What You Need Before You Start

A roof installation requires three things in place before the first nail goes in: a clear structural plan, the right materials for your climate, and safety equipment that actually fits. You cannot improvise these on the job.

Start by knowing what you are building on. Walk the perimeter of your structure and measure the footprint — length times width gives you square footage, which determines how much material you need. Measure the slope or pitch of your roof (the angle of the rise). A 4:12 pitch means the roof rises 4 inches for every 12 inches of horizontal run. Steeper roofs shed water faster but are harder to work on. Flatter roofs need more waterproofing layers.

Next, choose your material. Asphalt shingles are the most common for residential work — they cost less and install faster than metal, tile, or wood. Metal roofing lasts longer but requires different fastening and flashing. Your local building code may restrict what you can use based on fire rating or wind resistance in your area. Check with your building department before buying materials.

Key Takeaways

  • Measure your roof's square footage and pitch before ordering materials, because both affect how much you need and how the installation changes.
  • Remove old roofing down to the deck, inspect the wood underneath for rot, and replace any damaged boards before laying new material.
  • Asphalt shingles require underlayment, starter shingles along the edge, and proper nailing at specific points to prevent wind damage and leaks.
  • Flashing around chimneys, vents, and valleys must overlap correctly and seal with roofing cement to stop water from running under the shingles.
  • A typical residential roof takes two to five days depending on size and complexity, and mistakes in ventilation or water management cause leaks years later.

Removing the Old Roof and Inspecting the Deck

If you are replacing an existing roof, you must remove the old shingles and underlayment first. Use a roofing shovel or flat pry bar to lift shingles from the edge and work toward the peak. Shingles are nailed down, so you are fighting gravity and fasteners at the same time. Work in sections and throw debris into a dumpster or onto a tarp below — roofing waste is heavy and sharp.

Once the shingles are off, inspect the roof deck (the plywood or wood boards underneath). Press on soft spots with a screwdriver. If the wood is spongy or the screwdriver sinks in, that section has rot and must be replaced. Rot spreads, so cut out the damaged area and screw down new plywood in its place. Do not cover rot with new shingles — it will only get worse and eventually cause the roof to fail.

Check for nails sticking up from the old installation. Pound them flat or pull them out. A nail sticking up will puncture your underlayment and create a leak point. Sweep the deck clean of debris and loose nails.

Installing Underlayment and Starter Shingles

Underlayment is a water barrier that sits between the deck and the shingles. It catches water that gets under the shingles and directs it toward the gutters. Use synthetic underlayment (also called roofing felt or weather barrier) rated for your climate. In cold climates, use ice-and-water shield along the eaves and valleys — this self-adhering membrane prevents ice dams from forcing water under the shingles.

Roll the underlayment across the roof starting at the lowest edge, working upward. Overlap each row by 4 to 6 inches. Staple or nail it down every 12 inches along the edges and seams. Do not pull it tight — it should lay flat but with a little slack so it does not tear in wind.

Starter shingles go along the bottom edge of the roof, overhang the gutter by about half an inch, and provide a nailing base for the first course of regular shingles. You can buy starter shingles or cut the tabs off regular shingles and flip them upside down. Nail them down with four nails per shingle, placed about 5 to 6 inches from the bottom edge.

Laying Shingles in Rows and Nailing Correctly

Start the first course of shingles at one end of the roof, aligned with the starter shingles below. Asphalt shingles come in three-tab or architectural styles. Three-tab shingles are lighter and cheaper. Architectural shingles are thicker and look more textured. Both nail the same way.

Each shingle has four nailing spots marked by a dark line or adhesive strip. Nail at these spots only — nailing outside them can cause the shingle to tear or curl. Use roofing nails (short, wide-head nails) and drive them straight in, not at an angle. The nail head should sit flush with the shingle, not sunk into it. If you sink a nail, it will tear the shingle and create a leak point.

Offset each row by half a shingle width so the seams do not line up vertically. This staggering prevents water from running straight down a seam. Lay the second row so the bottom edge of its shingles covers the nails of the first row. Continue upward until you reach the peak.

Flashing and Sealing Valleys, Chimneys, and Vents

Flashing is metal or rubber material that directs water around obstacles like chimneys, vents, and roof valleys (the low points where two roof planes meet). Without proper flashing, water runs under the shingles and leaks into the house.

For valleys, lay valley flashing (a metal channel) down the center of the valley before you shingle. Nail it down and seal the edges with roofing cement. Shingle over it, cutting shingles to fit the valley shape. Do not nail through the flashing itself — nail into the shingles on either side.

For chimneys and vents, install flashing before you shingle the area around them. The flashing has a base that slides under the shingles above the obstacle and sits on top of the shingles below. Seal all edges with roofing cement. This overlap is what stops water from running sideways into the gap.

Roof vents (for attic ventilation) come with their own flashing collar. Nail the collar down and seal around it. Do not skip this step — a leaking vent is one of the most common sources of attic water damage.

Finishing the Peak and Edges

At the peak, two roof planes meet. Shingles from both sides overlap at the top. Cut shingles to fit the peak and nail them down. The last shingle on each side should cover the nails of the shingle below it.

Ridge caps (shingles made specifically for the peak) go over the seam. Fold them over the peak and nail them down with two nails per cap, one on each side of the peak. Seal the nail heads with roofing cement to prevent water from running in around them.

At the edges (the gable ends), shingles should overhang the wall by about an inch. Trim them straight with a utility knife. Some roofers install drip edge (a metal trim) along the gable to direct water away from the wall. This is not required but prevents water damage to the fascia and siding.

Ventilation and Long-Term Durability

A roof that does not breathe will fail early. Your attic needs intake vents (usually in the soffit, the underside of the overhang) and exhaust vents (at the peak or high on the roof). Air flows in through the intake, across the attic, and out through the exhaust. This airflow removes moisture and heat, which extends the life of the shingles and prevents ice dams in winter.

If your attic has no ventilation, add soffit vents and a ridge vent before you shingle. If you are installing a new roof on an existing structure with poor ventilation, this is the time to fix it. Ventilation problems show up as premature shingle failure and mold in the attic.

After installation, inspect the roof from the ground with binoculars. Look for shingles that are not lying flat, nails that are sticking up, or gaps in the flashing. These are the spots that will leak. Fix them now rather than waiting for water damage.

Frequently Asked Questions

How many nails does each shingle need?

Standard asphalt shingles need four nails, one at each marked spot. Some roofers use six nails in high-wind areas, but four is the manufacturer standard. Nailing more than required can actually cause problems by creating extra holes for water to enter.

Can I shingle over an old roof without removing it?

You can, but it is not recommended. A second layer of shingles hides problems in the deck, adds weight the structure may not be designed for, and makes future removal harder. Most building codes allow one layer of shingles over another, but removing the old roof first gives you a chance to inspect and repair the deck.

What is the difference between ice-and-water shield and regular underlayment?

Ice-and-water shield is self-adhering and sticks directly to the deck, creating a waterproof seal. Regular underlayment is just a water barrier that relies on gravity to shed water. Ice-and-water shield is thicker and more expensive but necessary in cold climates where ice dams form at the eaves.

How long does a roof installation take?

A typical residential roof takes two to five days depending on size, pitch, and weather. A small, flat roof might take one day. A large, steep roof with multiple valleys and penetrations can take a week. Weather delays the work — you cannot shingle in heavy rain or high wind.

Do I need a permit to install a new roof?

Most jurisdictions require a permit for roof work. The permit process includes an inspection of the deck and flashing before you shingle, and a final inspection after. This catches problems early and ensures the work meets code. Check with your local building department before you start.