The choice between a flat and a pitched roof is usually made by the building, not by the homeowner. Where a choice exists, it turns on drainage, the covering, the load, the maintenance each one needs and the cost of the scope. This guide compares the two side by side and sets out the order of the decision.
Key takeaways
- A flat or low-slope roof sheds water by a gentle fall, so drainage and seams decide its life.
- A pitched roof sheds water by gravity over shingles or another covering, so flashing and valleys decide its life.
- The roof's load and the City's snow-load value are checked for either type before the covering is chosen.
- Maintenance differs: flat roofs need regular drain and seam checks, pitched roofs need attention to edges, flashing and gutters.
- The scope decides the cost, and each type is quoted with its own lines.
How does each roof shed water?
A flat roof sheds water by a slight fall to a drain. A pitched roof sheds it by gravity over the covering to the edge.
A flat roof is built to drain by a slope that is barely visible, and it depends on drains and scuppers to carry the water away. When the fall is lost or a drain sits too high, water stands and works into the membrane. A pitched roof sheds water by its slope, and the covering carries it to the gutters without a drain to clog.
Because of this difference, the most important part of a flat roof is its drainage, and the most important part of a pitched roof is its edges and flashing. A replacement is designed around those parts.
- Flat: fall to the drains and scuppers
- Pitched: gravity over the covering to the edge
- Each type fails at its own weak point
What does each covering involve?
Flat roofs use a membrane system, such as single-ply or modified bitumen. Pitched roofs use shingles or another sloped covering, with underlayment and flashing.
A flat roof is covered with a membrane, which is usually laid in sheets and joined at seams. Its life depends on the membrane, the seams and the drainage, and it can be coated later to extend its life if the substrate is sound. A pitched roof is covered with shingles or another sloped covering, laid over underlayment, and the flashing ties it to the walls and penetrations.
Neither covering is better in every case. The right one depends on the building, the roof's shape and the maintenance the homeowner is willing to do, and the roofer explains the options for that building.
- Flat: membrane system with seams
- Pitched: shingles or sloped covering over underlayment
- Each covering has its own life and its own checks
What about load and the City's rules?
The load the roof must carry is set by the City of Albuquerque for the address, and the roof's shape changes how that load is applied.
Both types have to carry the City's load for the address, and the roofer confirms that value before the covering is chosen. A flat roof carries the load over its whole surface, and ponding adds to it, so its drainage is part of the load question. A pitched roof sheds much of the load by its slope, and the exposure and the slope are part of the calculation.
Ask the roofer to show the load and the framing check in writing. The City's permit and inspection steps apply to either type, and the permit question is answered by the Planning and Building Safety Division.
- Confirm the City's load value for the address
- Flat roofs: ponding adds to the load
- Pitched roofs: slope and exposure are part of the calculation
How should the choice be made?
In order: the building, the drainage, the load, the maintenance you will do, and then the scope and the cost. Write the reasons down.
Start with the building. If the existing roof is flat, a flat replacement is usually the honest choice unless the structure is changed. If it is pitched, the same logic holds. Where a change is being considered, the roofer should explain what it involves for the framing, the drainage and the permit.
Then look at the drainage and the load, then the maintenance you are prepared to do, and last the scope and its cost. Write each reason down in the quote, so the choice can be explained later to an insurer, a buyer or a future roofer.
- The building and its existing roof type
- Drainage and load for the address
- The maintenance you will do, then the scope and cost
What does a flat roof replacement usually include?
A tear-off to the deck or the insulation, a check of the drainage and the deck, the membrane system, the seam and edge detail, and the new drains or scuppers where they are needed.
A flat replacement is a system, not a single material. The old membrane and any loose insulation are removed, the deck is checked for soundness and any wet insulation is replaced, and the new membrane is laid to a fall that carries water to the drains. The edges, the parapets and the penetrations are detailed one by one, because those are the points where a flat roof most often fails.
The drainage part of the scope is often the largest single change. A drain that sat above the low point of the old roof is moved, and a scupper that dumped water down the wall is re-formed. These lines should be written in the quote so the homeowner can see what changes the roof's behaviour as well as its covering.
- Tear-off to the deck or the insulation
- Deck and insulation check, with wet areas replaced
- Drains and scuppers set to the low points
What does a pitched roof replacement usually include?
A tear-off of the shingles, a check of the deck, new underlayment, new flashing at the walls and penetrations, new valley metal, and the covering laid to the manufacturer's detail.
A pitched replacement is most often a covering job, but the parts that decide how long it lasts are underneath and at the joints. The old covering is removed, the deck is checked for soft or split boards, and a new underlayment is laid to shed any water that gets past the covering. The flashing at walls, chimneys and vents is renewed, because the old flashing rarely comes off in one piece.
Valleys and the ridge are the other places a pitched roof is most often weak. Valley metal is replaced where it is worn and the ridge vent is checked for its fixings. A written scope names each of these, so the homeowner knows the job is not only the shingles on the field of the roof.
- Tear-off and deck check
- New underlayment and flashing at each joint
- Valley metal and ridge detail renewed
How does maintenance differ over the life of each roof?
A flat roof needs regular drain, seam and parapet checks; a pitched roof needs attention to its edges, flashing, valleys and gutters. Both need records.
The maintenance a roof needs is one of the most practical differences between the two types. A flat roof asks for regular attention at its drains, its seams and its parapets. Those parts are easy to neglect because they are not seen from the street, and a drain that is not cleared becomes a pond that wears the membrane. A pitched roof asks for attention at its edges, its flashing, its valleys and its gutters, which are easier to see from the ground and therefore easier to keep up.
Whichever type is chosen, the homeowner should keep a simple record: the date of each check, what was seen and what was done. That record is what makes a later decision, about repair or replacement, rest on evidence rather than on memory.
- Flat: drains, seams and parapets checked regularly
- Pitched: edges, flashing, valleys and gutters watched
- Both: a dated maintenance record
What should a homeowner ask about warranties?
Who gives the warranty, what it covers, what it requires in the way of inspection and what voids it. Get the terms in writing before the work is booked.
A warranty on a roof can come from the manufacturer of the covering, from the installer or from both. They cover different things. A manufacturer's warranty usually concerns the material, and an installer's warranty concerns the workmanship. Ask which is which, and ask for each in writing.
Ask also what the warranty requires. Some require an inspection at set intervals, some require that only approved products are used in repairs and some are voided by a change to the roof. The terms decide whether a later repair keeps the warranty in force, so they should be read before the job is signed.
- Who gives the warranty and what it covers
- What it requires, such as inspections
- What voids it, including later repairs
Frequently asked questions
Can a flat roof be changed to pitched?
Sometimes, but it is a structural change with its own load and permit questions. The roofer and the City should be consulted before any decision.
Which type lasts longer?
Life depends on the material, the workmanship and the maintenance, not only the type. The roofer can explain the expected life of each option.
Is a flat roof cheaper?
The price depends on the scope, and no figure is given on this site. Compare written quotes line by line.
Does the choice affect insurance?
It can. Ask the insurer what it needs for the roof type you choose, and keep the written scope with the policy papers.
Sources
This guide is general information for Bernalillo County homeowners, not a substitute for an on-site inspection. Prices are estimates. See our disclaimer.