What kind of plastic goes under a house slab, and does it go under the gravel or on top of it?

Use 6-mil (minimum) polyethylene sheeting meeting ASTM D4397 (IRC R506.2.3). In many jurisdictions, 10-mil or 15-mil (e.g., Stego Wrap, Viper) is required for better puncture resistance and lower permeance (<0.01 perms).

Placement: On top of the gravel (capillary break), directly under the slab.

Assembly order (bottom up):

1. Compacted subgrade.
2. 4+ inches of clean, graded gravel/crushed stone (no fines; AASHTO #57 or similar). This is the capillary break and drainage layer.
3. Polyethylene vapor retarder/barrier.
4. Slab.

Critical details (IRC R506.2.3 / ACI 302.1R):

• Overlap seams 6–12 inches, seal with manufacturer’s tape (not duct tape).
• Seal all penetrations (pipes, conduits, footings) with tape/boots/mastic.
• No sand/blotter layer on top of the plastic. Place concrete directly on the vapor barrier to prevent trapped water bleeding up later (ACI 302.1R). Use a low-w/c mix (≤0.45) and proper curing to minimize curling/cracking risk.
• Extend poly up footings/grade beams and tape to foundation wall (or use termite barrier tape per local code).

Why not under gravel? Gravel angularity punctures thin poly; stone movement during pour tears it. The gravel layer must drain freely—plastic under it traps water against the subgrade.

Check local amendments: Radon zones (IRC Appendix F), termite shields, or energy codes (IECC) may dictate specific thickness, permeance, or sealing methods.

Under a concrete slab, the plastic used is typically 6-mil polyethylene vapor barrier (also called a vapor retarder or moisture barrier), meeting ASTM E1745 Class A, B, or C standards. This is required by most building codes (e.g., IRC R506.2.3) to prevent ground moisture from wicking up into the slab, which can cause flooring damage, mold, or efflorescence.

Placement: On top of the gravel, not under it.

Here’s the correct sequence from bottom to top:

1. Compacted subgrade (native soil, properly graded and tamped)
2. 4–6 inches of crushed stone or gravel (typically #57 or similar clean aggregate) — this provides drainage, load distribution, and a stable base
3. 6-mil polyethylene vapor barrier — laid directly on top of the gravel, overlapping seams by at least 6 inches and taped or sealed
4. Concrete slab (typically 4” thick, reinforced with rebar or wire mesh)

Why not under the gravel? Placing plastic under the gravel traps moisture against the soil, creating a hydrostatic pressure zone and defeating the purpose of drainage. The gravel layer must be free-draining; plastic beneath it would turn it into a bathtub. The vapor barrier must be above the aggregate to block rising moisture while allowing the gravel to drain laterally.

Important notes:

• Seal all penetrations (pipes, conduits) with tape or boots.
• In radon-prone areas, add a passive radon vent system beneath the plastic.
• Never use thin construction-grade poly (e.g., 4-mil) — it tears easily and fails long-term.
• Always check local amendments — some jurisdictions require specific vapor barrier classes or insulation under slabs in cold climates.

Bottom line: 6-mil poly on top of gravel, under the slab. Do not skip it — it’s cheap insurance against costly moisture problems. If unsure about soil conditions or local code, consult a building inspector or licensed contractor.