Underlayment on Roofs. What You Should Know
When a roofer first walks onto a job, unless he’s tearing off an old roof-covering material, he’s faced with a bare roof deck. The first component to be installed on the roof is underlayment.
Underlayments are manufactured with different properties designed to meet the needs of homes in different climate zones. An underlayment that works well under metal roofing in a hot, humid place like New Orleans, Louisiana, may not work well beneath wood shakes in a cold, dry climate like Jackson, Wyoming.
The different types of roof-covering materials may also have specific underlayment requirements.
Confirming that the proper type of underlayment was used on your roof if you see problems with the roof, and understanding the basic properties and general installation requirements of underlayment may give you a clue as to the source of the problem.
Although underlayment is typically required in new construction by building codes, in the past, roof-covering material manufacturers haven’t always required it on slopes of 4:12 and steeper.
PURPOSES of UNDERLAYMENT
Most roof-covering materials are not waterproof, but water-resistant, and are designed to be installed over a waterproof or water-resistant membrane of some type. “Underlayment” is the general term used to describe these membranes.
Even though the underlayment is the first material to be installed on the roof deck, the roof-covering material — the shingles, tiles, metal or slate — is the primary barrier against roof leakage. Underlayment is a secondary barrier.
Water-resistant underlayment may allow the passage of moisture vapor, but will prevent the passage of water in its liquid form. Waterproof underlayment will prevent the passage of both liquid water and water vapor.
Waterproof underlayment is typically used on parts of the roof that are more likely to leak or suffer moisture intrusion. This includes penetrations in areas where roof-covering materials change or end, and low-slope sections of roof. It’s not unusual to use combinations of underlayment on a home’s roof.
The permeability of underlayment is the extent to which it allows the passage of water vapor. Although all underlayments are designed to prevent the passage of moisture in its liquid form, they can have different levels of resistance to the passage of water vapor.
Underlayment permeability ratings are provided by the manufacturers, and are less important in roof underlayment than they are in housewrap. Underlayments with a perm rating of 1 or less are moisture barriers. Underlayments rated above 1 are moisture retarders.
Temporary Protection From Underlayment
Underlayment provides temporary protection of the building interior and the roof deck before the roof-covering material is installed.
Ideally, the roof-covering material would be installed as soon as possible, but in the real world, the roof may be protected by only the underlayment for days, weeks, or sometimes months.
Protecting the building interior is especially important when an old roof-covering material is being replaced and the home interior is finished. During that time, the underlayment may be under attack from weather elements such high winds, UV radiation, and precipitation. It also needs to resist the wear and tear that occurs when the roof-covering material is being installed.
Preventing Chemical Degradation From Underlayment
Underlayment also provides a layer of separation between the roof sheathing and the roof-covering material.
Newer homes use plywood or an engineered panel called oriented strand board (OSB) for roof sheathing
For many years, pine and fir boards were used as sheathing, and many older homes still have these boards in place. Resin pockets in these boards can react chemically with some roof-covering materials, such as asphalt shingles. In these situations, missing underlayment can cause accelerated deterioration and premature failure of the roof-covering material.
Fire Resistance of Underlayment
Underlayment materials are available for wood roofs which increase their resistance to fire. In fact, without special underlayment, wood shakes and shingles cannot achieve a Class A fire rating, which is the highest available.
FACTORS AFFECTING UNDERLAYMENT
A number of factors can affect the performance of underlayment and determine which types are appropriate:
For purposes of determining optimum roofing material, depending on location, climates in North America can be separated into two basic categories:
- hot or cold dry climates and;
- hot or cold humid climates.
Hot and dry climates will affect bituminous underlayment by accelerating the loss of volatile’s.
In humid climates, older felt underlayment will absorb more moisture which, in turn, can be absorbed by the substrate, causing it to expand. In cold climates, underlayment will become brittle and more easily damaged by footfall and impact.
Each of these climate types should have underlayment installed which has performance characteristics compatible with that particular climate.
Some designs shed runoff quickly. Some have design features which may actually trap runoff and expose the underlayment to more moisture.
Manufacturers produce underlayment of different types for use with the different types of roof-covering materials. The use of underlayments that are not compatible with the roof-covering material with which they’re installed can cause problems.
Roof-covering materials in poor condition which expose underlayment to weather, especially to UV radiation from sunlight, can accelerate deterioration.
Although underlayment is typically required in new construction by building codes, in the past, some manufacturers have not required it on roofs of 4:12 and steeper. Unless you know for certain that the roof-covering material on the home you’re inspecting required underlayment, you should refrain from calling missing underlayment a defective installation.
Determining whether underlayment was required means finding the manufacturer’s installation instructions for that particular roof-covering material, and also finding out what jurisdictional requirements were in place at the time the home was built.
UNDERLAYMENT INSTALLATION METHODS
Installation methods vary with the pitch of the roof, with the requirements of both the underlayment and roof-covering material manufacturers, and with jurisdictional requirements.
At headlaps and sidelaps, all underlayments should extend up the wall for at least several inches.
Unless the underlayment is self-adhering, it’s attached to the roof with fasteners, which are a disadvantage because they make holes in the underlayment.
One of two fasteners is usually used. Staples are the most common, but in high-wind areas and with synthetics, underlayment is often fastened with plastic caps.
“Plastic caps” is the industry term for nails that come with plastic gaskets attached. They are typically used for conditions where wind damage to underlayment is a possibility. They also help seal against moisture intrusion.
In high-wind areas, it’s not unusual for roof-covering materials to be blown off while the underlayment remains in place. In these situations, the remaining underlayment can make a big difference in limiting interior water damage. Underlayment fastened with plastic caps instead of staples is a lot more likely to remain in place.
In areas where there is a history of ice forming along the eaves, causing melt-water to back up under the shingles (or ice dams), the ice barrier underlayment should be installed at the roof edge. An ice barrier is typically a self-sealing, self-adhering waterproof underlayment.
The International Residential Code (IRC) is the residential building code most widely adopted in the U.S. According to the IRC, the ice barrier should extend from the lower roof edge to a point at least 24 inches in from the outside of the exterior wall, measured level.
All other roofing industry organizations specify that the 24 inches be measured from the inside of the exterior wall.
On roofs with steep pitches, this may require up to four courses of underlayment. Depending on the roof-covering material and the installation method, you may not be able to confirm proper ice barrier installation. Ice barrier requirements are the same no matter what roof-covering material is installed.
FYI, roofing underlayment is an essential component to the roofing materials’ ability to withstand the elements, protect a home’s interior, and prolong its service life. The more we understand about a roof’s components, the better we can spot problems and deficiencies.
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Tag: asphalt shingles
, covering material
, covering materials
, ice dams
, metal roofing
, moisture barrier
, moisture intrusion
, roof deck
, roof leakage
, water resistant
, wind damage