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Installing Urethane Mortar Flooring Over Plywood

Urethane Mortar Flooring provides impressive properties that simply cannot be achieved using other types of finishes. The material is designed to be installed over a well-prepared concrete substrate. But what if the job calls for urethane mortar over a plywood sub-floor? It’s not an ideal situation, but careful planning, attention to detail and the below guidelines can help:

wineries-and-distilleries-antimicrobial-protectionThe Plywood Subfloor

The plywood subfloor must be structurally sound and able to support the installation. FloroCrete floors are heavy and FloroCrete is a hard material. It can break or dislodge (just like a tiled floor) if the surface bends under the load. Joists should be 16″ on center.

The plywood must be installed over an adequate subfloor and must be at least 1 1/4″ thick when combined with the subfloor. Use two layers of exterior grade or better plywood, with the top layer screwed every 8″ on the edge and 6″ in the field. Make sure the joints are well away from the joints of the bottom layer. Leave a 1/8” gap for the joints. When screwing down the top layer go no further then the bottom layer of plywood.

Interior-grade plywood and particleboard are not considered a strong enough for a FloroCrete installation. You will need an exterior grade or better. 1/2″ concrete board may be used.

Once the sub-floor is completed, it should be checked for bounce.

Calculating Deflection

Many experienced contractors can judge a floor’s deflection by walking around on it. However, to calculate a specific value, divide the total span of the floor joists (in inches) by 720 to determine the maximum amount the floor can give in the middle under a live load of 300 lb./sq. ft., plus any long-term deflection due to the weight of the floor. For example, the maximum deflection for a joist span of 15′ is [15 * 12] / 720 = 1/4″.

Reducing Deflection

According to TCNA, there are several factors that affect deflection, including the condition of the sub-floor and joists, joist size, type and spacing, and how the sub-floor was fastened.


Adding another layer of plywood can reduce deflection and curvature of the sheathing between joists. Take care to stagger the joints and gap the panels 1/8″ on each edge. Turning panels perpendicular to joists and not aligning joints with joists (pg. 23 of DITRA Handbook) can also help. In extreme cases, additional joists may need to be put in place.

Treatment of Joints

To help reduce plywood joint movement, the joints should be filled with a urethane elastomer like Florock 6500. Fiberglass reinforcement embedded into the elastomer may be added to the joint to help prevent joint transfer lines. Joint transfer lines are cracks that develop in the FloroCrete from excessive movement of the substrate after coatings are applied.

Once the sub floor has been properly addressed, including joints, FloroCrete may be installed using the same standard methods as over a concrete substrate. Please refer to the Florock Technical Data Sheets on the FloroCrete system you will be installing.

To learn more about FloroCrete Cementitious Urethane Flooring or to get a quote on installation for your facility, simply contact us today.
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