How many storeys do you need for a clean rooftop-to-rooftop hop?

Somebody on the RF side asks how many storeys the building two blocks over needs before the hop clears, and there's no single number that answers it. It depends on the gap between the two rooftops, what's sitting in the path between them, and how much clearance the link needs at the frequency you're planning to run. Storey count is a stand-in for height, and height is only half the problem.

Storeys are a proxy, not the spec

A link budget doesn't care about floors. It cares about meters above ground at the antenna mount, plus whatever parapet, lift shaft, or rooftop plant is already up there blocking the aperture. Two buildings with the same storey count can clear very differently depending on floor-to-floor height, which runs anywhere from under 3 m in older residential stock to well over 4 m in a commercial tower with plant rooms between tenant floors. That's why a planner working a dense CBD corridor can't just count windows from the street and call it done.

What sets whether a hop clears is Fresnel zone clearance along the path, not a visual guess at how tall something looks. For a short urban hop you're trying to keep the first Fresnel zone, or at minimum 60% of it, clear of rooftops, trees, and clutter between the two ends. Add earth curvature on longer hops and the required mount height creeps up again. None of that math starts until you have a real height number for both ends, which is the part that usually stalls the desk review.

Working backward from the candidate list

In practice the planner isn't solving one hop. There's a shortlist of twenty or thirty candidate rooftops for a given corridor, and half the job is ruling out the ones that are obviously too short before anyone books a survey. The old way was pulling the local planning record for each address and hoping the permitted storey count matches what got built, which it often doesn't once you account for rooftop additions, mezzanines, or a building that went up a storey short of what was approved. The other old way was sending a surveyor to eyeball every candidate on the list, which is fine for the two or three finalists and a waste of a field day for the rest.

What closes that gap is a height estimate tied to the building footprint itself rather than a document about it: a per-building storey count derived from the rooftop height in current imagery, so the planner can rank candidates by clearance potential before committing a survey crew to anything. It won't replace the final walk-up measurement on the two buildings that make the shortlist, but it cuts the shortlist down from thirty to three.

Where this breaks down without current data

The failure mode planners run into most is a building that's changed since the last time anyone looked at it. A rooftop addition, a new plant enclosure, or a demolished floor all shift the real clearance number without touching the planning file. If the height data you're working from is a few years stale, you're back to guessing which candidates are worth a field visit, which is exactly the step this whole exercise was meant to skip.

Storey Estimation fills that gap with a per-building height and storey value pulled from very high resolution imagery and refreshed annually, so the number on your candidate list reflects current rooftop conditions rather than a decade-old planning approval.

If your next hop depends on knowing which rooftops clear before you book a survey, that's worth a look.