How your roof works

Water runs downhill, and a roof is built so that every layer overlaps the one below it like scales, always shedding water toward the gutter. Most roof problems come from something that interrupts that overlap: a gap at a wall, a cracked pipe boot, a nail in the wrong place, or ice that dams the water and pushes it uphill.

The structure: rafters or trusses

The roof's skeleton is either rafters (individual sloped boards, common on older Colorado homes) or trusses (factory-built triangles, standard on most newer homes). They carry the weight of the roofing plus snow, and they set the roof's pitch. You'll rarely deal with them unless there's sagging, rot from a long-term leak, or a heavy new material like tile that needs an engineer's check.

Decking

Decking, also called sheathing, is the layer nailed across the framing that everything else attaches to. Newer homes use sheets of OSB or plywood. Many older homes have spaced boards (1x4s or 1x6s with gaps) left over from wood shake roofs; most shingle manufacturers require solid decking, so a re-roof on those homes often adds new sheets over the boards.

Decking is the one part of a re-roof nobody can price for certain until the old roof is off. Soft, delaminated or rotted sheets get replaced, usually billed per sheet. A good quote tells you that price up front.

Underlayment

Underlayment is a water-resistant sheet over the entire deck. It's the backup layer: if wind-driven rain or melting snow gets under the shingles, underlayment keeps it off the wood. Older roofs used asphalt-saturated felt ("tar paper"); most new roofs use synthetic underlayment, which is lighter, stronger and doesn't wrinkle when it gets wet.

Ice and water barrier

At the eaves, in valleys and around chimneys and skylights, roofers lay a self-adhering rubberized membrane directly on the deck. It seals around the nails that pass through it, so even standing water can't leak through. In Colorado it matters most at the eaves, where ice dams form, and in valleys, where water from two slopes concentrates.

Whether it's required depends on where you live. Fort Collins requires an ice barrier at all eaves, and unincorporated Larimer County requires one countywide. Lakewood and Arvada don't require one on residential re-roofs, and Denver's 2019 published guidelines didn't either. Many roofers install it anyway; ask what your quote includes.

The roof covering

Shingles, metal panels, tile or slate are the weather surface. They shed water, block UV and take the hail. On asphalt shingles, the mineral granules on top are what protect the asphalt from the sun; when granules wear or get knocked off by hail, the shingle ages faster. Each material behaves differently, so we cover them one by one in the materials guide.

Shingles are designed to be nailed in a specific strip and to seal to the course below with a strip of adhesive that bonds in the sun. Nails placed too high or too few, or shingles that never sealed because they went on in cold weather, are a common cause of wind damage.

Flashing

Flashing is the metal that seals every place the roof meets something else: walls, chimneys, skylights, valleys and vent pipes. Leaks start at flashing far more often than in the middle of a shingle field.

• Drip edge runs along the eaves and rakes and sends water into the gutter instead of behind it onto the fascia.
• Step flashing is a series of small L-shaped pieces woven in with each shingle course where a roof meets a wall.
• Counter flashing covers the top of the step flashing at chimneys and masonry walls.
• Kickout flashing at the bottom of a roof-to-wall joint turns water into the gutter. Missing kickouts are a classic cause of rotted siding below.
• Pipe boots seal around plumbing vents. Rubber boots crack in strong sun and are one of the most common small leaks.

Ventilation

An attic needs air moving through it: cool air in at the soffits (intake) and warm, moist air out near the ridge (exhaust). Good ventilation keeps the attic close to outdoor temperature in winter, which prevents the warm-roof, cold-edge pattern that creates ice dams, and it lets moisture from the house escape before it condenses on the decking. In summer it lowers attic temperatures, which helps shingles last.

Intake and exhaust should be roughly balanced. Too little intake, or mixing different exhaust types on one roof, can short-circuit the airflow. Insulation and air sealing at the attic floor matter just as much: they keep house heat from reaching the roof in the first place.

Gutters and downspouts

Gutters aren't technically part of the roof, but they finish its job by carrying water away from the foundation. Clogged gutters back water up under the drip edge and feed ice dams. Dents in gutters and downspouts are also one of the easiest signs of hail you can check from the ground.

Where water usually gets in

In rough order of how often we hear about them:

1. Flashing at chimneys, walls and skylights
2. Cracked or loose pipe boots
3. Valleys, especially where debris collects
4. Ice dams at the eaves
5. Wind-lifted or missing shingles
6. Hail damage that cracked the shingle mat
7. Exposed or backed-out nails

That order is a rule of thumb from the trade, not a measured statistic. It's a useful map of where to look first when you see a ceiling stain. See spotting roof damage for what each looks like.