Patching things up

The ongoing challenge of substrate preparation

By Christopher Capobianco

With so many options in patching and levelling compounds, choosing which product to use is confusing enough. In addition, how these compounds get used, or shall I say misused, is still the cause of many complaints. It’s a problem, especially on resilient-flooring installations. Proper specification and preparation for any substrate that will have a “glue-down” floorcovering is critical.

Flash the floor

ASTM  F710 states: “Patching or underlayment compound shall be moisture-, mildew-, and alkali-resistant, and, for commercial installations, shall provide a minimum of 3000 psi compressive strength ….” When I started in the business in the 1970s, we had Flash Patch, and the slang “flash the floor” was born. Many still use that term for trowel-applied patch. Back then it was mostly white gypsum-based compounds. There was some awareness of moisture-related problems in concrete floors, so we used grey cement-based products, although those products were sandy and not nearly as easy to apply and fast to dry as today’s products.

As vinyl-backed sheet vinyl hit the residential market in the 1980s, staining problems from gypsum patching compounds became a common complaint. Manufacturers scrambled to include anti-bacterial agents into their products to prevent this, and then the polymer-modified cement products arrived. These really changed the industry because they did not allow bacterial growth, were much harder than gypsum, spread like butter, dried very fast and could allow you to feather a floor easily.

So what happened to gypsum compounds? I don’t see much gypsum floor patch these days, but poured self-levelling underlayments are definitely still in wide use, especially on high-rise construction projects like residential properties and hotels. Properly used and prepared, they are still suitable for most flooring applications, as long as they meet the ASTM F710 criteria.

Of course, cement-based patching and levelling products are dominating the industry today. They are tough, easy to use, they dry hard and fast, and adhere well to a variety of substrates. But they are often misused.

Like a recipe

It’s all in the mix, whether you are baking a cake or prepping a floor. Most gypsum underlayments are site-mixed from sand, water and gypsum compound, so getting the ingredients right is key. Too much sand makes for a much weaker finished product. Cement-based products are already mixed, so you just add water. In both cases, over-watering is a huge problem because too much water makes the finished product more porous than it should be, and that can affect the adhesive bond in a big way.

I see so many cases where installers are mixing patch by eye, and almost always with too much water. That makes it easier to spread, but thicker is better as far as the hardness, strength and porosity of the substrate you will adhere to. Follow the recipe and don’t use too much water.

Just a few days before writing this, I had a call about how to install rubber over gypsum. I was asked, ”will the gypsum suck up all the adhesive?” Great question, because that’s exactly what I always worry about. Gypsum products are more porous than cement products, which is why the manufacturers have sealers that are made specifically for floors where an adhesive will be used. The problem is: who does the sealing? The underlayment subcontractor or the flooring installer?

If you can clear that up, you will solve a lot of problems. Most of these projects have multiple floorcoverings: areas of adhered flooring like wood or resilient and other areas of non-adhered like carpet over pad or floating floors. In that case, it’s probably best for the flooring installer to be the one to seal the gypsum substrate where needed. If you glue to it, you have to seal it!

Even though today’s patching compounds are engineered with great adhesion, and even with additives to allow for encapsulation of asphalt adhesive residue, this topic is the biggest area of misuse for patching and levelling compounds. Here are some things to avoid.

Adhesive residue: If it’s anything but black, take it off. Carpet, VCT and most other adhesives are water based, so if you add water, they can break down. Since patching compounds have water in them, this can be a big problem. As far as the asphalt adhesive, it has to be a thin, almost transparent layer — no trowel ridges. Don’t just pick up VCT and patch right over the old adhesive. Scrape it smooth.

Other contamination: Paint, oil, dirt, sealers and all kinds of other substances can be on a concrete slab when you arrive. The patching compound and the floor that goes over it can pull on those substances, and when that happens, the whole floor comes up. Make sure the slab is clean before you patch it. One floor killer I saw recently was pipe-cutting oil on a construction site. When that gets into a slab, it’s very hard to remove, and almost nothing sticks to it.

I have written about proper selection and installation of wood underlayments here before. Especially for resilient floors, use a product that’s made to be an underlayment, such as a good Canadian multi-layer plywood. Avoid lauan, fiberboard or OSB at all costs. Install the underlayment with underlayment staples or underlayment nails, and avoid screws. When it comes to patching the joints, it may or not be necessary, so check the manufacturer’s specifications. Some products require just a light sanding, and patch is not needed.

Whether you are an installer on your knees, a dealer supplying material and labour, or an architect selecting a flooring system, proper specification, use and preparation before application is key for patching and/or levelling a floor. Go by the book and you will rarely have a problem.

Christopher Capobianco is a flooring expert currently working with Sorest Hill, Md.-based Spartan Surfaces, a distributor of commercial hard-surface flooring. Chris’s family lineage in floorcovering goes back for generations. His career includes time as a retailer, architectural sales rep, technical support manager, consultant, instructor, columnist and active volunteer in several organizations.

 

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