Concrete Subfloors
- Subfloor Must Be Level
- Ensure the concrete slab is flat, with the tolerance being 3/16” over 10’.
- If the slab is out of specification, consider grinding, floating, or both. Many high spots can be removed by grinding; depressions can be filled with approved patching compounds; and slabs can also be flattened using self-levelling concrete products.
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When sanding or grinding concrete, be sure to wear a mask. Apply water to the floor before sanding to limit dust.
- Subfloor Must Be Dry: Refer to Chapter 6 (Moisture Testing)
Understanding Concrete
Is your Vapor Barrier Properly Installed below the slab?
Concrete Preparation
• Minimum: 30 day old concrete
• Concrete must be free of any contaminates such as: wax, oil, grease, paint or curing compound
• May be necessary to scar, shotblast, grind
• Removal must be deep enough to eliminate all contaminants and produce necessary profile.
• Majority of concrete floors will require patching to flatten or repair
• Portland based products recommended.
Concrete Moisture MYTHS
NWFA — National Wood Flooring Association
- Moisture vapor can't occur in a 30-year-old concrete slab
- Depending on the vapor barrier, the structure of the concrete, the design of the base materials and the placement of the concrete, older concrete can also emit high levels of moisture.
- Moisture-induced flooring failures can't happen on a suspended slab
- Quite the contrary, many problems occur in slabs above grade with a non-perforated steel pan construction. Free water has no place to go other than up into the flooring installation.
- It can't be moisture, it was dry when I installed it
- A dry slab today does not guarantee a dry slab tomorrow. Conditions change like weather, landscaping/irrigation, maintenance and interior environments. All this can drive moisture into your flooring installation through a concrete slab.
Concrete Moisture FACTS
NWFA — National Wood Flooring Association
- Moisture migration and concrete permeability are affected by the water-to-cement ratio when it is mixed.
- More water in the cement mixture means more permeability.
- Age of the slab is also a critical factor
- 90% free water when first mixed
- Water used in the mix (free water) leaves within 9 months
- In normal circumstances this slab will reach balance within 90 days
- Concrete curing compounds slow down the release of free water
- Don’t assume a slab is dry because of its age. These chemicals can also be a cause of adhesive bond failure.
