What You Need Before You Start
Roof installation requires specific tools, materials, and safety equipment before the first nail goes in. You will need a hammer or pneumatic nailer, a utility knife, a tape measure, a chalk line, a ladder rated for your roof height, and roofing nails or fasteners matched to your shingle type. Most residential roofs use asphalt shingles, which come in bundles that cover roughly 33 square feet per bundle.
Safety equipment is not optional. Wear a harness connected to a rope anchored above the roof line, non-slip shoes with good grip, and work gloves. A fall from even a single story can cause serious injury. If your roof pitch is steeper than 6:12 (6 inches of rise per 12 inches of run), or if you are uncomfortable at height, hire a licensed roofer instead. The cost of a professional installation is usually less than the cost of an emergency room visit.
Before ordering materials, measure your roof in squares — one square equals 100 square feet. Measure the length and width of each roof plane, multiply them together, and divide by 100. Add 10 percent for waste and roof penetrations like vents and chimneys. Order shingles, underlayment, flashing, ridge caps, and nails all at once so you do not run short mid-job.
Key Takeaways
- Roof installation begins with removing old shingles and inspecting the deck for rot or damage, which must be repaired before new material goes down.
- Underlayment goes down first as a water barrier, followed by starter shingles along the eaves, then full courses working upward toward the ridge.
- Each shingle is fastened with four nails placed in the nail line marked on the shingle, not higher or lower, to may support the next course covers them properly.
- Flashing around chimneys, vents, and valleys requires careful sealing with roofing cement to prevent leaks at these high-risk points.
- Ridge caps finish the peak and are nailed and cemented the same way as field shingles, then the entire roof should be inspected for missed fasteners and gaps.
Removing the Old Roof and Inspecting the Deck
Start by removing all old shingles, flashing, and nails down to the bare roof deck. Use a flat shovel or roofing shovel to pry shingles up, working from the ridge downward. This is the most physically demanding part of the job. Rent a dumpster or arrange for debris removal before you start, because old shingles are heavy and take up space quickly.
Once the deck is bare, inspect every inch for soft spots, rot, or missing sections. Press a screwdriver into the wood — if it sinks in easily, that section is compromised and must be replaced. Mark damaged areas and cut out the bad wood with a circular saw, then nail in new plywood or OSB (oriented strand board) of the same thickness as the existing deck. The deck must be solid and flat before underlayment goes down, because bumps and gaps will show through the finished shingles and can trap water.
Check that all roof penetrations — vents, chimneys, skylights — are still in place and not damaged. If flashing is rusted or cracked, note it for replacement. Sweep the deck clean of debris and nails.
Installing Underlayment and Starter Shingles
Underlayment is a water-resistant barrier that sits between the deck and shingles. Roll it out horizontally starting at the eaves, overlapping each course by 4 to 6 inches as you work upward. Nail or staple it every 12 inches along the edges and every 24 inches in the field (the middle section). Trim excess at the edges with a utility knife. Underlayment prevents water that gets under shingles from reaching the wood deck.
Starter shingles go along the bottom edge of the roof, along the eaves. These are either full shingles with the tabs cut off, or purpose-made starter strips. Nail them with the adhesive strip facing up, using four nails per shingle placed in the nail line. The starter course overhangs the eave by 1/2 inch and the gable edge by 3/4 inch. This overhang directs water into the gutter instead of down the fascia.
Once the starter course is complete, snap a chalk line 5 inches above the top of the starter shingles to mark where the first full course goes. This spacing accounts for the 5-inch exposure (the visible part) of standard asphalt shingles. Use the chalk line as your guide for the first course, and every course after that will stay aligned if you nail consistently.
Laying Shingles Course by Course
The first full course goes on with the tabs pointing down. Nail each shingle with four nails placed in the nail line — the line printed on the shingle where the next course will cover. Do not nail above or below this line. Nails that are too high will be exposed; nails that are too low will not be covered and will leak. Space nails about 1 inch from the edges and 1 inch apart within the nail line.
Offset each course so the seams do not line up vertically. Most roofers offset by half a shingle width — cut the first shingle of every other course in half so the seams stagger. This pattern is called a running bond and is stronger than aligned seams. Snap a new chalk line for every third or fourth course to keep rows straight, especially on longer roof planes.
Continue nailing courses upward until you reach the ridge. On the last course before the ridge, trim shingles so they fit the remaining space without overhang. The ridge itself is covered with ridge caps, which are shingles cut into thirds and folded over the peak.
Flashing Around Chimneys, Vents, and Valleys
Flashing is metal or rubber material that directs water around obstacles. Install flashing as you shingle, not after. When you reach a vent pipe, slide the flashing boot over the pipe and nail it to the roof deck before nailing shingles around it. The shingles above the vent overlap the flashing; the shingles below do not. Seal the nail heads and the top edge of the flashing with roofing cement.
Chimneys require step flashing — individual pieces of metal that alternate between the roof and the chimney, creating a staircase pattern. Install step flashing as you shingle, nailing one piece to the roof, then nailing the next piece to the chimney, overlapping each piece by 4 inches. Once all step flashing is in place, install a counter-flashing (a cap that covers the step flashing) and seal it with roofing cement.
Valleys are where two roof planes meet. Install valley flashing before shingles reach the valley, then trim shingles to fit the valley without covering the flashing edges. Seal all flashing seams with roofing cement rated for your climate. In cold climates, use a cement that remains flexible when frozen.
Installing Ridge Caps and Final Inspection
Ridge caps finish the peak and are made from shingles cut into thirds. Cut a shingle into three equal pieces along the lines printed on the back. Fold each piece over the ridge and nail it with two nails on each side of the ridge, placed so the next cap covers them. Overlap each cap by 5 inches, the same as field shingles. Seal the nail heads of the last cap with roofing cement so water does not run under it.
Once the last ridge cap is nailed, walk the entire roof and look for missed nails, gaps between shingles, or flashing that is not sealed. Press down on shingles to make sure they are adhered — the adhesive strip under each shingle should bond to the shingle below it within 24 hours in warm weather, longer in cold. If shingles are not adhered, explore roofing cement under the tab to bond them.
Check gutters and downspouts to make sure they are clear and properly attached. Debris in gutters can trap water and cause leaks. If you installed new flashing, inspect every seam one more time for gaps or lifted edges. A small gap now becomes a leak in the first heavy rain.
Common Installation Mistakes and How to Avoid Them
The most common mistake is nailing shingles outside the nail line. Nails placed too high will be exposed to weather; nails placed too low will not be covered by the next course and will leak. Mark the nail line with a pen if the printed line is hard to see. Use a pneumatic nailer set to the correct depth — too shallow and nails will back out, too deep and they will tear the shingle.
The second mistake is not overlapping underlayment and flashing enough. Water runs downhill and sideways, so every overlap must be at least 4 inches and must be sealed with roofing cement at high-risk points like valleys and chimneys. Skimping on overlap is the fastest way to develop leaks that are hard to find and expensive to fix.
The third mistake is installing shingles in cold weather. Asphalt shingles are stiff when cold and the adhesive does not set up. If the temperature is below 50 degrees Fahrenheit, wait for warmer weather or use a heat gun to warm shingles before nailing. Cold installation leads to shingles that blow off in wind or curl at the edges.
Frequently Asked Questions
How long does a roof installation take?
A single-story house with a straightforward roof shape takes two to four days for one or two people working at a steady pace. A two-story house or a roof with many penetrations and valleys takes longer. Weather delays are common — rain stops work, and wind makes it unsafe to work at height.
Can I install shingles over an old roof?
Some building codes allow one layer of new shingles over one layer of old shingles, but only if the old roof is in good condition and not curling or buckling. Most roofers recommend removing old shingles because you cannot inspect the deck for rot, and the extra weight stresses the structure. Check your local building code before deciding.
What is the difference between three-tab and architectural shingles?
Three-tab shingles are flat and uniform, with a single tab pattern. Architectural (or laminate) shingles have multiple layers and a textured appearance that mimics wood or slate. Architectural shingles last longer and look better, but cost more and weigh more. Both are installed the same way.
Do I need to seal the nails on every shingle?
No. Roofing cement is only needed where water is likely to find a path under shingles — at flashing seams, ridge caps, and valleys. Field shingles are covered by the course above them, so nail heads are protected. Overusing roofing cement can actually trap moisture and cause problems.
What should I do if I find rot in the roof deck?
Cut out the rotted section with a circular saw, cutting back to solid wood on all sides. Replace it with new plywood or OSB of the same thickness, nailed every 6 inches to the surrounding framing. If rot is widespread, the roof structure may be compromised and you should have a structural engineer inspect it before proceeding.