An inspector should never make a determination on how long a roof will last, since there really is no way to tell. I am aware of one roof which was meant to last for twenty years, that is still going strong after almost fifty years. I also know of a roof that is five years old, which now needs to be replaced. Why is there a difference? Both roofs were made of similar materials, but the material behaved differently and was treated differently once they were installed.
In Houston heat and humidity cause a good deal of damage to our exteriors, but storms during hurricane season leave their toll as well. Roofs with steeper pitches have a tendency to do better here because of their ability to shed water faster. To help control the effects of the heat and humidity, you will want to have a system of ventilation in place for your attic. The most common type of ventilation is having vents in the soffits with a vent in the ridge. This allows air to flow through the underside of the roof (attic), cooling it while moving the humidity out. The magic formula is one square foot of ventilation for every one hundred and fifty square feet of attic space. However, you will want the vents to be spread around the building, instead of located in one area. For example, my son’s room had no soffit vents to the attic around it. The builder thought that they would look bad on the front of the house. The roof above his room received a good deal of afternoon sun, so his room was quite hot at this time of day. By adding the vents, I helped reduce the temperature in his room. Powered attic vents can be efficient, but the trade off is the cost in electricity.
Slate shingles and clay tiles experience the same problems in Houston. Slate and clay tiles are brittle, so you will see broken pieces, which leads to water getting into the house. These tiles break easily during hail storms or hurricanes, and when someone is walking on them to work on the roof. These broken pieces can be replaced, but new pieces will not have weathered like existing ones, so repairs are obvious. Some breaks may not be bad, but eventually any break can lead to more serious damage.
Metal roofs are making a comeback, and they can be long lasting. With new installation methods, they can be a great option. The concern will be the movement in the roof. Metal expands and contracts more in the heat and cooler temperatures than other roofing materials. This movement can cause problems to the seam and the fasteners holding the roof in place. Some new fastening technology prevents this from occurring. If the roof feels like it is moving when you walk on it, you have a problem with the roof. Look for moisture stains in the attic and the ceiling to see if there is a leak. Checking for moisture stains in these areas applies to all roof types though.
The most common type of roof is the asphalt shingle. The granules on this shingle are meant to protect the roof from the sun’s rays, which causes the asphalt to break down. You will be able to see this if the shingles look worn down (no granules), or if you are seeing the granules in the gutter. Blistering of the shingles happens when there is too much moisture. These blisters eventually cause the shingle to break. When you are able to see fasteners going through the shingle, you will have problems. Installing nails and screws through the main surface of the shingle is an issue for any type of roofing material, but it seems to happen more frequently with asphalt shingles. A temporary remedy for these fasteners is to place an appropriate roof caulk over the head of the fastener. If the shingle feel loose, a failure is starting to occur.
Built-up roofs are used on flat roofs or roofs with low slopes. They are made from layers of asphalt and roofing felt with pebbles on top. It experiences much of the same problems that you will see with asphalt shingles: blistering and the wearing down of the asphalt from the sun. These roofs last for less time than the others.
Wood shakes and shingles are not as common any more, but many people do like the look. Cracks and broken shingles represent a problem. Also loose shingles can occur.
For all roofs you will want to look for branches of trees or large shrubs hitting the roof. This causes a path for insects into the roof system. Branches moving during a storm can break the roofing material. Shade from the leaves allows moisture to stay on the roof, promoting algae growth and material breakdown. I have seen one limb which removed the roofing material down to the rafters. You will also want to look for debris on the roof for these same reasons.
Currently innovative materials are coming into the roofing industry, which hold the promise of longer life. There are now composite materials imitating the look of slate and clay tiles, but they do not have the same issues with brittleness. Improvements are being made to the asphalt shingle and to flat roofing systems. I have even recently read of a plastic roof sheets on a vacation cottage. However, the same rules apply when examining them: look for breaks, cracks, loose pieces, blistering, fasteners showing through, debris, and excessive shade leading to moisture. Then make sure that trees or large shrubs have their branches clear from the building.