Laser Scanning for Roof Inspections: What the Scan Records

A roof is measured less often than the floors below it, and it keeps changing: units are set, curbs cut, drains moved, conduit run across the membrane, and little of it reaches a drawing. When a reroofing is designed, a rooftop unit replaced or a leak traced, the roofing consultant starts from a plan that may be thirty years old and a walk with a tape. This guide explains what a laser scan of a roof, a roof survey made of measured points, records, how it is done, what the consultant, the engineer and the roofing contractor get from it, and what it costs. The scan is a measured record of the roof as it stands; the inspection, the condition assessment and the design remain the consultant’s work, done on that record.

What a roof scan records

A terrestrial scanner of the Trimble X7 class, by its datasheet, measures up to 500,000 points a second over a range of 0.6 to 80 m, with a 3D point accuracy of 2.4 mm at 10 m and 3.5 mm at 20 m (one standard deviation). Set on the roof, it records every surface in its line of sight: the membrane and its low spots, every penetration, curb, drain and scupper, the parapet with its coping and its inner and outer faces, the units, ducts, conduit, gas lines and pipe supports that stand on the roof, the skylight frames and hatches, and the walls and equipment screens that rise above it. The result, once the stations are registered, is one point cloud in which any dimension can be taken afterwards: the distance between two curbs, the height of a parapet, the elevation of a drain rim against the membrane around it.

Because the scanner measures elevation at every point, the cloud shows where the roof is low. Water that stands after rain does so where the membrane sits below its surroundings, and that geometry is in the cloud as measured points; elevations can be read from the cloud at any point, and a spot-elevation or contour plan can be drawn from it on request. Whether the fall is enough for the drainage design is the consultant’s call.

Where the roof is dark, as black EPDM and asphalt are, the scanner runs in the high sensitivity mode its datasheet lists for dark and reflective surfaces, which takes about twice as long for the same point spacing. Standing water and glass return little of the beam, so a puddle and a skylight appear as gaps in the cloud with their edges measured.

How the scan is done

The scanner stands on a tripod on the roof and is moved from station to station, each taking a few minutes without images, depending on the point density chosen and on whether the dark-surface mode is needed. A flat roof needs more stations than its area suggests, because every unit, curb and screen hides what stands behind it, and each hidden face needs a station with a view of it; a roof dense with equipment is scanned from between the units. Where part of the roof cannot be walked, it is scanned from adjacent height where there is one, and the scope says what was seen from where. The stations are registered to each other in the field, with no targets to place, and the registration is checked before we leave the roof, so a missed face is rescanned the same day. When the roof is scanned together with the building, the roof stations are registered into the same cloud as the floors below, so a roof drain and the leader it feeds inside, where the leader is exposed, are measured in one coordinate system.

The scanner levels itself to survey grade, and the registration report delivered with the cloud lists the average error of each station and the overlap between scans. The accuracy of the cloud is stated in the quote in the U.S. Institute of Building Documentation’s Level of Accuracy terms, as a standard deviation at the 95 percent confidence level; the drawings are usually represented at LOA30, a deviation between 15 mm and 5 mm. The 3D laser scanning page describes the scan itself.

What is drawn from it

The scan on its own is delivered as the registered point cloud in E57, the ASTM exchange format for 3D imaging data, and in RCP/RCS for Revit, with the registration report and the panoramic images from each station, which give dated panoramic photographs of the roof on the day of the scan. What is drawn on top depends on the project:

• A roof plan in DWG and PDF with every penetration, curb, drain, scupper and unit located and dimensioned, the parapet heights and the roof edge, for a reroofing bid or a unit replacement. The as-built drawings page describes the sheet set.
• Sections through the parapet and the roof edge at the places the consultant names, with the coping, the parapet faces and the roof edge as they stand; what is under the membrane or behind the coping is not in the cloud.
• A spot-elevation or contour plan of the roof surface, on request, for the drainage design.
• A Revit model of the roof and its equipment, when the project is a new unit or a rerouted main that has to clear what is already there. The MEP BIM page describes that scope, and the scan-to-BIM page the model.

Price and time

A roof scanned on its own is priced on its area and the stations it needs. The registered point cloud starts at about $1,200, and a roof plan or sections on top are quoted by the roof. When the roof is scanned with the building, the price falls within the building ranges from recent projects: a house or an apartment with drawings or a model, $2,000 to $4,500; interiors with the facades and the site, $2,500 to $5,000; an office or a commercial floor, $3,000 to $9,000; large buildings from $12,000, quoted on their own. The exact price comes in a quote within one business day of your sending the address, a photograph or an aerial view of the roof and the deliverables the project needs. The scan is scheduled as soon as the project needs it, often within days of the order, and a roof is usually scanned in a day; the registered cloud usually follows within a few days and the drawings usually in two to three weeks. We scan in all 50 states.

Sources

• Trimble X7 3D laser scanning system datasheet: scan speed, range, 3D point accuracy, high sensitivity mode, self-leveling, scan times, registration report.
• USIBD Document C120, Level of Accuracy Specification Guide, version 2.0 (2016): the LOA levels as standard deviations at the 95 percent confidence level.
• ASTM E2807, Standard Specification for 3D Imaging Data Exchange: the E57 file format.