Hardwood flooring can be installed successfully on either on-grade or above-ground concrete slabs. When installing hardwood floors over concrete subflooring, appropriate MC levels must be achieved.
Wood and concrete both have independent, changeable moisture levels. In fact, MC in both materials fluctuates with changes in ambient relative humidity RH. High humidity causes wood and concrete to absorb water vapor; low humidity causes both to release water vapor into the air. However, it can also transfer from other sources. If a ground leak or faulty appliance introduces additional moisture to the concrete, it can upset the balance between ambient conditions and the conditions necessary to have hardwood look and perform at its best.
The reverse is also true. Therefore, installers must make sure that MC levels are stable in all building materials before combining them. Building experts recommend that flooring installers allow time for wood flooring products to equalize to their surrounding environment before installation. The same general rule-of-thumb holds true for concrete subfloors, so moisture content levels can stabilize. Moisture meter technology makes for precise and efficient moisture measurement in wood and concrete.
For best results, several areas of each room should be tested for excess moisture. If tests indicate too much moisture is in the slab, do not install hardwood floors. A new moist slab should be allowed to dry naturally or the process can be hastened with heat and ventilation. Slabs less than 60 days old are generally too wet to install floors.
Test for Dryness New concrete is heavy with moisture, and it must be a minimum of 30 days old before moisture testing begins. Determining moisture content of both the wood flooring and the subfloor is an important part of quality control in the flooring installation process.
Detailed qualitative and quantitative testing methods are provided by the National Wood Flooring Association. Flat, Clean Slab The slab must be flat with a trowel finish, free of grease, oil, stains and dust.
Grind off any high spots and fill any low spots. Vapor Retarder With solid hardwood floors, a proper vapor retarder must be installed on top of the slab to make sure normal slab moisture does not reach the finished floor. Layers of asphalt felt with asphalt mastic or 6 mil polyethylene sheathing are some of the choices.
Subfloors The National Wood Flooring Association recommends two choices in sub-flooring systems for concrete installations: plywood-on-slab and sleeper. Either subfloor system is satisfactory for three-quarter inch flooring. Asphalt felt or building paper: first, prime the slab and apply cold, cut-back asphalt mastic with a notched trowel 50 sq.
Allow to set for two hours. Unroll lb. Over this, apply a second similar coating of mastic and roll out a second layer of asphalt or paper in the same direction as the first, staggering the overlaps to achieve an even thickness. Polyethylene: Cover the entire slab with 4 to 6 mil polyethylene film, overlapping the edges 4" to 6", and allowing enough film to extend under the baseboard on all sides.
When moisture conditions are more severe, prime the slab and apply cold, cut-back mastic with a straightedge or fine-tooth trowel sq. Lay 4 to 6 mil polyethylene film over the slab, overlapping the edges 4 to 6". In either case, roll the film flat or "walk" it in, stepping on every square foot to ensure adhesion. Puncture bubbles to release trapped air. Install the plywood after the vapor retarder is in place.
Lay plywood diagonally across the direction of the finished floor to help prevent cracks along panel edges. This article, however, focuses only on the glue-down application. Nail-down installations are covered in this article. Regardless of whether or not the flooring is glued, nailed, or floated, engineered wood flooring needs to be acclimated to its environment before installation — just like solid wood flooring.
This can be done by periodically measuring the moisture content of the wood until it meets the guideline specs for installation. There are two types of wood moisture meters available; pin meters and pinless meters. The latter being chosen more often for their non-destructive electromagnetic measurement capability. Another thing to check is the moisture levels within the concrete slab.
Excessive moisture in a slab can damage an engineered wood floor and cause the plies to separate. Most flooring manufacturers require moisture tests for concrete moisture to validate their warranty requirements. Scientific research from leading academic and industrial institutions confirms the reliability and accuracy of the ASTM F RH test at these depths. Slab preparation is critical for glue-down and includes sanding, scraping, leveling, and filling low spots because the slab must be flat so the planks can fit correctly.
Before applying adhesives, you must fill in any voids or deflections in the slab with a cementitious patch or a self-leveling underlayment. An adhesive is not intended to fill voids or deflections. Use adhesives specially formulated for wood flooring so they conform to the natural characteristics of wood — expanding and contracting. That is, their elasticity ensures that the engineered wood has an ample amount of space to expand and contract without causing the glue to crack or separate.
Be advised: using the wrong adhesive or applying incorrect amounts can lead to a failed flooring installation. For example, use a wood floor adhesive that contains no water — like a moisture cure urethane or modified silane adhesive.
These products are a good choice for these installations because they increase the strength of the bonding agent and offer a degree of structural flexibility. A glue-down installation requires premium wood adhesives be properly troweled over the concrete slab and the engineered wood planks laid onto the adhesive and locked together at their tongue and groove joints.
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