What installation actually consists of
Ask ten property owners what a roof installation involves and nine will describe removing the old shingles and nailing on new ones. That is roughly a quarter of the work and none of the difficulty. The parts that decide the outcome are the deck you find underneath, the water barrier you build before the covering goes on, the metal you form at every interruption, and the air path you leave underneath the whole thing.
A tear-off and a new-construction install differ mainly at the start. On new construction, the deck arrives clean and square and the roofing schedule has to be coordinated against framing and dry-in, because a deck left open to weather is a deck that will move. On a replacement, the deck is the unknown: it can be sound, it can be soft in patches around old leaks, or it can be an original board deck with gaps too wide for the fasteners the new covering requires. Everything after that point is the same discipline.
Sequence, start to finish
| Stage | What happens | Why it matters |
|---|---|---|
| Protection and staging | Ground cover over landscaping and hardscape, plywood against siding, disposal positioned, magnetic sweep planned for each day's end. | Most of the damage complaints in roofing are not roof damage. They are the flowerbed and the gutter face. |
| Tear-off | Covering, underlayment and old flashing removed down to bare deck, in sections small enough to dry in the same day. | Stripping more roof than you can cover before the evening is how a replacement turns into an interior repair. |
| Deck inspection | Every sheet walked and probed. Soft, delaminated or rotted sheathing is cut out and replaced, and the replacement count is photographed. | Fasteners in compromised sheathing never reach rated withdrawal strength, which is what fails under wind uplift. |
| Edge metal and barrier | Drip edge at eave, ice and water barrier at eaves, valleys, and around every penetration, then field underlayment. | This is the layer that keeps the building dry when wind drives rain under the covering, which happens on every roof eventually. |
| Flashing | New step flashing at wall intersections, counter-flashing at masonry, valley metal, pipe boots and curb detail. | Reused flashing is the single most common cause of leaks on an otherwise new roof. |
| Covering | Installed to the manufacturer's pattern for the wind zone, with starter course, correct exposure and proper nail placement. | High nailing on an architectural shingle is invisible from the ground and voids the wind warranty entirely. |
| Ventilation | Ridge or static exhaust balanced against soffit intake, calculated in net free area rather than by unit count. | Exhaust without matching intake pulls conditioned air out of the house and moisture into the deck. |
| Cleanup and handover | Full magnetic sweep, gutter clearing, debris removal, photograph set and documentation package delivered. | The file is what you hold anyone to later, including us. |
Underlayment is not a formality
Underlayment is the secondary water barrier — the layer that keeps the building dry during the days between tear-off and covering, and during every wind-driven rain for the rest of the roof's life. There are three broad categories and they are not interchangeable.
Asphalt-saturated felt is the traditional option, cheap and still code-compliant in most places. It tears in wind during installation, it wrinkles when it gets wet, and it degrades quickly if left exposed. It is adequate under asphalt shingles on a straightforward roof and unimpressive everywhere else.
Synthetic underlayment is a woven polymer sheet: lighter, far stronger in tear resistance, dimensionally stable when wet, and safer to walk on. It carries a longer exposure rating, which matters on any job where the covering will not go down the same week. Check the temperature rating before running it under metal, because panel surfaces reach temperatures that will degrade some products.
Self-adhering membrane, usually called ice and water barrier, is a rubberised asphalt sheet that bonds to the deck and seals around fasteners driven through it. This is what goes at the eaves, in the valleys, and around every penetration — the places where water is either standing or concentrated. In cold climates code typically requires it to extend from the eave to a point at least two feet inside the exterior wall line, and stopping short of that line is a very common shortcut.
Ventilation, in the terms that matter
Attic ventilation is the part of a roof installation most often done by habit rather than by calculation. The habit version is to add a ridge vent and call it done. The calculated version starts from net free area — the actual open square inches a vent provides, which is far less than its physical size — and balances exhaust at the ridge against intake at the soffit.
The common ratio requirement is one square foot of net free area per one hundred and fifty square feet of attic floor, halved to one per three hundred where a vapour retarder is present and the vents are balanced high and low. What matters more than the exact ratio is the balance. If exhaust exceeds intake, the ridge vent cannot draw from the soffit, so it draws from the path of least resistance instead: the ceiling penetrations of the living space below. You end up exhausting heated, humid indoor air into the attic, where it condenses on the underside of cold sheathing all winter.
- Blocked soffit intake from blown insulation is nearly universal in retrofitted attics, and baffles are the fix.
- Mixing vent types on one roof plane — ridge vent plus powered fan, or ridge vent plus gable vents — usually short-circuits the airflow.
- Bathroom and dryer exhaust terminating into the attic instead of through the roof adds an enormous moisture load. It gets found during tear-off more often than anyone would like.
Flashing: where the money is
If a roof leaks, look at the flashing first. Not because covering never fails, but because flashing is where the roof stops being a simple plane and someone has to think. Every chimney, every wall intersection, every valley, every vent stack and every curb is a place where two surfaces meet and water has to be persuaded to keep moving.
The reliable details are all mechanical rather than chemical. Step flashing at a sidewall means an individual piece of metal woven in with each course, lapping the one below, so the water path never depends on adhesion. Counter-flashing at masonry means metal let into a cut reglet in the mortar joint rather than surface-sealed against the brick. Valley metal means a formed pan carrying concentrated water down the intersection. Every one of these has a caulk-based imitation that looks identical from the driveway and fails in about four years.
Reusing flashing on a replacement roof is the version of this that costs property owners the most. The metal is bent, fatigued, often corroded at the fastener holes, and it has to be re-integrated with new courses it was never formed for. The saving is small. The failure is guaranteed and it happens under a covering that still has twenty years of life in it.
Ask for the dry-in photographs. Any roofer can promise new flashing and full ice barrier. Photographs of the bare deck, the barrier at the eaves and valleys, and the step flashing before the covering went on cost nothing to take and are the only proof that exists. A contractor who will not take them is telling you something.
Installation is where every other roofing decision either pays off or does not, which is why National Roofing Center writes the assembly before quoting a price. If the covering is still undecided, compare metal against asphalt shingle systems first.