--- title: "Why Polythene Sheet Is Used Between Walls and on Slabs" date: 2022-10-21 author: "Pawan Khatri" url: https://oceangeosynthetics.com/why-we-use-polythene-sheet-between-two-walls-on-slab/ --- October 21, 2022 | [Insights](https://oceangeosynthetics.com/category/insights/) # Why Polythene Sheet Is Used Between Walls and on Slabs A polythene sheet on a slab or between two walls is there to stop one of two things: water moving from one material into another, or two materials bonding when they need to move separately. Under a ground-floor slab it is a damp-proof membrane. Where a roof slab sits on a brick wall it is a slip layer. Between a new wall and an old one it is a bond breaker. This post explains each use, how to choose the sheet, and the site mistakes that make it useless. ## The five places polythene sheet goes in a building ### 1. Under a ground-floor slab or floor PCC Soil holds moisture, and concrete wicks it upwards. Without a barrier, damp rises through the floor, lifts tiles, stains flooring and keeps the room humid. A polythene sheet laid over the compacted, blinded sub-base before the concrete is poured stops that path. It does a second job during casting: it keeps the cement slurry from draining into the dry soil, so the concrete keeps the water it needs to hydrate and the underside does not end up weak and honeycombed. ### 2. Where an RCC roof slab bears on a masonry wall In load-bearing construction, the top slab heats up in the sun and expands far more than the shaded brickwork below. If the slab is cast directly onto the wall, it drags the top courses with it, and you get the familiar horizontal crack just below the slab in the top-floor rooms, often at the corners. Laying a polythene sheet (often two layers) on the wall’s bearing surface before casting lets the slab slide over the wall as it expands and contracts. This is a long-standing practice in Indian load-bearing buildings and costs very little. ### 3. Between two walls that must move separately When a new wall is built against an existing one, or two blocks of a building meet at a separation joint, the two walls settle and move differently. If mortar or plaster bonds them, the joint cracks unevenly. A polythene sheet between them stops the bond, so the crack forms where you planned it, and the joint can be sealed and finished neatly. In a cavity or double-leaf wall, the sheet works as a damp barrier instead. Rain driven into the outer brick leaf stops at the sheet, and the inner leaf and its room-side plaster stay dry. ### 4. On a roof or terrace slab, under screed or waterproofing A sheet laid over the slab before the screed, brickbat coba or tiles separates the layers. The top layers can then move with temperature without cracking the slab, and any water that gets through the top layer is stopped before it soaks the concrete. ### 5. Over fresh concrete for curing Covering a freshly finished slab with polythene traps the water that would otherwise evaporate in the sun and wind. It is a standard curing method on Indian sites where ponding or wet hessian is impractical. Lap the sheets and weigh down the edges so wind cannot lift them. ## Why polythene and not something else Polyethylene film lets almost no water or vapour through, survives decades buried in damp soil without rotting, and costs little per square metre. Low-density polyethylene (LDPE) is the usual choice for slabs and walls because it is flexible: it drapes over an uneven sub-base and folds around corners without cracking. High-density polyethylene (HDPE) is stiffer and more chemically resistant, and it is used where the sheet is a long-term waterproofing layer, for example on basement walls and in water-retaining structures. Bituminous membranes and liquid-applied coatings are alternatives for roofs and basements, but for a simple damp-proof membrane or slip layer, polythene does the job at the lowest cost. ## Choosing the sheet ### Thickness Indian sites quote polythene thickness in microns or in gauge. One micron is about four gauge, so a 250 micron sheet is roughly 1000 gauge. The right thickness depends on the job: - **Curing:** a thin sheet is enough, since it is temporary. - **Slip layer on wall bearings and separation joints:** moderate thickness, laid smooth. - **Damp-proof membrane under slabs:** a heavier sheet, because workers, rebar and concrete pumps will be on it before it is covered. - **Basement or below-ground waterproofing:** a geomembrane-grade sheet, often HDPE, with welded or taped joints. Follow the thickness on your drawings or specification. If none is given, choose heavier where puncture risk is high. Our post on [geomembrane thickness](https://oceangeosynthetics.com/what-is-the-thickness-of-the-geomembrane/) explains how thickness relates to puncture and durability. ### Virgin or recycled Black recycled polythene is common on Indian sites and sells cheaply. Its thickness can vary across the roll, it may carry pinholes, and its strength is unknown. For a damp-proof membrane that will be buried under a slab for the life of the building, ask for virgin LDPE with a stated thickness. BIS covers polyethylene films for construction and other uses in IS 2508 (fourth revision, 2024), and IS 3067:1988 is the code of practice for damp-proofing and waterproofing details in buildings. ### Check the weight, not only the label Thickness is easy to mislabel and hard to check on site without a micrometer. Weight is easy. LDPE has a density of about 0.92 g/cc, so: - 1 micron of LDPE over 1 sq m weighs about 0.92 g. - A 250 micron sheet should weigh about 0.25 × 0.92 = 0.23 kg per sq m. - A roll that weighs much less than its area × thickness × 0.92 is thinner than claimed. ## A worked example (illustrative) The numbers below are made up to show how to order sheet for a ground-floor slab. A ground floor measures 12 m × 10 m, with the DPM turned up 150 mm at the walls and sheets supplied in 4 m widths, lapped 200 mm. - Floor area: 12 × 10 = 120 sq m. - Upturn at the perimeter: 44 m × 0.15 m = 6.6 sq m. - Laps: three joints along the 12 m length, each 0.2 m wide = 3 × 12 × 0.2 = 7.2 sq m. - Subtotal: about 134 sq m; add 5% for cutting and damage, so order about 141 sq m. - At 250 micron, that is about 141 × 0.23 = 32 kg of sheet. ## Laying it so it works - **Prepare the base.** Compact the sub-base and blind it with sand or lean concrete so no sharp stones or brickbats poke through. - **Lap and tape.** Lap sheets by at least the amount on your specification and tape the laps. Untaped laps let moisture creep through. - **Turn up at the edges.** Bring the sheet up the wall face to meet the wall’s damp-proof course, so moisture cannot bypass the edge. - **Seal penetrations.** Pipes and conduits through the slab are the usual leak points. Tape a collar of sheet around each one. - **Protect it.** Use bar chairs with wide feet, lay boards for barrows, and repair any tear with a taped patch before the pour. - **Keep it out of the sun.** UV exposure weakens polythene left on site for weeks. ## Ocean products and projects Ocean supplies [LDPE sheets](https://oceangeosynthetics.com/products/ocean-geomembrane/low-density-polyethylene-ldpe-sheets/), [waterproofing sheets](https://oceangeosynthetics.com/products/ocean-geomembrane/waterproofing-sheet/) and [HDPE sheets](https://oceangeosynthetics.com/products/ocean-geomembrane/high-density-polyethylene-sheets-hdpe/) in custom thicknesses, and [dimple drain board](https://oceangeosynthetics.com/products/dimple-drain-board/) for basement walls. Project examples include an [LDPE liner used as a moisture barrier in building construction](https://oceangeosynthetics.com/ldpe-liner-in-construction-moisture-barrier/), [Ocean LDPE liner 300 micron on a road project in Yadgir, Karnataka](https://oceangeosynthetics.com/road-infrastructure-ldpe-liner-300-micron/), and [waterproofing work in Dwarka, Delhi](https://oceangeosynthetics.com/advance-waterproofing-solution-dwarka-delhi/). ## Frequently asked questions ### Why is polythene sheet laid before casting a slab? Under a ground-floor slab it stops ground moisture rising into the floor and stops the cement paste soaking away into the ground during setting. On a masonry wall bearing it lets the slab slide as it expands and contracts, which prevents cracks in the wall below. ### What thickness of polythene sheet is used under a slab? Use the thickness on your specification. Damp-proof membranes need heavier sheet than curing covers because they take foot traffic, rebar and concrete before they are covered. Check the delivered thickness by weight: a 250 micron LDPE sheet weighs about 0.23 kg per sq m. ### Why do builders put polythene between the slab and the wall? So the slab can move. A roof slab expands in the sun much more than the wall under it. The polythene stops the two bonding, so the slab slides instead of cracking the top courses of brickwork. ### Can black recycled polythene be used as a damp-proof membrane? It is cheap but its thickness and strength vary and it can have pinholes. For a membrane that stays under the slab for the life of the building, virgin LDPE of a stated thickness is the safer choice. ### Is polythene enough to waterproof a basement? For a dry site with low groundwater, a heavy sheet with taped joints plus good drainage may do. 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