Dry film thickness (DFT)
Dry film thickness is the coating thickness after solvent flashes off, measured in microns. AS 4654 sets minimums; DFT verification is the install-acceptance gate.
Ask Chalkline about this →Dry film thickness (DFT) is the thickness of a liquid-applied coating or waterproofing membrane after the solvent and water have flashed off and the coating is cured, measured in microns (µm) or, for thicker membrane systems, millimetres (mm). It is distinct from wet film thickness (WFT), the thickness measured immediately after application while the coating is still wet. The two differ by the volume of solids in the product: a 1.0 mm wet film of a 50%-solids coating dries to 0.5 mm DFT.
The DFT specification is the contractual and statutory acceptance gate for the installed coating. For external above-ground waterproofing, AS 4654.1 / .2:2012 sets minimum DFT per exposure class (verified 2026-05-16):
| Exposure class | Application | Minimum DFT (indicative, check current AS 4654.1 product table) |
|---|---|---|
| Cat I | Sheltered, low-traffic | 1.5 mm DFT |
| Cat II | General external above-ground | 2.0 mm DFT |
| Cat III | Trafficable, high exposure | 2.5 to 3.0 mm DFT |
For internal wet areas under AS 3740:2021, the DFT is set in the manufacturer’s installation data for the specific membrane system. Standard wet-area liquid membranes commonly require 1.0 to 2.0 mm DFT delivered in two or three coats.
Why multiple thin coats, not one thick coat. Liquid-applied coatings have a maximum single-coat WFT before they pinhole, run, or cure unevenly. The DFT specification is reached by building up coats: first coat goes down at WFT well under the maximum per-coat limit, dries to its DFT contribution, second coat overlaps, dries, third coat where required. Applying a single coat at 3x WFT to reach the spec produces a defective coating: pinholes on vertical surfaces, uneven cure depth, surface skin over uncured interior.
How DFT is verified at install:
- Wet film gauge: a hand-held comb dragged through the wet coating shows the lowest tooth that registers wet. Sets a target WFT during application.
- Dry film gauge: a hand-held thickness meter (electromagnetic for ferrous substrates, eddy current for non-ferrous, or destructive cross-section sampling) measured at a sample density per AS 4654.2 (commonly one reading per 10 m² in residential).
- Documentation: the dry-film readings are logged on the waterproofing certificate that the trade issues at completion. The certifier checks the certificate at the critical stage inspection.
Also known as: DFT; dry coating thickness; cured film thickness.
Category: Materials.
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Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.