how to extreme

DIY Repair of an Asphalt Shingle Roof

Construction How-To, Repairs, Roofing February 4, 2013 Sonia

By Clint C. Thomas, Esq.                  Photos by Zoe Thomas



The roof is one critical component of a house. Every house has one, but very few homeowners know what to do when it begins to wear-out, other than call a roofing contractor. Minor leaks can often be repaired with silicone or roofing cement, commonly called “tar,” or as it is known in the industry, “bull.” But what about an area of a roof that has multiple leaks that result from hail damage or a roof that has simply exceeded its life expectancy?

One solution to this dilemma is to re-roof just the problem area. Roofs are often divided into different sections as a result of a home’s construction. Adjoining roofs create valleys, as do the intersection of gables with the main roof. Ridges are formed at the apex where two roof planes meet. Each side of a ridge or valley is actually independent of the roof on the other side of that same landmark. Furthermore, some rooms in a house may have different heights than other portions of the house, especially if there has been an addition that has been made to the original construction. These different roof areas help to simplify re-roofing jobs by enabling repair of only the problem area rather than the home’s entire roof system. There are two ways to re-roof a house. One is the remove all of the existing asphalt shingles, install new roofing felt, which is commonly referred to as “tar paper,” and then reapply new flashing and shingles. If you have ever done any roofing work at all you know that it is not technically difficult but it is very labor intensive. Stripping old shingles off a roof is hard, hot and nasty work and should not be approached lightly.

The second alternative when re-roofing a portion of a house is to leave the existing shingles and flashing in place and install new shingles over the old. This method avoids the labor involved in a tear-off, the old layer of shingles helps to add an additional layer of protection from the elements, and the old shingles also provide a ready-made grid that can be followed when installing the new shingles. Without the old shingles to follow, it is necessary to lay out horizontal and vertical control lines to follow when installing the new shingles. Without these lines or some other guide, such as the old shingles, the new shingles will not be straight and will look like a twisting snake.

Roofing over an existing roof can be done up to two times without any problem. This means that the original roof can be roofed over once and then once more for a total of three layers of shingles. It is a bad idea to put more than three layers of shingles on any roof because of the shear weight of the shingles. If too much weight is applied to a normal roof it is likely to collapse under the strain.


Prep Steps

Before launching into nailing down new shingles it is necessary to estimate the number of new shingles that will be needed. There are two common ways to estimate the shingles needed for a roof.

Use a chalk line to mark a straight line on the back side of any shingles that need to be cut.

Sweep off the old roof before starting work so it is free of debris.

The easiest method is to begin by finding the roof’s area. The formula for determining area is length multiplied by width. In my case, the roof measured approximately 10 by 21 ft. for a total of 210 square feet.

The next step is to divide the total number of square feet—in my case 210 sq. ft.—by 32 (some people use 29), which is the approximate number of shingles in a bundle. The resulting number indicates how many bundles are needed. In my case, when 210 is divided by 32 the answer is 6.5625. Always round up when estimating shingles. If the quotient to the above formula ends up as a decimal, such as my 6.5625, round-up to the next whole number which was 7 in my case.

Another method is to determine how many “squares” are in a roof. Roof measurements are often figured by the “square.” A “square” is 100 sq. ft., e.g., 10 by 10 ft., or any other measurement that arrives at a total of 100. To use this method you must first determine the total number of square feet of the roof to be repaired. Again, on my project there were 210 square feet. This total square footage is then divided by 100, e.g., 210 / 100 = 2.1. The quotient of this last computation is 2.1 and this is then multiplied by 3 because it takes three bundles of shingles to cover one “square.” When 2.1 is multiplied by 3 the product is 6.3. This means that 6.3 bundles of shingles will be needed. Again, round the product up to 7 bundles to be safe.

The only supplies that should be necessary are shingles and flat-head roofing nails. Normally 1-in. nails are used when installing shingles, but I chose to use nails that measured 1-1/4 inches because they had to go through two layers of shingles, the new layer and the original layer, before penetrating the plywood decking that is under all of the shingles.

Lay the new starter row of shingles upside-down so the tabs are on the top.

The easiest way to cut shingles is to cut them on the back side with a utility knife.

As an illustration of the money that can be saved, I spent less than $150 to re-roof my 210 square feet. The shingles were $22.99 per bundle and a 100 count box of nails cost about $10. The total $150 is a fraction of what a roofing contractor would have charged me.


How to Repair

Before starting the installation, sweep off the old roof with a broom. If there are any missing or broken shingles, make sure to fill in the holes and gaps with pieces of new shingle. It is important that the shingles on the old roof are all on the same horizontal level. You should try to avoid having any dips in the old shingles because this will create visible low spots in the new roof.

With all of the measurements made and the prep work complete, it is time to begin installing the new shingles. Begin at the bottom of the roof and install what is called the “starter row.” The first row of new shingles will be installed over the first and second rows of existing shingles, and—this is very important—this new row of shingles will be installed upside-down. Shingles are divided into two sections. The upper section is solid while the lower section is divided into three “tabs” by open spaces between each tab. Shingles are normally installed with the tabs facing down and the solid portion on top. A starter row is the exact opposite. The solid row is on the bottom to keep water out. If the tab side were on the bottom of the first row then water would simply run between the tabs and your roof would leak.

Stagger all of the tabs to keep water out of the roof system.

The second row of new shingles is laid with the tab facing down and should abut the bottom edge of old third row of shingles.