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A methane mitigation system is mostly not membrane. Under a building in a mapped Los Angeles methane zone, the barrier is one layer in an assembly that also includes a vented rock blanket, a grid of perforated collection pipe, gas-tight boots at every pile and utility penetration, and risers carrying collected gas above the roofline. The membrane is the part that gets argued about in submittals, by material and by mil thickness. The layers around it are what keep gas from ever reaching the underside of the slab.
That distinction shows up in plan review, in inspection, and in the schedule, because all three track the full assembly rather than the sheet. In the City of Los Angeles, methane requirements sit in Division 71 of the Los Angeles Municipal Code, and the mitigation engineer of record designs to a site design level, I through V, assigned from the soil gas investigation. Two projects on the same block can carry identical membrane callouts and completely different vent and control systems.
The vent layer does the work in a passive system
The operating principle is a permeability contrast. Soil gas moves toward whatever path offers the least resistance. A clean open rock blanket under the slab, tied to atmosphere through risers, is a far easier route than a welded geomembrane, so gas travels laterally through the rock and out above the roof instead of collecting against the barrier and building pressure.
The usual build is crushed rock free of fines, perforated pipe laid on a grid inside it, and solid pipe carrying up through the structure to daylight. Where the section between subgrade and finish floor is tight, a manufactured gas venting geocomposite can take the place of part of the rock, which also gives the barrier crew a flat clean surface to deploy over. Pipe diameter, grid spacing, and riser locations come out of the mitigation design. Risers terminate above the roofline and away from mechanical air intakes, which is a coordination item with the mechanical engineer more often than teams expect.
None of that hardware needs power or an operator. That is what passive means here: the system runs on the pressure difference between soil gas and outside air.
Where the passive line ends
Passive protection holds when concentrations and subslab pressure are modest, and LADBS ties the step up to measured numbers rather than to judgment. Subslab gas pressure at or above 2 inches of water column pushes a site into the higher design levels on its own. Above 12,500 ppmv, which is 25 percent of methane's lower explosive limit, the site lands at Design Level V and a passive vent system is no longer accepted as the whole answer.
That is why the soil gas report controls the scope rather than the architecture. A small residential remodel over a former oil field can carry heavier mitigation than a large commercial pad two streets away. Nobody on the team can size the system, price it, or schedule it before the probes are read.
What an active system adds
An active system puts a blower on the vent network and holds the space under the slab at negative pressure, the same idea as sub-slab depressurization in radon work. The equipment list grows accordingly: a blower with a power feed and controls, methane sensors in enclosed spaces such as elevator pits, trench vaults, and unventilated utility rooms, an alarm panel, and in many designs an interlock that starts mechanical ventilation when a sensor trips.
Active hardware brings obligations that outlast construction, and design teams tend to underweight that. A blower needs power, a maintenance interval, sensor calibration, and a named person who knows what the panel means when it goes off at two in the morning. Handing a building over with an alarm nobody owns is a real failure mode, and it is a documentation problem long before it is an equipment problem.
Where these systems fail
On finished work the sheet itself is rarely the weak point. Punctures happen constantly during installation, and they get found and patched while the barrier is still exposed and the Deputy Inspector is standing there. The defects that survive to the pour are penetrations and damaged vent pipe.
Penetration count is the honest measure of difficulty on a methane job, more so than square footage. A podium slab threaded with grade beams, pile caps, and hundreds of plumbing and electrical stub-ups is harder than an open pad twice its area. That count is also what should drive material selection. Sheet membrane welds fast across long open runs. Spray-applied liquid boot over a geotextile carrier, which cures into a continuous barrier with no factory seams, earns its place where the detailing is dense and every transition would otherwise be a hand-cut boot.
Vent damage is the worse problem because it hides. Membrane defects are caught by smoke testing and seam testing before concrete goes down. A run of perforated pipe crushed under a loader track, or a riser packed with fines during backfill, passes every test the barrier gets, because none of those tests look at the vent network. It surfaces later as gas pressure under an occupied building, at which point the fix is on the other side of a slab.
What this changes for the construction sequence
The vent layer is placed before the barrier contractor mobilizes, usually by the grading or underground crew, and its condition sets the barrier crew's first day. Rock carrying fines, rutted subgrade, or pipe left proud of the blanket means the liner installer opens by repairing someone else's work while the membrane clock runs. A short walk of the vent layer with the barrier foreman before it is signed off costs an hour and saves days.
Inspection sequencing is the other schedule lever. LADBS requires continuous special inspection by a certified Deputy Inspector rather than one call at the end, and no concrete goes over the barrier until that certification is complete. Bringing the inspector in as penetration boots and seams are being finished, instead of after, is the difference between a signed certification and a pour date that slides while defects are chased across a fully installed barrier.
Worth being clear about scope, since it splits across trades in a way that surprises owners. EC Applications installs the membrane portion of these systems, seams or sprays it into a continuous gas-tight layer, seals the penetrations, and carries it through Deputy Inspector certification. The rock blanket, the collection piping, the blower, and the alarm panel are typically other contractors' scope. Crews out of the Anaheim headquarters have been doing the membrane side of this work since the company was founded in 2006.
So read the mitigation drawings for what the vent system asks of the site, not only for the membrane callout. Design level, riser locations, whether a blower and alarm panel are in the scope, and who owns that equipment after turnover are settled before the liner crew mobilizes, and every one of them is more expensive to change once the slab is down.
