Radon Sub-Slab Suction Points From a Communication Test

A test hole is drilled, vacuum is applied at a candidate location, and a micromanometer is read at holes at increasing distances: the field reaches the ones that show a measurable pressure difference and does not reach the ones that do not. That bracket is the design input, and this converts it into a covered area and a point count. THE MEASUREMENT IS THE DELIVERABLE, because nothing about the field is calculable in advance. It depends on the aggregate under the slab, on how much fine material has migrated into it, on the slab's own cracks and penetrations, and on the soil below -- and those vary between houses on the same street and between ends of the same house. Clean gravel can carry a field twenty-five or thirty feet from one point; a slab poured directly on compacted fines can give a field of a few feet. A BIGGER FAN DOES NOT FIX POOR COMMUNICATION. On a tight sub-slab the fan simply pulls harder on the same small area, and the far corner of the slab stays at no measurable pressure difference regardless. More suction points is the answer, and the number of them is what the test determines. SHAPE MATTERS AS MUCH AS AREA, which a coverage figure alone hides. Points cover a circle, so a long narrow slab needs points spaced along its length even where the total square footage would suggest fewer, and both checks are reported here with the larger governing. A compact rectangle and a corridor of the same area are different jobs. AND THE TIMING IS THE PRACTICAL POINT. The test costs about an hour before installation and settles the point count; the same finding after a single-point system fails its post-mitigation measurement costs a second mobilisation, another roof or rim penetration, and the homeowner's confidence in the diagnosis. This converts entered test readings into a point count. It does not predict field extension, design the pit itself (its size and the material excavated under the slab affect communication substantially), locate points against the building's layout, foundation walls, or finished areas, size the fan or the piping, address sealing of cracks, sumps, and floor drains -- which is part of the same system and often the larger effect -- evaluate a radon measurement, or determine what any standard requires. ANSI/AARST RMS-LB and SGM-SF, a diagnostic communication test, and the certified mitigation professional govern.

area per point = pi x the confirmed radius squared; points on area = slab area / that; points along the longest run = ceil(run / two radii); the LARGER of the two governs.

A diagnostic communication test -- vacuum applied at a candidate point, a micromanometer read at test holes at increasing distances -- converted into a point count. The reach is MEASURED because nothing about it is calculable.

Circle area and a spacing count on entered test readings.

Estimate. AHJ and licensed professional govern.

Field names used by the API: slab_area_ft2, reaches_ft, fails_ft, slab_length_ft, slab_width_ft, existing_points, effective_radius_ft, area_per_point_ft2, points_required, coverage_ratio, longest_dimension_ft, points_by_length, governing_points

• The confirmed radius is the design input not the distance at which vacuum was absentthe communication test
• Circular field assumed footings and slab joints compartmentalise it in realitythe mitigation professional
• Points spaced two radii apart the standard non-overlapping spacingsub-slab depressurisation practice