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Roofing Materials and Roof Structure

In this section, you will learn what goes into your roof and the differences in the materials that may be used.  This is not designed to be a comprehensive discussion of all possible roof types, just those dealing with cedar shake or composition surfaces.

In North America, most roofs are built on a frame of heavy boards that may include beams, trusses, rafters, and various other components.   The part that will eventually hold the roof is most often referred to as the rafters; they proceed from the eve of the house to the apexes of the roof.  How rapidly these rafters rise, that is the angle, is determined by a number of factors, including common weather patterns, interior spaces, and architectural considerations.  This angle is referred to as the roof slope or pitch and is calculated by taking any point on the roof surface and comparing the number of units it is above a flat plane to the number of units from the eave.  If you are mathematically inclined, each rafter can be considered the hypotenuse of a right triangle, with the short side being the overall height of the roof from a flat plan formed by the long side.   For example, a roof that rises 4 feet for every 12 feet from the eave is said to have a slope or pitch of 4 to 12.

The roof slope is very important for two reasons.  First, certain products are not effective roofing materials if the pitch is too low.  Cedar shakes, for example, should not be used on roofs with a pitch smaller (that is flatter) than 4:12.  There is too much of an opportunity for water to be blown or wick up inside and under the roofing surface.  A completely flat roof (0:12) cannot use composition shingles either, but must generally be of the hot tar variety so that it can be completely sealed.  The other reason roof pitch is important is that it becomes a factor in the cost of roofing or re-roofing a house.  Pitches higher than 10:12 will require toe boards for the workers.  Roofs with pitches higher than 12:12 are generally applied from the side, requiring the use of scaffolding or cranes.

Attached laterally to the top of the rafters is the sheathing (also sometimes called sheeting) that will eventually form the roof surface.  A composition roof uses plywood or particle board sheathing that forms a solid surface.  Shakes use a series of 1 by 4 boards that is called skip sheathing, since it does not form a solid surface but only a place to which the shakes can be nailed.  Quality composition roofs only use CDX grade plywood or higher, with the X indicating that one side of the plywood has been treated to make it suitable for exterior applications.  Generally 1/2 inch or 9/16 inch are used.  Thicker plywood is not used because it is heavier, requiring more supports in the rafters and in the walls of the home.

Immediately above the sheathing is applied the underlayment, commonly called felt, black paper, shake liner, or tar paper.  Recent building codes now require that all felt be approved by the American Society for Testing and Materials and must carry an ASTM mark.  Previous codes, as recently as early 2004, still allowed the use of uncertified paper that is cheaper.  Felt comes in several grades, the most common being 15 lb and 30 lb.  Highest quality roofs use 30 lb felt, since this is an extremely important roof component.  Felt is tacked on using a light weight stapler.  Eventually, it will be held in place by the roof surface fasteners that also penetrate the felt paper.

In some areas, an additional water permeable membrane may be applied.  This water and ice barrier is self adhesive over its entire surface, providing a moisture and air tight barrier.  Because this material is expensive, it is usually used only in potential problems areas where ice, snow, or water might buildup.  These areas are generally up-roof of skylights, chimneys, and other holes in the roof.

Next is applied the roofing surface material.  See the section below for a discussion of shakes versus composition.

Cedar shakes come in various sizes and grades.  Most common are 24 inch shakes.  These shakes have a width that varies from 6 inches or so to 10 inches or so, but the length is always 24 inches.  Cedar shakes are applied in overlapping rows so that the nailing area of a lower shake is covered by the un-nailed part of the shake row above.  The nailing portion lines up directly over the skip sheathing.  The part of each shake that is exposed (not covered by a higher row), is called the weather exposure and the ICBO recommends that this not exceed 10 inches for a 24 inch shake.  At the edges of a shake roof, shake starter materials must be applied.  These are simply the first course of shakes, so they do not have a weather exposure, as this entire area is covered by the next row.  At the very top is another special type of shake called the ridge cap.  This special shake sits on the ridge or apex of each roof and is also used on the hips of the roof.  A roof hip is like a ridge, but is on a slope itself.  Shakes are fastened to the skip sheathing using either staples or nails.  Both must be of sufficient length to penetrate the shake, underlayment, and the sheathing.  Some shakes may be treated by fire retardant or anti-algae chemicals.  These usually require a stainless steel fastener to prevent a chemical reaction between the treatment and the fastener.  Treated shakes can last much longer than regular untreated shakes if applied and maintained properly.

Composition shingles also come in various sizes and grades.  The most common is about 3 feet by 12 inches.  Like shakes, these are nailed on in overlapping rows.  Composition shingles should only be fastened using nails, since the material can tear around the fastener head.  Also, since asphalt is more flexible, composition shingles are more prone to wind damage.  Most manufacturers recommend that a minimum of 6 nails should be applied to each shingle to provide the best win resistance, although code requires only 4.  Hot dipped galvanized nails are the best available for this purpose.  They must be of sufficient length to penetrate the sheathing beneath.  Hip and ridge materials may be fashioned by using a regular shingle; however, most architectural grade shingles have a special ridge product that is designed to enhance the overall appearance of the roof when applied to ridges and hips.  Composition shingles are now being offered with 30, 40, 50 year and even lifetime limited warranties.  Generally, the longer the warranty, the thicker the shingle.

Flashing is metal that is applied to certain areas of the roof where shingles may meet other surfaces or edges.  The new ICBO code requires 26 gauge flashing be used in all areas.  Flashing is used in valleys, where two or more roof surfaces join to shed water downhill.  It is also used along gutter edges, drip edges, and anywhere a roof surface meets a vertical wall.  Flashing is also used anytime the roof surface is penetrated, as is the case with plumbing vent pipes, chimneys, and skylights.  Lead flashing around plumbing vents is still considered the best as it will not rust or decay.  Flashing is labor intensive and therefore costly to apply.  It is a common area where some contractors attempt to cut costs, either by re-using old flashing or by leaving it out altogether.  Re-using flashing is never recommended as the old flashing will have holes that do not line up with the new fasteners.  A roof without drip edging may look OK when finished, but will show rot along the facer boards as water seeps into the wood over time.

Ventilation is another very important component to a roof system.  Contrary to popular belief, a good roof does not seal in an attic space, but allows for the free flow of air underneath the roof surface.  This prevents moisture buildup that can shorten a roofs life.  Because warm air rises, air flow is usually obtained by placing inflow vents in the soffit area (under the eaves) and outflow vents at the highest point on the roof to prevent warm air from being trapped.  These highest vents may be placed on the sides of the house, but are commonly placed on the roof structure itself.  Roof vents are simply holes in the roof that allow air to escape.  They are covered by a plastic or metal cap and are commonly seen on roofs of all types, although sometimes an attempt is made to place them on the back or where they cannot be seen from the street.  The number of vents required can be calculated by considering the roof area, volume, and the size and number of inflow vents.  When in doubt, it is much better to have too many vents rather than too few.  In recent years, a system called ridge venting has gained in popularity.  This system, applied directly beneath a ridge cap, provides one long continuous vent in the roof ridge, providing superior ventilation and eliminating the need for the unattractive roof vents.  Due to structural considerations, some house cannot use this system, but it is generally preferred where possible.

Re-Roofing

If you are replacing a shake roof with a new shake roof, the old shakes, paper, and flashing are all removed and must be disposed of legally.  This cost should not be underestimated as dumping fees will average about $500 for a typical home.  The skip sheathing is examined and repaired if necessary and all protruding fasteners are either removed or nailed down.  From here, the process is as described above for new shake roofs, starting with felt and ending with ridge caps.

If you are replacing a composition roof with a new composition roof, you must first decide if you want to remove the old roof.  Under some conditions, a new composition roof may be placed over an existing composition roof.  While this can save money, the new roof will not be as reliable as one installed over new felt.  The added weight may also overstress the underlying roof structure and sometimes requires a special license to meet building codes.  Metroplex does not recommend applying new composition over old composition.  Once the old roofing is removed, the sheathing is examined and repaired as necessary.  Again, the process is as described above.

If you are replacing a shake roof with a new composition roof, first the old shake roof is removed, but the skip sheathing remains.  New solid plywood sheathing is applied on top of the skip sheathing and the process proceeds as above for a new composition roof.

Shakes vs. Composition

Metroplex Roofing is expert in both types of products and generally does not recommend one above the other.  The following paragraphs are designed to help you understand the advantages and disadvantages of each system so that you can make an informed decision.

Cedar has been used as a roofing material for thousands of years.  The wood contains natural oils that help to protect against decay and can be pressure treated with chemicals that can prolong the life of a cedar shake roof for up to 50 years.  Cedar can withstand high impacts, including hail and hurricane force winds.  Cedar is a renewable, natural resource that is most highly prized for its beauty and aesthetic appeal.  Cedar shakes are still very popular in the Pacific Northwest on high-end homes and help to reinforce the idea of quality and value for the homeowner.  Many people believe that a cedar shake roof helps to maintain home values and makes it easier to sell a house.  In fact, some home owner's associations require that all new roofs must be cedar shake.  On the downside, cedar shakes are generally more expensive and require more routine maintenance than a composition roof.  They can be attacked by moss if not properly treated and must be treated with a fire-retardant to be as fire resistant as composition shingles.

Composition shingles represent the current state of considerable research into roofing materials and wear patterns.  They are usually cheaper to apply, install, and maintain and represent an excellent value for the cost conscience homeowner.  Architectural Grade shingles have appeared recently that offer manufacturer's warranties of 30 to 50 years.  They are available in a variety of styles and colors from many manufacturers and can be applied to roofs with slopes as low as 2:12.  Composition shingles are less subject to damage by insects, but may be damaged more than shakes by impact from hail.  They must also be treated to prevent algae.  Maintenance on composition roofs is generally much lower than on shake roofs.

A new and special type of composition shingle deserves special mention.  This is the architectural grade lifetime product exemplified by the Presidential TL.  These very high quality shingles are triple layered and so sturdy that the manufacturer is now offering a lifetime limited warranty.  This product represents the highest level of technology currently available and has even been accepted by many home owner's associations that were formally restricted to cedar shake.  The Presidential TL is probably the most reliable roof system available today.  However, this system is not particularly cheap.  Because plywood sheathing must be added, replacing a cedar shake roof with Presidential-TL can cost as much as a new cedar shake roof.

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Copyright © 2004 Metroplex Roofing, Inc.
Last modified: 06/07/06