Understanding your result
The headline is the roofing dead load per square foot after the work, with the total weight and the change from today. The stats give the total, the change and the load with the deck; the bars compare before and after.
The number to give an engineer is the change: the extra pounds per square foot the framing must carry. The materials themselves are counted by the tile calculator, the slate calculator and the synthetic slate calculator; the old roof’s weight going to the dumpster is in the tear-off calculator.
How we calculate this
The mid-points of the manufacturers’ ranges are used because the range for each material is wide; for a specific product, its data sheet weight is better. The material weights table lists the ranges.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Material weights per square | as in the material weights table | Manufacturers’ installed weights |
| Sheathing and underlayment | 1.7 lb per sq ft | ½ in plywood about 1.5; underlayment about 0.2 |
| Overlays | one at most | IRC R908.3.1.1 |
Assumptions last reviewed October 7, 2026.
The calculator does not add snow, wind or live loads, check spans or size members. The guide on tile and slate weight covers what an engineer looks at.
Two worked examples
Concrete tile replacing architectural shingles
2,000 square feet.
- Tile: 10 lb per sq ft; shingles: 3
- After: 10 lb per sq ft, 10 tons; change +7 lb per sq ft
- With the deck: 11.7 lb per sq ft; an engineer should check the framing
Metal over one layer of shingles
The same roof, exposed-fastener metal as an overlay.
- After: 3 + 1.15 = 4.2 lb per sq ft, 4.15 tons; change +1.2
A modest increase, within the margin most framing has, but still worth confirming under snow.
Where to find your inputs
Roof area. From the roof area calculator.
Materials. The new and the existing roofing.
Overlay. Whether the old roof stays.
Common mistakes
- Tile on shingle framing without a check. It is three times the load.
- Forgetting an overlay’s old roof. Both weights count.
- Ignoring snow. Dead load uses up the margin snow needs.
- Using the heaviest figure as typical. Use the product’s data sheet.
- Treating weight as capacity. Only an engineer answers capacity.
- Two overlays. Not allowed.
Questions people ask
- How much does a roof weigh per square foot?
- Roofing alone, at the mid-point of manufacturers' figures: asphalt shingles 2.4 to 4 pounds per square foot, metal about 1 to 1.5, cedar shakes 3.5, synthetic slate 3.3, concrete tile about 10, clay tile 8.5 and natural slate about 10. Add about 1.7 pounds for plywood sheathing and underlayment.
- Can my roof hold tile?
- Concrete tile adds about 7 pounds per square foot over architectural shingles, a 2,000 square foot roof gaining 7 tons. Framing sized for shingles may not carry that with its snow load. A structural engineer reviews the rafters or trusses and specifies any reinforcement; lightweight tile is an alternative.
- How much does a metal roof weigh?
- About 80 to 150 pounds per square, 1 to 1.5 pounds per square foot, a third to a half of asphalt shingles. That is why metal is often laid over one layer of shingles where codes allow.
- Can I lay a new roof over the old one?
- Codes allow at most one overlay (IRC R908.3.1.1), not over wood shakes, slate or tile, and only if the existing roof and deck are sound. The weights add up, which the calculator shows when overlay is ticked.
- What is dead load?
- The permanent weight of the roof structure and roofing, as opposed to live loads (people, equipment) and environmental loads (snow, wind). Framing is designed for all of them together; changing the dead load changes what is left for snow.
- Does the calculator check the framing?
- No. Whether a roof can carry a load depends on span, spacing, lumber grade and snow load, which the span tables in the code or an engineer answer. The calculator reports the load.