Asphalt Roofing Installation Primer
Asphalt shingles are by far the most prolific roofing material used in residential construction today. Virtually 80 percent of all residential roofing materials are manufactured by asphalt roofing product companies. The industry has come a long way since its beginning in 1893. Today's asphalt shingle products are far superior to products made just 25 years ago.
The construction industry (of which I was a participant for 20 years) has a major flaw. The learning process in virtually every field is either visual or verbal. Young people beginning to work in the industry simply learn by doing what they are told or by watching more experienced individuals. There is very little reading that takes place. The only widespread exposure of written learning material that I am aware of is the textbook material available to vocational students or those few who go through apprenticeship programs (plumbers, electricians, some HVAC).
Relying on verbal or visual information is simply not acceptable in today's world. Professionals in fields other than building and remodeling are formally educated and continue to read on a regular basis about changes in their profession.
A construction worker who is just entering the field may be taught by an individual who has been doing something wrong or against manufacturer's recommendations for years. In many instances, these individuals do not even realize that they are making mistakes.
Many individuals who install asphalt roofing shingles do not follow written, established roofing practices as outlined by manufacturers and roofing associations. This is prevalent in the industry. In fact, in my own hometown, I am aware of only one roofing contractor who correctly installs asphalt shingles! That means that 98 to 99 percent of all the asphalt roofs installed in Cincinnati are susceptible to leaks or premature failure. The same thing quite possibly is happening in your city or town.
The performance of asphalt roofing shingles is dependent upon several factors. A mistake in any one area can cause failure of the system which manifests itself to you as a leak. Some of these factors are roof slope, ventilation, roof deck to which shingles are applied, drip edges, flashings, fasteners (nails or staples), application of fasteners, quantity of fasteners, placement of fasteners and condition of asphalt shingles prior to application.
Asphalt roofing shingles cannot be successfully installed on each and every roof. These products depend heavily on gravity to keep water from entering your house. As the slope or pitch of a roof increases (gets steeper), gravity can pull the water off of your roof faster. Thus, the steeper your roof, generally, the better asphalt shingles perform.
Roof slope is commonly called out as the amount of vertical rise per amount of horizontal run. By convention, the amount of horizontal run is commonly called out as 12 inches. A common roof slope (pitch) used in many ranch houses is 4 inches of vertical rise for every 12 inches of horizontal run. This is called out as a 4/12 pitch roof.
Asphalt shingles can be applied on roofs with a slope (pitch) as low as 4/12 and as high as 21/12 using standard installation practices. Low slope roofs between 2/12 and 4/12 require special application methods, as do roofs that exceed 21/12 pitch.
Poor attic ventilation can adversely affect shingle life. Prolonged exposure to high temperatures can accelerate the aging process of asphalt shingles. Excessive moisture buildup in attic spaces can cause water vapor to penetrate asphalt shingles. This water vapor in turn is boiled by the sun and causes blisters in shingles. Excessive attic moisture can also cause the roof framing and decking members to swell. Excessive swelling can buckle shingles, which in turn can fracture them. Leaks may develop as a result of fractures. Proper ventilation of attic spaces will remove excess heat and moisture. The minimum ventilation requirements are a net free ventilation area of 1 square foot for every 150 square feet of ceiling area. Net free area refers to the unobstructed area of ventilation systems. Remember that insect screening restricts air flow! You can't simply use the area of the holes you cut in your roof to determine the free net area. Virtually every well made ventilation cover or system tells you the net free area of the vent or per linear foot of ridge vent.
The roof deck or surface to which the shingles are going to be applied must be solid and rigid. It has to be able to support with minimal deflection the weight of the roofing materials, snow loads and anyone who is walking on the roof. The decking must also be installed properly. Plywood installed improperly can and will buckle. I addressed this subject in this plywood article.
Water can easily penetrate roofs at the bottom and side edges of roofs. This is especially true when rain is accompanied by wind. Drip edges are corrosion resistant metal which protects the wood roof decking from this water. They usually project a minimum of 3 inches onto the deck. The metal at the edge of the roof decking is then bent down a minimum of 1 inch onto the rake trim and gutter boards.
My guess is that over 90 percent of roof leaks are flashing-related failures. Flashings are the transition materials used to blend roofing materials into things that are not roofing materials (chimneys, sidewalls, skylights, plumbing vent pipes, etc.) The installation of flashings and the use of proper flashing materials is vitally important. For example, I have seen countless chimney flashing failures due to the use of aluminum. A roofer, thinking that he is providing a maintenance free flashing, installs aluminum flashing around a masonry chimney. Little does he (she) know, aluminum is rapidly corroded by the alkalies in the mortar between the bricks! After several years, the aluminum simply dissolves. Also, aluminum cannot be soldered. Chimney flashings almost always require soldering at the corners of the base and head flashings! The only ones that do not are pre-engineered chimney flashings, and these are rare.
Flashings require skill to fabricate and install. Step flashings, for example, need to be specific sizes and need to be laced and overlapped into each layer of roofing. They need to be bent just right so as not to hold up a shingle. Step flashings are an integral part of a two component flashing system. They require a separate free-floating counter flashing. I addressed flashings in this related article. Do not underestimate the importance or necessity of proper installation of metal roof flashings. Trust me, those difficult and troublesome leaks are probably being caused by a flashing.
Fasteners are the nails and staples used to apply asphalt shingles to a roof deck surface. Virtually every manufacturer of asphalt roofing approves the use of staples. However, the Asphalt Roofing Manufacturers Association recommends that roofing nails are the preferred fastening system for asphalt shingles.
Believe it or not, you just can't use any nail or staple to install asphalt shingles. Nails and staples need to be corrosion-resistant. Moisture does collect beneath shingles, both in liquid and vapor form. This moisture will rust non-protected iron nails.
The shank diameter of nails must be a minimum of 0.105 inch (12 gauge) and the head of the nail must not be less than 3/8 inch. Staples must have a minimum outside crown (width) of 15/16inch. They should be made from wire that is a minimum of 0.0625 inch (16 gauge).
Fasteners must be of a sufficient length to completely penetrate at least 3/4 inch into the roof deck. If the roof decking material is less than 3/4 inch thick, the nail must penetrate the decking completely and extend a minimum of 1/8 inch past the inner surface of the roof decking.
Fasteners must be placed correctly. They should be driven so that the bottom of the nail head or staple crown is flush with the shingle. Fasteners driven too deep, crooked or not far enough are simply not acceptable.
Quantity & Placement of Fasteners
There are many different shingle types available today. The standard 3 tab shingle is common to many homeowners, however dimensional type asphalt shingles are becoming ever more popular. Most of these shingles are designed to expose 5 inches of the shingle to the weather. In cases such as these, the manufacturers usually require that you place the fasteners in specific locations along a line 5 5/8 inches from the bottom (exposed) edge of the shingle.
In all shingles that have a thermoplastic asphalt self sealing compound, the nails are to be placed below this strip!
Virtually, every manufacturer requires a minimum of four per to determine the proper location of the fasteners.
The location of the fasteners is critical for a very simple reason. As each successive course of shingle is applied, it is offset from the course below. This offset is needed to adequately cover the butt joint of the course below. This butt joint is where water leaks into your house. Fastener locations are designed so that they receive the maximum protection from the course of shingles immediately above them. Any deviation from the nailing pattern increases your chances of a leak!
High Wind Area Nailing Pattern
Asphalt shingles applied in areas subject to high winds need two extra fasteners (total of six) per shingle. These two extra fasteners are attached along the same line as the original four. The two fasteners within the body of the shingle are moved slightly to the left and/or the right and an extra fastener is applied so that it is 2 inches away. Once again, the locations are specific to each type of shingle and you (or your roofer) need to read the shingle wrapper to determine the correct location.
Condition of Materials
Asphalt shingles can absorb water. Waterlogged shingles cannot be successfully applied to a roof and achieve maximum performance. Thus, it is imperative that they be stored in such a way as to remain dry prior to application. They should always be stored on a flat surface. Buckled shingles should never be applied. Modern asphalt shingles also become very brittle at temperatures below 40 degrees F. Care must be taken so as not to fracture them if applying them in cold weather conditions.
Reroofing over existing roofs
There are many considerations which must be studied before applying new asphalt shingles over an existing roofing material. Weight, fastener holding capability, smoothness of existing roof, local code requirements, etc. must be investigated. Do not assume that you can simply apply a new layer of asphalt shingles over an existing roof. Remember that asphalt is a plastic material. When the shingles get hot, they will conform to the surface upon which they lie.