October, 2013
Feature
Inspection News and Views from the American Society of Home Inspectors



Wood Roofing

FROM ASHI@HOME BY CARSON DUNLOP

Types of Wood Used

WOOD ROOFS WERE MOST COMMON shedding roof system used prior to the advent of asphalt shingles. In some areas of North America, wood shingles are still among the most common roofing materials. Several types of wood have been used. Most of the roofs we’ll come across are cedar. Cedar is a relatively strong wood given its light weight, with a relatively low rate of expansion and contraction. Cedar contains natural resins that resist rot, distinguishing cedar from many other softwoods.

Shingles and Shakes

Wood shingles are typically sawn smooth and wood shakes are typically rougher in texture and thicker. Shakes may be split, sawn or both. Common flashing materials include galvanized or stainless steel, aluminum, copper or lead.

Maximum Number of Layers Recommended

There should only be one layer of wood roofing. While it’s unwise to put a second layer of wood shingles over a first layer, it is common to find asphalt shingles laid over wood. This is rare, however, with shakes because they are too uneven to have asphalt shingles installed over top. In general, this practice is not recommended. You should advise your client of potential long-term problems if you see asphalt shingles installed over wood shingles or shakes.

Life Expectancy

Life expectancy is approximately 20 to 40 years. Several factors affect the life expectancy, including climate, roof pitch, roof exposure, overhanging trees, sheathing and underlayment (which affect the ability of the wood to dry quickly), quality of shingles, exposure, installation technique, maintenance, etc. While it is not common practice to add preservatives to wood roof shingles and shakes, doing so about every five years will prolong their life, particularly if the preservative includes fungicides. Pressure-treated cedar and pine shingles are available.

Common Failure Modes

Curling, cupping, splitting, rotting, wear-through and burn-through.
Failure Causes
Failure is caused by:
1. rot
2. cupping, curling or splitting
3. burn-through or wear-through

The material quality and installation technique have a bearing on how quickly failure in any of these modes occurs. The amount of water the roof sees and its ability to dry quickly also affect how quickly failure will occur.

Water Sources
Water may get into a wood roof system by:
1. driving through the roofing materials
2. leaks through flashings or joints
3. being drawn up through capillary action
4. accumulate as a result of condensation
5. swings in relative humidity as discussed earlier

Wicking
The more water that wood fibers soak up, the more likely the wood will rot. Think of wood as a bundle of straws. It is the end of the straws that soak up the water. Shingles have more exposed straws than split shakes because shakes are made by pulling apart the bundles of straws; shingles are made by sawing through these straws. Wicking is more important on shingles than shakes but occurs with both. Moisture drawn up between shingles or shakes may be absorbed by the wood or deposited on the sheathing. In some cases, there is building paper to catch and retain this moisture.

Fast Drying Is Important
For wood roofing to last a long time, it must be able to dry quickly after getting wet. This is the reason that open or skip sheathing is popular in many areas. The wood is able to dry from the front and the back. Staining on the underside of the shingles is often visible from the attic on a roof with skip sheathing. This may simply be an indication that the roof is drying properly from the back. It does not necessarily indicate leakage. In many cases, the dark staining that you see on the underside of wood shingles in the attic is not really water staining but hardened resins from the wood. Next time you see them, touch them – they feel like a hardened glaze, not water staining.

Age of Wood Roof
It is difficult to pinpoint the age of wood roofs. It’s also not helpful in many cases, since roofs can last for as few as 10 years or as many as 60. It’s more important to determine whether the roof is serviceable. If you go beyond the basics, you may also give the client some idea of the remaining life. If more than 10% to 15% of the roof requires replacement, consider complete re-roofing.

Let’s first look at curling, cupping and splitting

Curling, Cupping and Splitting


These conditions are caused by:
1. uneven drying, resulting from solid sheathing or felt underlays, for example
2. shingles or shakes that are too wide (more than 8 inches)
3. excessive nailing or re-nailing of shingles that have curled or cupped to flatten them out
4. low-quality materials

Strategy

Shingles that are curling or cupping are usually visible from the ground. A close look is often required to identify split shingles that may simply look like a joint between adjacent shingles at first glance. Shingles are more prone to curling, cupping and splitting than shakes because they are thinner.

Several shingles have curled, cupped and split in various locations of this roof

Close-up of a curled shingle

Burn-Through and Wear-Through

Now let’s have a look at burn-through and wear-through. This may be the result of:
1. mechanical action from rain, snow, wind, hail, foot traffic, as well as owners using power washers
2. freeze/thaw action
3. ultraviolet light
4. low-quality materials

Rain and weather soften the wood fibers and wash away loose fibers, exposing the layer below to weathering. Freeze/thaw cycles also loosen the wood fibers. Ultraviolet light breaks down the wood fibers. The popularity of power washers in recent years has motivated owners to deal with moss growth or other debris on wood roofs with these tools. This is by far the biggest catalyst in premature aging of the roof and will unquestionably speed up the process of burn-through and wear-through.

Check the butt thickness of the shingles or shakes. You’ll need to know how thick the shingle or shake was when it was new, to be able to evaluate the wear. Flat-grain shingles have much more softwood exposed and are more susceptible to weathering. Watch particularly close for burn-thorough and wear-through on these. The wear is often near the middle of the exposed part of the shingle or shake.

IMAGE Burn-through, wear-through, splitting and general deterioration on this wood roof

Keep in mind that there is a risk in damaging wood roofs by walking on them. Newer inspectors, especially, should not try to walk on wood roofs without proper footwear. It is also worth noting that there is inherently an added risk for fire associated with wood roofs, and there are potential implications on insurance. Some areas, in fact, ban wood roofs altogether. Find out what the rules are in your area.

We have briefly introduced the topic of wood shingle and shake roofing, and have also outlined some of the conditions that are found during home inspections. More information on the implications and the strategies for inspection can be found in the ASHI@HOME training program.