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Test the Scratch Coat with a Scratch


Q: The building inspector uses a 16 penny common nail to test the scratch coat for proper hardness. If the scratch does not crumble, the scratch coat has properly hydrated. If not, the inspector makes us wet down the exterior walls for a day and tests it the same way. Is this the correct way to inspect exterior stucco scratch coats?

A: When a building inspector is involved, I think the best bet is to see what the code says before going too far a-field, lest I launch into yet another wild goose chase.

The code language in the 2009 International Building Code offers the following on the application of portland cement plaster (aka stucco):

“2510.3 Installation. Installation of these materials shall be in compliance with ASTM C926 and ASTM C1063.

“2512.6 Curing and interval. First and second coats of cement plaster shall be applied and moist cured as set forth in ASTM C926 and Table 2512.6.

[From Table 2512.6] “a. The first two coats shall be as required for the first coats of exterior plaster, except that the moist-curing time period between the first and second coats shall not be less than 24 hours. Moist curing shall not be required where job and weather conditions are favorable to the retention of moisture in the cement plaster for the required time period.

“2512.8.2 Curing. Curing of the first coat is permitted to be omitted and the second coat shall be cured as set forth in ASTM C926 and Table 2512.6.”

So far, no direct mention of a field test for checking the cure of the scratch or brown coats. Next step is to look at the referenced standard, ASTM C 926, which offers the following on curing of the scratch coats:

“7.1.9 Each coat shall be permitted to set before the next coat is applied. (See X1.4.2.)

“ The first (scratch) coat shall become sufficiently rigid to support the application of the second (brown) coat without damage to the monolithic continuity of the first (scratch) coat or its key.

“X1.4.2.1 The timing between coats will vary with climatic conditions and types of plaster base. Temperature and relative humidity extend or reduce the time between consecutive operations. Cold or wet weather lengthens and hot or dry weather shortens the time period. Moderate changes in temperature and relative humidity can be overcome by providing additional heating materials during cold weather and by reducing the absorption of the base by pre-wetting during hot or dry weather.”

Still no mention of how one actually determines whether the scratch coat is sufficiently set up to successfully accept the brown coat. So, where next? Google, of course. It took a couple of attempts to find the passage below, but eventually I came up with results using the search term: “field test stucco scratch coat with nail.” That combination brought up only 198,000 articles to peruse, but on the second page I found an article from the Journal of Light Construction (May 2006) titled “Stucco that Works” by Clayton DeKorne at

The salient passage reads as follows: “Before applying the brown coat, Webber [the stucco contractor] verifies the hardness of the scratch coat by dragging a nail over the wall. If the nail does not dig in but leaves a white line, the stucco is hard enough for the brown coat.” This is clearly not a slam-dunk, code- or ASTM-approved test method, but it does indicate that it is one test method relied upon by someone who was featured by a reputable publication as competent applicator.

Q: We are having an addition put on a house in Ottawa, Ontario, and the contractor is currently applying EIFS. The concern I have is with respect to temperature. In Ottawa it is approximately +4 degrees Celsius (39.2 degrees F) as a high and overnight it drops to minus 3 degrees (26.6F). As EIFS uses a cement-based product, is it acceptable to apply this at these temperatures?

A: A quick look at several technical data sheets offered these or similar passages regarding conditions for EIFS installation: “Ambient and surface temperatures shall be minimum 5ºC (41ºF) during installation. When installing in climatic temperatures below 5ºC (41ºF), tarping, heating and ventilation shall be provided to maintain proper installation temperatures. Ambient temperature shall be maintained above 5ºC (41ºF) for a minimum of 24 hours after installation to ensure that drying is complete. Allow for extended drying times in cool, higher humidity conditions.”

However, I recommend you find out which system is being installed and then download the technical data sheet from the manufacturer’s website to determine the specific recommendations for that system.

Lee G. Jones is AWCI’s director of technical services. Send your questions to, or call him directly at (703) 538.1611.

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