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I’m a second-generation roofing contractor, a certified installer of most common types of roofing. In this article, I’ll discuss what it takes to apply a guaranteed, watertight asphalt-shingle roof. While basic installation guidelines are printed on every shingle wrapper, there’s a lot more to know if you’re really interested in preventing callbacks. About 80 percent of our asphalt-shingle installations are replacement jobs working directly for the homeowner in which we use laminated “architectural” shingles — by far the most popular roofing choice here in New England.

No Go-Overs

Roofs age differently under different exposures and in different climates, but the roofs we replace are generally between 20 and 30 years old. Although shingling directly over an old roof eliminates the considerable effort and expense of stripping and disposing of the shingles (building codes permit a second layer only), we strongly discourage the practice, for several good reasons.

First, old step flashing can’t be inspected and replaced as needed. Second, adding another layer of shingles piles a lot of extra dead load on the roof structure — a particular concern in the case of older homes whose framing may not be up to snuff (see “Reroofing Over Asphalt Shingles,” Practical Engineering, 2/96). It’s a lot better to stick with what’s worked for that roof over time than to assume it’ll handle the extra weight.

Third, if you don’t strip the roof, you don’t know what you’re walking on. The sheathing may require repair in areas that can’t be reliably detected from the attic. Also, for homeowners looking to spruce the home up for sale, multiple layers of shingles on its roof are sure to prompt a lower assessment of value.

Finally, some roofers argue that a go-over has a shorter service life because the thicker the layer of shingles, the more solar heat is absorbed and held. When I explain these issues to the homeowner, it’s rare that they still insist on going over the old roof.

Tools for stripping. Stripping a go-over is particularly brutal work (yet another reason not to layer shingles). The tried-and-true dedicated shingle stripper, a spadelike tool with serrated teeth and a prying wedge on the underside, is our tool of choice. We particularly like the Shingle Eater (508/248-7800, shingleeater.com). When stripping, the most comfortable and secure way to work on most slopes is in a seated position, working from the ridge down to the eaves. Two or three workers can typically clear a four-square area in about an hour.

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It’s important to protect foundation plantings and the face of the building from falling debris. We cover shrubs with tarps tacked up under the roof overhangs and keep one worker on the ground continually cleaning up and loading the truck. Tarps don’t last long around these loads. I like to recycle those big lift wraps from the lumberyard that are otherwise tossed in the dumpster. They’re almost as tough as standard poly tarps, and some of them will last through several jobs. But even if we get only one use out them, they’ll have done double-duty and saved me a little dough.

We generate a lot of disposable debris. As often as possible, I truck old asphalt shingles to a regional depot that charges $90 per ton and sends the material to be recycled into asphalt roadbed.

We strip only as much as we can cover again with underlayment that day. To define the strip area, we slit a line through the old shingles with a hook blade, straight up the roof, and tear off up to that line. Sheffield hook blades (516/746-5352, sheffield-tools.com) are by far the best, outlasting every other brand we’ve used by a wide margin. I buy them in bulk at a local lumberyard.

Renail the sheathing. It’s important to renail the roof sheathing before installing underlayment, because the violent action of stripping the roof can easily loosen the original sheathing nails. We use 8-penny galvanized ring-shank nails and hammer the old nail heads back down to prevent nail pops through the roofing. If any of the sheathing is in bad shape, now’s the time to replace it.

Underlayments and Flashing

An asphalt roof is not a waterproof barrier; it’s a durable water-shedding system that must be installed to strict standards in order not to leak. I’ve seen torn and poorly applied felt underlayment shingled over as if it were only a temporary dry-in measure. But properly installed underlayment is the last line of defense against leaks.

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Instead of 30-pound felt paper, we like to use CertainTeed’s Roofer’s Select, a fiberglass-reinforced asphalt-impregnated organic felt. It seems to have higher asphalt content than competing products. It goes down easily, resists tearing around fasteners in windy conditions, and provides a better grip underfoot than ordinary felt paper.

Although Roofer’s Select is less absorptive than ordinary felt — which readily absorbs atmospheric moisture, producing wrinkles that interfere with chalk lines and prevent shingles from lying flat — it too can hump up when wetted. As a quick and effective remedy, we dry wrinkled underlayment with a leaf blower that we carry with us as standard equipment. It works like a charm and is also great for clearing out gutters, a routine aspect of our work.

No staples. When it comes to securing the underlayment, I don’t find pneumatic cap tools to be worth the bother or expense. We hand-nail with plastic roofing caps. Since hammer-tackers and staples do more harm than good, they aren’t allowed on our jobs. Staples don’t have the holding power of the caps, and they set the underlayment up for tear-throughs and blowoffs. If an overlooked nail head pops through while we’re installing the underlayment, we pull the nail and repair the breach with a patch, making sure it sheds water. We never leave a hole or a void anywhere in the underlayment.

On slopes from 4/12 to 2/12, we use only self-adhering membranes. Actually, I prefer to treat a 2-in-12 slope as a flat roof and apply an impermeable EPDM membrane — but that’s not comfortable for every budget.

With self-adhering membranes, Grace Ice & Water Shield (617/498-4997, graceconstruction.com) is the one to beat. But it costs twice as much as CertainTeed’s WinterGuard — our standard waterproof membrane — so I use it only on high-budget jobs. I like WinterGuard because it makes a better sealed overlap than other granulated membranes (and I’ve tried them all).

We always install continuous water and ice membrane along eaves to a point 2 feet inside the exterior wall plane. Valleys receive full-length, 36-inch-wide membrane centered on the divide. We run membrane up the rakes, too, covering the top edge of the trim with a 12-inch strip. We also use self-adhering membrane around all penetrations like chimneys and roof vents. At chimneys and dormer cheeks (when accessible), we fold the membrane up the vertical face as a backup to conventional metal step flashing.

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Self-adhering membranes become exceptionally sticky in the heat of the sun and require careful handling to prevent frustration and wasted material. Never leave the rolls in the sun. Immediately store them in the shade or in the basement if possible, right up until use.

Applying the membrane is a two-person performance. First, cut it to length, roll it out, and position the edge accurately against a chalk line. Then, roll it back halfway along its length, carefully score the backing paper across the roll, and peel it away as you redistribute the roll. Repeat this process from the opposite end, and the membrane will end up right where it belongs.

Step Flashing

You can’t always inspect or predict the condition or quality of existing flashing along dormers and sidewall cheeks. After tearoff, we often find the typical paper-thin aluminum step flashing worn through from a couple of decades of acid rain. Depending on the type of siding and how tightly it was installed, worn-out step flashing can be difficult to replace. In some cases, I have to present the client with a change order to strip and replace the siding and trim before proceeding. Removal is certainly the best case, since it gives us the opportunity to run self-adhering membrane between the roof and the wall before installing new step flashing.

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It’s important to note that 5-inch-by- 7-inch aluminum steps aren’t suited to the wider coursing of metric-size shingles, now the prevalent size. However, most suppliers carry appropriately sized step flashing for metric shingles. Custom-bent flashing made of heavier aluminum or copper stock is best, but it can tax the average roofing budget.

We often find aluminum flashing folded around outside dormer corners in a tortured wrap that stands little chance of keeping water out. This is a common leak location. Here, we absolutely have to remove the corner trim and shape a piece of lead around the corner transition.