What happens when you build a roof, and why the order matters
Roof construction follows a fixed sequence: the roof frame (rafters or trusses) goes up first, then the roof deck (plywood or OSB sheathing), then the underlayment (a water barrier), and finally the roofing material (shingles, metal, tile, or other covering). Each layer depends on the one below it. If you skip underlayment or rush the fastening, water will find its way in within a few years, even if the shingles look fine. The entire process typically takes two to four weeks for a single-story house, depending on size, weather, and crew experience.
Most homeowners do not build their own roofs — the work is steep, the mistakes are expensive, and building codes require specific nailing patterns and material grades that inspectors will check. But understanding what the crew is actually doing helps you spot problems, know what to expect, and make decisions about materials and timing.
Key Takeaways
- Roof framing (rafters or trusses) must be sized and spaced according to your local building code and your roof's slope and snow load.
- Roof sheathing is nailed to the frame in a specific pattern, and all edges must be supported — gaps or misalignment will cause shingles to fail early.
- Underlayment goes down before shingles and acts as a second water barrier; it is not optional and must overlap correctly to shed water downhill.
- Flashing around chimneys, vents, and valleys must be installed before shingles and sealed properly, or water will leak at those exact spots.
- Shingles are nailed in a set pattern with a specific number of fasteners per shingle, and the pattern affects how wind and water move across the roof.
Roof framing: rafters versus trusses and why it matters
The roof frame is either stick-built rafters or prefabricated trusses. Rafters are individual boards cut and assembled on site; trusses are pre-engineered triangular frames built in a factory and delivered ready to install. Most new homes use trusses because they are faster and cheaper. Rafters are more common in older homes and in situations where the attic space needs to be open (like a cathedral ceiling).
The size and spacing of rafters or trusses depend on three things: your local building code, the roof slope (pitch), and the snow load your area receives. A steep roof in Arizona needs different framing than a shallow roof in Colorado. The building department will not issue a permit without engineered plans that show the frame can handle the load. If you are replacing a roof, the existing frame usually stays; if you are building new, the frame is designed before construction starts.
Rafters or trusses are fastened to the top of the wall (the top plate) with nails or bolts, and they must be braced and tied together so they do not spread under load. Collar ties or ceiling joists run horizontally between rafters to prevent them from pushing the walls outward. This bracing is not decorative — it is structural, and building inspectors will check it before you cover it up.
Roof sheathing: the base for everything that comes next
Once the frame is up, roof sheathing — usually plywood or oriented strand board (OSB) — is nailed to the rafters or trusses. The sheathing is typically 1/2 inch or 5/8 inch thick, and it must be fastened with nails spaced 6 inches apart around the edges and 12 inches apart in the field (the middle of each sheet). The spacing matters because it affects how the shingles will perform in wind.
All four edges of each sheet must be supported by a rafter or truss. If a sheet edge hangs over empty space, it will flex when someone walks on it or when wind pushes on the roof, and that movement will crack shingles and work fasteners loose. The sheets are staggered so that joints do not line up, which distributes stress and prevents a weak line running across the roof.
The sheathing is also where the roof slope becomes real. If the slope is wrong — too shallow for the climate, or installed at the wrong angle — water will not shed properly, and you will have leaks even with perfect shingles. The slope is set by the rafter angle and cannot be changed without rebuilding the frame.
Underlayment: the water barrier that most people never see
Underlayment is a layer of material — usually synthetic (plastic-based) or felt — that goes down on top of the sheathing and under the shingles. It is a second line of defense against water. When rain hits the shingles, some water runs off the surface, but some gets under the shingles (especially at the edges, around penetrations, or in high wind). The underlayment catches that water and directs it down and off the roof.
Underlayment is installed in horizontal strips, starting at the eaves and working up toward the peak. Each strip overlaps the one below it by at least 2 inches, so water always flows downhill over the overlap. The underlayment is fastened with nails or staples, but the fasteners are covered by the next strip, so they do not leak. At the peak, the strips meet and overlap, again with the upper strip on top.
Synthetic underlayment is more durable than felt and does not degrade in sunlight, so it can sit exposed for a few weeks if shingle installation is delayed. Felt is cheaper but can tear and deteriorate if left uncovered for months. Either way, underlayment is not optional — building codes require it, and skipping it voids most shingle warranties.
Flashing: the detail work that prevents leaks at weak points
Flashing is metal (usually aluminum or galvanized steel) that directs water away from places where the roof meets something else — a chimney, a vent pipe, a skylight, a wall, or a valley where two roof slopes meet. Flashing is installed before shingles, and it must overlap the underlayment above it and be sealed with roofing cement or sealant.
Valleys are especially important. A valley is where two roof slopes meet at an angle, and water concentrates there. Valley flashing must be wide enough to handle the volume of water, and it must be installed so that water flows down the center of the valley and off the edge, not sideways into the shingles. There are different valley styles (open, closed, woven), and each has different flashing requirements.
Chimney flashing is complex because it has to handle water coming from four directions. The flashing sits in a groove cut into the chimney (called a cricket or saddle), and it must be sealed with mortar or sealant. If the seal fails, water runs down the inside of the chimney and into the house. Vent fllanges sit on top of the underlayment, and shingles are nailed over the flange, creating a seal. If the nails are in the wrong place or if the flange is not sealed, water leaks around the vent.
Shingle installation: nailing pattern and overlap
Shingles are nailed to the sheathing through the underlayment. Each shingle is held down by a specific number of nails — usually four for standard asphalt shingles, sometimes six in high-wind areas. The nails must go through the nail strip (a band of adhesive on the shingle) and into the sheathing, not just into the underlayment. If a nail misses the sheathing, it will not hold the shingle down.
Shingles are laid in horizontal rows, starting at the eaves and working up toward the peak. The first row is often a starter strip — a full shingle or a special starter shingle — that hangs over the edge of the roof and provides a surface for water to run off. Each subsequent row overlaps the one below it, usually by 5 to 6 inches, so that water flows down over the overlap.
The shingles are also offset so that the joints between shingles in one row do not line up with the joints in the row above or below. This staggering distributes stress and prevents a weak line running up the roof. The offset is usually 6 inches, which means every other row starts with a full shingle and the rows in between start with a half shingle.
At the peak, the last row of shingles meets, and a ridge cap (a special shingle or a folded shingle) covers the joint. The ridge cap is nailed down and sealed so that water does not run into the joint. The same process happens at any hip or valley where two roof slopes meet.
Ventilation and soffit details during construction
As the roof goes up, the soffit (the underside of the roof overhang) and the fascia (the board at the edge) are installed. The soffit often has vents that allow air to flow into the attic, and the roof itself may have ridge vents or other exhaust vents that let air flow out. This ventilation prevents moisture from building up in the attic, which can rot the frame and shorten shingle life.
The vents must be installed before shingles go down, and the shingles must be cut and nailed around them. If a vent is installed after shingles, the seal will be poor and water will leak. The soffit vents must not be blocked by insulation or debris, or the ventilation system will not work.
Inspections and final details
Building inspectors check the roof at two points: after the sheathing is down (to verify the frame and sheathing are correct) and after the shingles are installed (to verify the nailing pattern, flashing, and overall installation). Some jurisdictions also inspect after underlayment is down. If the inspector finds problems, they must be fixed before the roof is considered complete.
After shingles are installed, the crew cleans up nails and debris, installs gutters and downspouts if they are part of the job, and does a final walkthrough. The homeowner should walk the roof with the contractor (or have someone do it for them) to verify that all flashing is sealed, all vents are properly installed, and no nails are sticking up or missing.
Frequently Asked Questions
Can I install shingles over an existing roof without removing the old one?
Yes, in most places you can install new shingles over old ones, but there are limits. Most codes allow only one layer of old shingles under new ones. If you already have two layers, the old roof must come off. Removing the old roof lets you inspect the sheathing for rot and install new underlayment, which is why many contractors recommend it even when layering is allowed.
What is the difference between asphalt, metal, and tile roofing during installation?
Asphalt shingles are nailed down and overlap like scales. Metal roofing is fastened with screws or clips and seams are sealed; it requires different flashing details. Tile is heavy and requires a reinforced frame; tiles are nailed or clipped to battens (horizontal strips) rather than directly to sheathing. Each material has different installation steps, but all require underlayment, flashing, and proper ventilation.
How long does a roof take to build from start to finish?
A typical single-story roof takes two to four weeks from framing to final inspection. A two-story or complex roof can take longer. Weather delays the work — rain stops shingle installation, and high winds can delay work for safety reasons. The crew size and the roof size also matter; a small crew on a large roof takes longer than a large crew on a small roof.
What happens if the roof frame is not level or square?
The roof slope is set by the rafter angle, not by leveling the frame. However, if the top plate (where the rafters attach) is not level, the roof will not look right and water may not shed properly. The frame is checked for square and level during construction, and problems are fixed before sheathing goes down. Once sheathing is on, fixing frame problems is much harder.
Do I need to ventilate the attic space under the roof?
Most building codes require attic ventilation to prevent moisture buildup and heat damage to shingles. Soffit vents bring in air, and ridge or gable vents let it out. The vents must be sized and spaced according to code, and they must not be blocked. Some newer homes use unvented attics with spray foam insulation instead, but this requires a different design and is not common in older construction.