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Overlays over Wood Surfaces
- Question:
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We have experience resurfacing concrete with microtoppings and polymer overlays, and we are now just starting to do this process over wood—either interior floors or upper-level patios that do not have enough clearance for concrete. What is the best practice for this process?
We currently staple down a vapor barrier, screw down metal lath, and then incorporate some fiber mesh into an underlay material about a 1/4 inch thick. After this sets up, we apply the microtopping or stamp mix to the required depth to take the stamp imprint and then spray apply antiquing color, acid stain or dye. This is followed by application of a high-traffic water-based sealer.
We have had two overlay failures; one cracked and the other leaked. What could be the problem? Also, can you recommend dependable products to use for the vapor barrier and the metal lath?
- Answer:
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From what you wrote, I have no issues with the surface preparation and systems you are using. I typically tell people to prep a wood or non-concrete surface as you would for tile: place a mortar bed with lath (as you are doing) or use cement board (such as HardieBacker) with seams taped and mudded.
As to your problems and questions:- Did you cut any control joints into the surface? Cracks are caused by stress or movement. Just because it is a microtopping over a mortar bed does not mean it will not shrink or move due to stress and temperature. I would not be surprised to see that the crack had followed a seam in the plywood or decking in the sub-floor. Utilizing control joints along long seams or large areas is important. Another thing to consider is the stability of the wood deck or floor. Additional bracing or stabilization may be needed, especially on exterior raised decks.
- A step often missed when installing exterior overlays is caulking. Water is amazing in its ability to migrate through a solid surface, usually finding its way through cracks, seams and edges. I recommend caulking or grouting all saw cuts and control joints. I would also caulk all the edges where the overlay comes in contact with a wall or another edge—pretty much anywhere water can get under or around the overlay.
- As far as a water barrier, I recommend using roofing felt or tar paper. And don't forget to tape your seams. Avoid using flexible rubber-based materials. The more rigid the sub-floor the better the surface will resist water penetration. A cement-based waterproofing mortar is also a good option. For metal lath, a stucco-grade material is fine. I have seen anything from chicken wire to concrete wire mesh used successfully.
- Did you cut any control joints into the surface? Cracks are caused by stress or movement. Just because it is a microtopping over a mortar bed does not mean it will not shrink or move due to stress and temperature. I would not be surprised to see that the crack had followed a seam in the plywood or decking in the sub-floor. Utilizing control joints along long seams or large areas is important. Another thing to consider is the stability of the wood deck or floor. Additional bracing or stabilization may be needed, especially on exterior raised decks.
Author Chris Sullivan, ConcreteNetwork.com technical expert and vice president of sales and marketing for ChemSystems Inc.
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