Measuring parapets, flashings and flat roofs from aerial imagery

From aerial imagery you can measure a parapet's length, every flashing run where a roof meets a wall, and a flat roof's area. You cannot measure parapet height, curb height, drains or what sits under the membrane. A top-down image gives you everything that runs across the roof and nothing that stands up from it.

Parapet walls: length yes, height no#

The parapet is usually the easiest edge on a commercial roof to trace. The question is which side. The outer edge of the coping is the building line; the inner edge is where the roof field stops. Trace the inside face. The membrane covers the field and turns up the inside of the wall; the outside is cladding, not roofing.

What the wrong line costs on a building that measures 100 × 80 ft outside, with a 12 in parapet wall:

• Outside line: 100 × 80 = 8,000 sq ft
• Inside face: 98 × 78 = 7,644 sq ft
• Difference: 356 sq ft, about 3.6 squares of membrane and insulation for a roof field that isn't there

The inside perimeter, 2 × (98 + 78) = 352 ft, is your parapet flashing length. In RoofStruct, label those edges Parapet Wall in the "What is this edge?" picker and the length lands in the report as parapet wall flashing.

Height is the number the image can't give you, and it drives the flashing material. If your detail takes the membrane up the inside face and over the top under the coping, each linear foot needs the wall height plus the 1 ft wall width:

• 18 in parapet: 352 × (1.5 + 1.0) = 880 sq ft
• 36 in parapet: 352 × (3.0 + 1.0) = 1,408 sq ft

Same roof, same trace, 528 sq ft apart: 5.3 squares of membrane riding on a number you get somewhere else. A street-level photo works (count block courses, 8 in nominal each), as do building plans, a previous job file, or the owner with a tape in the picture.

Flat roofs: counted flat, and the slope barely matters#

A flat roof has no rise, so its plan area is its true area: pitch factor 1.0. In RoofStruct, a facet with no pitch is "counted flat" (the builder shows how many facets have no pitch, so none get left flat by accident).

Most "flat" roofs are built with a small slope to drain, often 1/4:12. Here is why that doesn't change your squares:

Pitch factor = √(rise² + 12²) ÷ 12

• 1/4:12: √(0.0625 + 144) ÷ 12 = √144.0625 ÷ 12 = 12.0026 ÷ 12 = 1.0002
• 1/2:12: √(0.25 + 144) ÷ 12 = √144.25 ÷ 12 = 12.0104 ÷ 12 = 1.0009

On the 7,644 sq ft roof field above, 1/4:12 adds 7,644 × 0.0002 = about 1.7 sq ft, and 1/2:12 about 6.6 sq ft. Not a quarter of one 4 × 8 insulation board. The parapet line you trace matters about 200 times more than the drainage slope.

Flashing where a roof meets a wall#

Where a pitched roof runs into a wall, the line where shingles meet siding is visible from above. On an off-angle image you may see the wall face itself; the flashing line is at its bottom.

Label each run by how it meets the wall:

• Headwall: the slope ends against a wall face. It runs level, so the traced length is the order length.
• Step: the slope runs up alongside a wall. It climbs with the roof, so its true length comes from the pitch. A wall beside a 6/12 slope with 14 ft of plan run: 14 × 1.118 = 15.7 ft.
• Sidewall: a wall running along the side of a slope.
• Transition: where a slope changes pitch, such as a low-slope porch roof continuing off a steeper main roof.

A step flashing run on a facet with no pitch comes out at plan length, which is short. Set the pitch first.

Curbs and drains#

The image shows the rooftop unit, not the curb under it, and a unit often overhangs its curb. In RoofStruct, trace the curb as a Curb in Obstructions & Exclusions: its area comes out of the roof and curb flashing is counted around its perimeter. A 7 × 4 ft curb carries 2 × (7 + 4) = 22 ft. Its height is not in the photo.

Drains and scuppers may show as dark dots or parapet notches. Count them, but size, drain versus vent, and anything under ponding stay unknown.

Multiple roof levels#

Commercial buildings often step down to a lower roof over a loading dock or addition, and the wall between levels is flashing on the lower roof. In RoofStruct, draw the highest roof as the Main roof, then Add layer for each lower roof. Trace the lower roof up to the wall and label that edge (headwall, or parapet on a low-slope roof). On a leaning image the tall wall can hide a strip of the lower roof, so check that edge against a second view.

What top-down imagery can't show#

Parapet and curb heights, drain sizes, insulation thickness, tapered insulation and crickets, existing roof layers, deck type. Those come from a site visit, plans or core cuts. The image gets you the plan; the field gets you the vertical.

Where you trip#

• Tracing the outside of the parapet. You order roof for the top of the wall.
• Guessing parapet height. It moves membrane more than anything else on the roof.
• Tracing the unit instead of the curb. The unit is usually bigger.
• Missing the low roof behind the tall wall. It has its own flashing.

Measuring it yourself#

RoofStruct gives you free, unlimited self-serve measuring and PDF reports, no card, on satellite imagery or your own drone or phone photo. The measuring tools cover edge labels, obstructions and layers. Start with measure your first roof, then how to measure a roof from satellite imagery and how accurate satellite roof measurements are.

FAQ#

Can you measure parapet height from aerial imagery?#

Not reliably. A top-down image shows the parapet's length and coping, not its height. Get it from a street-level photo, plans or a site visit.

Should I add a pitch to a flat roof with a 1/4:12 slope?#

You can, but it won't change your order: the pitch factor is 1.0002, under 2 sq ft on a 7,644 sq ft roof. Counted flat is fine.

Which side of the parapet do I trace?#

The inside face, where the roof field meets the wall. The outside line adds the wall's thickness all the way around.

How do I count flashing around rooftop units?#

Trace each curb as a Curb obstruction. Its area comes out of the roof and curb flashing is counted around its perimeter. Confirm curb height on site.

Trace the inside of the wall, count the field flat, and get the heights from someone standing on the roof.