What You Need Before You Start

Building a roof requires three things in order: a solid plan, the right materials, and the structural frame already in place. You cannot start roofing until the walls and rafters or trusses are up and braced. The roof sits on top of that frame, so the frame has to be level, square, and able to carry the weight.

Before you buy anything, you need a roof design that matches your climate, your building's size, and local building codes. Your design should specify the pitch (the angle of the slope), the materials, and how much overhang you want at the edges. If you are working on a house or commercial building, your local building department will require you to show them plans and get a permit before you start.

The materials you choose depend on your climate and budget. Asphalt shingles are the most common and least expensive. Metal roofing lasts longer but costs more upfront. Wood shakes, slate, and tile are options in some regions but require specialized installation. Your choice also affects how many layers you can add later — some roofs can be re-shingled over an old layer, while others must be stripped to bare wood first.

Key Takeaways

  • Your roof frame (rafters or trusses) must be level, square, and braced before any roofing material goes on.
  • Most residential roofs start with underlayment (a water barrier), then sheathing, then shingles or other covering material.
  • Flashing — metal strips that direct water away from joints and penetrations — is installed before shingles and is critical to preventing leaks.
  • Local building codes set minimum requirements for pitch, overhang, fastening, and ventilation, and you need a permit before starting.
  • Asphalt shingles typically take one to three days to install on a residential roof, depending on size and complexity.

Installing Underlayment and Sheathing

Underlayment is a water-resistant barrier that goes directly on top of the rafters or trusses. It catches water that gets under the shingles and directs it down and out. Most building codes now require synthetic underlayment (also called roof wrap) rather than felt, because it resists tearing and lasts longer. Unroll it from the bottom edge of the roof upward, overlapping each row by at least 4 to 6 inches so water runs down, not up.

Sheathing is the layer of plywood or oriented strand board (OSB) that goes on top of the underlayment. It provides a solid surface to nail shingles to and helps the roof resist wind. Most residential roofs use 1/2-inch or 5/8-inch plywood. Nail it down with roofing nails spaced 12 inches apart around the edges and 16 inches apart in the field (the middle area). Leave a small gap between sheets — about 1/8 inch — to allow for expansion in wet weather.

Installing Flashing at Valleys, Ridges, and Penetrations

Flashing is metal (usually aluminum or galvanized steel) that directs water away from places where the roof changes direction or where something pokes through it — chimneys, vent pipes, skylights, and the valleys where two roof slopes meet. Without proper flashing, water pools in these spots and leaks into the building.

Valley flashing runs down the inside corner where two roof slopes meet. Install it before you shingle, nailing it every 12 inches along both edges. Ridge flashing caps the peak where the two slopes meet at the top. Chimney flashing has four pieces: a base pan that sits on the sheathing, side flashing on the downhill and uphill sides, and a counter-flashing that slides into the chimney mortar to overlap the side flashing. Vent pipe flashing is a rubber boot with a metal collar that slides under the shingles above it and over the shingles below it.

The rule for all flashing is: water flows downhill, so each piece overlaps the one below it. Install flashing before shingles go on top of it, so the shingles can overlap the flashing and direct water down and out.

Laying Shingles from Bottom to Top

Asphalt shingles come in bundles that cover about 33 square feet each (one "square" of roofing is 100 square feet, so you need three bundles per square). Start at the bottom edge of the roof and work upward. The first row is a starter strip — a row of shingles with the tabs cut off or a special starter shingle that has adhesive on the top edge. This row sits along the drip edge (the metal trim at the bottom of the roof) and is nailed every 6 inches.

The second row and all rows above it are standard shingles, laid so the tabs of each row cover the nails of the row below. Offset each row by half a shingle width so the seams do not line up vertically — this is called a staggered pattern and it makes the roof stronger and more water-resistant. Nail each shingle with four nails, one above each tab, driven straight in about 5/8 inch above the tab line. Nails that are too high or too low will not hold properly or will tear the shingle.

Continue upward until you reach the ridge. The last row of shingles on each slope will meet at the peak. Cut the shingles to fit, then install ridge cap shingles (special shingles made to fold over the peak) with four nails per cap, two on each side of the ridge.

Ventilation and Soffit Details

Most building codes require roof ventilation to prevent moisture buildup in the attic. Ventilation works by drawing cool air in through soffit vents (openings in the eaves) and letting warm, moist air escape through ridge vents or gable vents at the top. The soffit is the underside of the overhang where the roof extends past the wall. It needs to be vented so air can flow up into the attic and out through the ridge.

If you are installing a new roof, check whether the soffit already has vents. If not, you may need to cut them and install vent panels. The ratio of intake to exhaust matters — most codes require one square foot of vent opening for every 150 square feet of attic space, split evenly between intake (soffit) and exhaust (ridge or gable). If the attic is not properly ventilated, heat and moisture will shorten the life of the shingles and can cause rot in the framing.

Sealing Penetrations and Final Details

After shingles are down, seal any gaps where water could get in. Around vent pipes, use roofing cement or a compatible sealant under the flashing collar. Around chimneys, the counter-flashing should be embedded in the mortar, but you can add sealant along the top edge for extra protection. Do not seal the edges of shingles to each other — they need to move slightly with temperature changes, and sealing them can cause buckling.

Check all flashing one more time to make sure it is nailed down and sitting flat. Walk the roof carefully and look for any lifted edges, missing nails, or gaps. These are the spots where water will find its way in. If you see a gap between flashing and shingles, use roofing cement to fill it, but do not seal the entire edge — leave room for movement.

Timeline and When to Call a Professional

A residential roof with asphalt shingles typically takes one to three days to install, depending on the size of the house, the pitch of the roof, and the number of penetrations and valleys. A steep roof or one with many chimneys and skylights takes longer. A two-story house with a complex roof might take a crew of three to four people five to seven days.

Roofing is physically demanding and dangerous — falls from roofs are a leading cause of serious injury. If you are not experienced working at heights, have done roofing before, or have the right safety equipment (harness, rope, ladder stabilizers), hire a licensed roofer. Most states require roofers to be licensed, and licensing usually means they have passed an exam and carry liability insurance. Get at least three quotes and ask for references from recent jobs in your area.

Frequently Asked Questions

Can I put new shingles over old ones?

Yes, if the old roof is in decent shape and your local code allows it. Most codes permit one re-roof (one layer of new shingles over old ones), but not two. The old shingles must be nailed down flat — if they are curled or missing, strip them off first. Re-roofing is faster and cheaper than a full tear-off, but it hides problems underneath and adds weight to the structure.

What pitch do I need for my roof?

Pitch is the rise in inches for every 12 inches of horizontal run. A 4:12 pitch means the roof rises 4 inches for every 12 inches forward. Most residential roofs are 4:12 to 8:12. Steeper pitches shed water faster and are better in snowy climates. Flatter roofs (2:12 or less) require special materials and more flashing. Your local building code sets a minimum pitch for your area based on climate.

How do I know if my roof needs ventilation?

If your attic has insulation and is enclosed, it needs ventilation. If your attic is vented to the outside (has gable vents or ridge vents), check that soffit vents are open and not blocked by insulation. If you see ice dams in winter or dark stains on the underside of the roof deck, ventilation is probably inadequate. A roofer or home inspector can assess your attic and tell you what you need.

What fasteners should I use?

Use roofing nails — they are shorter and have a larger head than framing nails, so they hold shingles without tearing through. Galvanized or stainless steel nails resist rust. The nail length depends on what you are fastening to: 1 1/4 inches for shingles over plywood, 1 3/4 inches if you are nailing through two layers. Never use screws on asphalt shingles — they can tear the shingle and create a leak point.

How long does a roof last?

Asphalt shingles typically last 15 to 25 years, depending on climate, ventilation, and maintenance. Metal roofs last 40 to 70 years. Wood shakes last 20 to 40 years. Slate and tile can last 50 to 100 years or more. Regular maintenance — clearing debris, checking flashing, and fixing small leaks — extends the life of any roof.