Wooden flooring brings warmth and elegance to any space, but installing it over a concrete slab requires careful planning and execution. Moisture can be a significant issue, potentially leading to swelling or buckling of the wood. This guide provides detailed steps and precautions to ensure a successful installation that stands the test of time.
Installing wooden flooring over concrete isn’t as straightforward as it might seem. Concrete slabs can harbor moisture, which can seep into the wood, causing damage. Proper installation techniques are essential to prevent moisture-related problems and to maintain the durability and beauty of the wooden floor.
Importance of Vapor Barriers
A crucial step in the installation process is the use of polyethylene vapor barriers both beneath and atop the concrete slab.
- Purpose of Vapor Barriers: They act as a shield against moisture penetration from the ground into the concrete slab. By blocking this moisture, you prevent potential swelling, warping, or buckling of the wooden flooring.
- Placement: The vapor barrier should be laid directly on the ground before the concrete slab is poured and again on top of the cured concrete slab before installing the wooden flooring.
Moisture Testing of Concrete Slab
Before proceeding with the installation, it’s imperative to test the concrete slab for moisture content. Wood is highly sensitive to moisture, and any excess can compromise the flooring’s integrity.
The Oak Flooring Institute of the National Oak Flooring Manufacturers Association recommends several tests to assess moisture levels:
A. Rubber Mat Test
- Method:
- Place a flat, non-corrugated rubber mat on the concrete slab.
- Add weight on top to ensure it remains in place and to prevent air circulation.
- Leave the setup undisturbed overnight.
- Outcome:
- Upon removing the mat the next day, check for water marks on the concrete surface.
- Presence of water marks indicates trapped moisture within the concrete, signaling that the slab isn’t ready for wooden flooring.
- Limitations:
- This test may not be effective on slabs with dark surfaces, as water marks might be less visible.
B. Calcium Chloride Test
- Method:
- Place a quarter teaspoon of anhydrous calcium chloride crystals inside a three-inch diameter putty ring on the slab.
- Cover the ring with a glass to seal the crystals from the air completely.
- Leave it undisturbed for 12 hours.
- Outcome:
- If the crystals dissolve within this period, it indicates high moisture levels in the slab.
- A dissolving of crystals means the slab is too wet for hardwood flooring installation.
C. Phenolphthalein Test
- Method:
- Apply a few drops of a 3% phenolphthalein solution in grain alcohol on various spots of the slab.
- Outcome:
- A color change to red within a few minutes indicates the presence of moist alkaline substances.
- If red coloration occurs, it’s advisable not to proceed with hardwood flooring, as the moisture levels are too high.
D. Polyethylene Film Test
- Method:
- Securely tape a one-square-foot piece of clear polyethylene film onto the slab, sealing all edges.
- Leave it in place for 24 hours.
- Outcome:
- After 24 hours, check for clouding or water droplets on the underside of the film.
- Presence of moisture signifies that the slab isn’t suitable for wooden flooring without further treatment.
Preparation of Concrete Slab for Wooden Flooring
Once the slab passes moisture tests, preparation methods are essential to ensure a stable base for the wooden flooring. Two common methods used, especially in residential settings, are:
A. Plywood on Slab Method
1. Installation Steps:
- Lay Polyethylene Film:
- Place a 6-mil polyethylene plastic film over the concrete slab.
- Overlap the edges by 4 to 6 inches to ensure complete coverage.
- The film acts as an additional vapor barrier and is laid loosely without bonding it to the slab with mastic.
- Install Plywood Subfloor:
- Use 19mm (approximately 3/4 inch) plywood sheets as the subfloor.
- Lay the plywood over the polyethylene film.
- Securing the Plywood:
- Fasten the plywood to the concrete slab using power-actuated concrete nails.
- Begin nailing from the center of each panel and work towards the edges.
- Use at least nine nails per panel to prevent movement and ensure the subfloor remains flat.
- Additional nails can be used for increased stability.
2. Panel Arrangement:
- Staggered Joints:
- Arrange the plywood panels so that the end joints are staggered every 4 feet. This enhances the structural integrity of the subfloor.
- Spacing:
- Leave a half-inch (approximately 12.7mm) expansion gap between the plywood edges and the walls to allow for natural expansion and contraction.
- Provide a 6mm (approximately 1/4 inch) space between the edges and ends of the panels.
- In areas without baseboards or shoe molding (like door openings), cut the plywood to fit within a 3mm (approximately 1/8 inch) gap to accommodate expansion without visible gaps.
3. Benefits:
- Enhanced Stability:
- The plywood subfloor provides a stable, uniform surface for the wooden flooring.
- Moisture Protection:
- Combined with the polyethylene film, this method offers excellent protection against moisture seeping up from the concrete slab.
B. Screed Method
1. Initial Slab Treatment:
- Asphalt Primer Application:
- Coat the concrete slab with an asphalt primer to prepare the surface.
- Allow the primer to dry thoroughly before proceeding.
- Asphalt Mastic Application:
- Apply hot or cold asphalt mastic over the primed slab.
- Ensure 100% coverage to create a continuous moisture barrier.
2. Laying Wood Screeds:
- Screed Specifications:
- Use wood screeds measuring approximately 1 inch (0.1cm) to 4 inches (10.2cm) in width.
- Lengths can vary from 8 inches (20.3cm) to 48 inches (121.9cm) to accommodate different floor dimensions.
- Installation:
- Lay the screeds over the mastic while it’s still tacky.
- Overlap the ends of the screeds by 4 inches to ensure continuity.
- Spacing and Alignment:
- Space the screeds 12 to 16 inches apart, depending on the flooring requirements.
- Maintain a gap of 1.9cm (approximately 3/4 inch) between the screeds and the walls for expansion.
3. Applying Vapor Barrier:
- Polyethylene Film Placement:
- After the screeds are in place, lay a 6-mil polyethylene film over them.
- Overlap the ends by 6 inches to prevent any gaps in moisture protection.
- Securing the Film:
- Avoid nailing or stapling the film, as punctures can compromise its effectiveness.
- The weight of the subsequent layers will keep the film in place.
4. Final Steps:
- Installing the Wooden Flooring:
- Attach the wooden flooring strips or planks perpendicular to the screeds.
- Use appropriate nails or staples, ensuring they penetrate the screeds for a secure hold.
5. Advantages:
- Elevated Flooring:
- This method creates a slightly elevated floor, which can be beneficial for insulation and acoustics.
- Moisture Control:
- The combination of asphalt mastic, wood screeds, and polyethylene film provides a robust barrier against moisture.
Conclusion
Installing wooden flooring over a concrete slab demands meticulous attention to moisture control and proper installation techniques. By conducting thorough moisture tests and choosing the appropriate preparation method—whether it’s the plywood on slab or screed method—you can ensure a beautiful, durable wooden floor that enhances your space for years to come.
Key Takeaways:
- Never skip moisture testing: It’s essential for the longevity of your wooden floor.
- Use vapor barriers effectively: Proper placement of polyethylene films can prevent moisture-related damage.
- Choose the right installation method: Assess your specific needs, and select the method that offers the best protection and structural integrity.
- Allow for expansion: Wood naturally expands and contracts, so providing adequate spacing is crucial.
By adhering to these guidelines and taking the necessary precautions, you can successfully install wooden flooring over a concrete slab, combining the strength of concrete with the timeless beauty of wood.