What happens when you install roof shingles
Installing roof shingles means fastening individual shingle pieces to the roof deck—the wooden base under your roof—in overlapping rows that shed water downward. The process starts at the bottom edge of the roof and works upward, with each row covering the nails of the row below it. A typical residential roof takes a crew of two to four people one to three days, depending on roof size and complexity.
The work requires climbing, bending, and working at heights, which is why many homeowners hire contractors rather than doing it themselves. If you are planning to install shingles yourself, you will need to understand the sequence of steps, the tools involved, and the safety precautions that matter most.
Key Takeaways
- Shingles are installed in rows starting from the bottom of the roof and moving upward, with each row overlapping the one below to direct water down and off the roof.
- You will need a hammer or pneumatic nailer, a utility knife, a tape measure, roofing cement, and safety equipment including a harness and non-slip footwear.
- The roof deck must be clean and dry, and any damaged plywood or underlayment should be replaced before shingles go down.
- Each shingle is held with four to six nails driven into the nail line—a strip marked on the shingle itself—not into the tabs or the exposed part below.
- Starter shingles at the eave and ridge shingles at the peak require different cutting and placement than field shingles in the middle of the roof.
Preparing the roof deck and underlayment
Before any shingles go down, the roof deck—usually plywood or oriented strand board (OSB)—must be solid and dry. Walk the roof and look for soft spots, which indicate rot. Press on the deck with your foot; if it gives, that section needs to be cut out and replaced with new plywood of the same thickness. Wet or damp wood will cause shingles to buckle and fail early, so wait for dry weather and allow the deck to dry completely if it has been wet.
Next, check the underlayment—a water-resistant barrier that sits between the deck and the shingles. If your roof already has underlayment in good condition, you can install shingles directly over it. If it is torn, missing, or old, roll out new underlayment across the entire roof, starting at the eave and working upward. Overlap each row by at least 2 inches and fasten it with roofing nails or staples every 12 inches along the edges and every 18 inches in the field (the middle area). At the eaves, the underlayment should extend down to cover the drip edge—a metal strip that directs water away from the fascia board.
Installing the starter course and first row
The starter course is a row of shingles placed upside down along the eave, with the adhesive strip facing up. This row does not show; its job is to fill the gaps between the tabs of the first visible row above it and to provide a solid nailing base. Cut a starter shingle in half lengthwise so the first piece is 18 inches wide, then nail it at the corner of the eave. Continue across with full-width starter shingles, overlapping each one by 6 inches and nailing every 12 inches.
The first visible row goes on top of the starter course, right-side up. Snap a chalk line 5 and 1/4 inches above the eave to mark where the top of the first row should sit—this measurement accounts for the 5-inch exposure (the visible part of each shingle) that is standard for three-tab shingles. Nail the first shingle at the corner, then work across. Each shingle gets four nails driven into the nail line—the strip marked on the shingle, usually 5 and 3/8 inches from the bottom. Nails must go through the nail line, not into the tabs or the exposed area below, or water will leak in around the nail holes.
Building rows and offsetting the pattern
The second row sits on top of the first, with the top of the shingles again 5 and 1/4 inches above the top of the row below. To prevent water from running straight down through the seams, offset each row by 6 inches—start the second row with a full shingle, the third row with a shingle cut 6 inches short, the fourth row with a shingle cut 12 inches short, and the fifth row with a full shingle again. This pattern repeats across the roof and prevents the tabs from lining up vertically.
As you work upward, check that shingles are lying flat and that nails are driven straight and flush with the surface—nails that stick up will catch water and cause leaks. If a nail misses the nail line or goes in crooked, pull it out and move it slightly to the side. Roofing nails are small and easy to bend; a pneumatic nailer (powered by compressed air) is faster and more accurate than a hammer, though a hammer works if you are careful.
Handling hips, valleys, and roof penetrations
A hip is the angled edge where two roof planes meet at an angle pointing upward; a valley is where two planes meet in a V pointing downward. Both require special treatment because water collects there. For a hip, shingles from both sides meet at the peak. Trim the shingles so they meet at the centerline, then nail a hip shingle (a shingle cut lengthwise into two pieces) over the joint, overlapping both sides. Nail every 5 and 1/4 inches up the hip.
A valley is usually lined with a metal valley flashing or a woven valley (where shingles from both sides overlap and interlock). If using metal flashing, snap a chalk line down the center of the valley and trim shingles so they do not extend past the line. If weaving, alternate which side's shingles go on top as you work upward. Seal the valley with roofing cement under the shingles to prevent water from working underneath.
Roof penetrations—pipes, vents, chimneys—require flashing, a metal or rubber collar that sits under the shingles above the penetration and over the shingles below. Install flashing before laying shingles around it, then cut and fit shingles to the flashing shape. Seal all edges with roofing cement.
Installing ridge shingles and finishing the roof
Ridge shingles cap the peak where the two highest roof planes meet. They are made by cutting standard shingles into three pieces (each piece is called a ridge cap). Start at one end of the ridge with a ridge cap nailed so it overlaps the shingles on both sides by 5 and 1/4 inches. Work along the ridge, overlapping each cap by 5 and 1/4 inches, and nail each one with two nails—one on each side, placed so the next cap will cover them. At the far end of the ridge, cut the last cap to fit and seal it with roofing cement.
Once the ridge is done, walk the entire roof and look for any nails that are sticking up, any shingles that are not lying flat, or any gaps where water could enter. Seal any exposed nail heads with roofing cement. Check that all flashing is sealed and that no shingles are torn or missing. If you are installing shingles over an old roof (rather than tearing it off first), you may need to trim the eaves and rake edges (the sides) so the new shingles do not overhang too far.
Tools and materials you will need
A basic shingle installation requires a hammer or pneumatic nailer, a utility knife with fresh blades, a tape measure, a chalk line, roofing nails (usually 1 and 1/4 inches long for new construction, 1 and 3/4 inches for re-roofing over old shingles), and roofing cement. You will also need a roof ladder or staging to work safely at height, a harness if you are working alone, and non-slip footwear. Wear long sleeves and pants to protect your skin from the rough shingles and the hot roof surface on sunny days.
The number of shingles you need depends on roof area. A square of roofing (100 square feet) requires three bundles of standard three-tab shingles. Measure your roof length and width, multiply them, divide by 100, and multiply by three to find the number of bundles. Add 10 percent for waste and cutting around penetrations.
Frequently Asked Questions
Can you install new shingles over old ones?
Yes, if the old roof has only one layer and is not severely damaged. New shingles go directly over the old ones, and you use longer nails (1 and 3/4 inches instead of 1 and 1/4 inches) to penetrate both layers and reach the deck. However, if the old roof is buckling, has missing shingles, or already has two layers, you must tear it off first.
What is the nail line and why does it matter?
The nail line is a strip printed on each shingle, usually 5 and 3/8 inches from the bottom. Nails must go through this line, not below it into the exposed tabs. If nails go below the line, water runs down the shingle and leaks in around the nail holes. The nail line is designed to be covered by the next row of shingles, hiding the nails from view and weather.
How long do asphalt shingles last?
Standard three-tab asphalt shingles last 15 to 20 years in most climates. Architectural or laminated shingles, which are thicker, last 20 to 30 years. Lifespan depends on climate, ventilation, and maintenance. Hot, humid climates shorten the life; good attic ventilation extends it.
What should I do if a shingle cracks or tears after installation?
Small cracks can be sealed with roofing cement. If a shingle is torn or missing a large piece, it should be replaced. Lift the shingles above it, remove the nails holding the damaged shingle, slide out the old one, slide in a new one, and nail it back down. Seal the nail heads with roofing cement.
Do I need to ventilate under the shingles?
Yes. Attic ventilation prevents moisture and heat buildup, which shortens shingle life and can cause rot. Soffit vents (at the eaves) and ridge vents (at the peak) work together to pull cool air in and hot air out. If your roof does not have adequate ventilation, consider adding it before installing new shingles.