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That depends on the member sizes, spans and connections, so only the design or a structural engineer can answer it. What is knowable is the weight you are adding: about 250 pounds per square for architectural shingles, 150 for standing-seam metal and 900 for standard concrete tile.
Roof loads are measured in pounds per square foot, and the difference between roofing materials is far larger than most people expect. A single ply membrane weighs about 1.2 psf and a metal panel around 1.5, asphalt shingles run 2.5 to 4.5 depending on grade, and concrete or clay tile reaches 9 to 12 — which is why switching from shingles to tile is a framing decision rather than a finish decision. This calculator sums the covering (multiplied by the number of existing layers), the sheathing, a framing and underlayment allowance, and an optional solar array at roughly 4 psf including racking, then adds your snow or live load to produce the combined load in psf and the total weight in pounds across the whole roof. It compares the live load against the design value you enter so you can see whether margin remains. It is a planning tool for the conversation with a structural engineer or solar installer, not a structural analysis, and it does not check your rafter size, span or grade.
Roof load capacity is a structural question with two halves: the dead load, which is the permanent weight of the roof itself, and the live load, which is snow, wind, people and anything added later.
| Covering | Weight per square | On a 25-square roof |
|---|---|---|
| Three-tab asphalt shingles | ~200 lb | ~5,000 lb |
| Architectural asphalt shingles | ~250 lb | ~6,250 lb |
| Standing-seam metal | ~150 lb | ~3,750 lb |
| Wood shake | ~350 lb | ~8,750 lb |
| Lightweight concrete tile | ~600 lb | ~15,000 lb |
| Standard concrete tile | ~900 lb | ~22,500 lb |
| Slate | ~800 – 1,500 lb | ~20,000 – 37,500 lb |
Add the sheathing, the framing and any existing layer that is staying on. A second layer of asphalt shingles adds its whole weight again to the dead load.
A calculator gives you the weight. Whether a specific frame carries it is a design question about member sizes, spans, connections and load paths, and it is not one to answer from a table.
It depends on the member sizes, spans, spacing and connections, so the honest answer comes from the design or from a structural engineer. What a calculator gives you is the weight you are proposing to add.
Around 250 pounds per square for architectural asphalt shingles, 150 for standing-seam metal, 900 for standard concrete tile and up to 1,500 for slate. On a 25-square roof that is a range from under two tons to nearly nineteen.
Usually yes — an array adds roughly 2 to 4 pounds per square foot including racking, which most roofs carry. In a heavy snow-load region the combined figure is worth checking against the design.
Most codes allow one overlay over a single existing layer, never a third. It adds the full weight of another layer to the dead load, so on an older frame it is worth checking rather than assuming.
Checking whether the roof can take tile? Price it with the clay tile roof cost calculator.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.