Sub-Floor Preparation?

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Sub-floor preparation is perhaps one of the most misunderstood parts of a professional floor covering installation. Many DIY folks, as well as some semi-professional installers and flooring companies skip (or minimize) this step; but this can unfortunately lead to disastrous results.

So what exactly is a sub-floor? It is the structural part of the home (the floor) which is beneath the finished floor covering. In Vancouver it is almost exclusively made of either plywood or concrete, depending on the type of home. Most wood-framed homes will have plywood sub-floors on the main and upper levels, with a concrete slab in the basement. Many newer homes with hydronic (hot water) radiant heat systems will utilize concrete or have a concrete “topper” on the plywood base. All condominium high rises will have sub-floors made of concrete.

Regardless of the type of sub-floor, the floor covering industry universally requires the same sub-floor specification for the installation of hardsurface flooring (i.e. everything other than carpet and sheet vinyl floors). Most of these specifications are fairly obvious: the sub-floor must be dry, relatively smooth, free of excessive moisture and any contaminants such as paint or drywall dust. The big challenge however is that the sub-floor must also be FLAT, with a deviation generally of no more than 3/16″ (5mm) over a 10′ (3m) span.

In order to check a sub-floor to ensure it is flat to within this specification, sometimes lasers are used but the use of an aluminum straight-edge bar is the most common. With this bar positioned all across the sub-floor, the installation contractor can determine where are the high and low spots, and come up with a game plan to eliminate those undulations so that the deviation over a 10′ span is not greater than the 3/16″ allowable.

There are two primary means of addressing the undulations: we take down the high-spots by grinding/shaving the peaks and fill up the low-spots with a fluid self-levelling compound. Now, this is a bit of an oversimplification, but it should suffice for this purpose. Again, the object is that after the work is done, the new floor covering can be installed on a flat surface and thereby allowing it to perform as intended.

When it comes to the application of the self-leveling compound to fill the low areas, there are are two primary methods used. The first is typically referred to as “flood-leveling”. In this method the highest point of the sub-floor is used as a reference and the entire space is “flooded” with self-leveling compound up to that point. The benefits of this method is that it is typically fairly quick and often times it creates a level (as well as flat) surface to work off of. The downside is that it can get very expensive very quickly – each bag of leveling compound supplied and installed is not cheap.

The second method, and the one most often used by Ethical Flooring, is called “patch-leveling”. In this method a more nuanced approach is taken. Rather than flooding entire area in self-leveling cement, each area of the sub-floor is assessed individually and rectified as is necessary for that – and the adjoining – areas. It also takes into account various height restrictions we may run into such as railings or other floor coverings we must meet up against (tile being a common example). After the preparation is completed, the sub-floor may appear to have had work in some areas but not others; but the goal is still to fall within the 3/16″ over 10′ tolerance. The benefit of this approach is that it is often less expensive than the full-pour method, as well as allowing us flexibility to consider the finished floor height in any given area in relation to other critical areas of the home. The potential negative of this method is that it often takes more time to complete. Rather than simply pouring a significant amount of self-leveling compound in one or two days, working section by section is more time consuming and sometimes only a small amount of work can be done in a given day before it must dry and before we can continue working on the next section.

What type of preparation work completed – patchwork or flood leveling – is determined on a case-by-case basis with input from the installation contractor, the Ethical Flooring project manager, and (if desired) the client. If you, the client, have particular outcomes you would like to see (such as a perfectly level floor) then it is critical to discuss this with you project manager before work begins. Otherwise, we proceed with our default standard, which is always to get to the 3/16″ specification at the lowest cost to our client.

So why are we so concerned about a flat sub-floor anyway? First and foremost, the flooring manufacturers who have made and warranty the product have done testing and have determined that in order for their products to perform as intended, this sub-floor specification of 3/16″ over 10′ is necessary. To put it plainly; if you want a warranty for your floor (and you do) then your floor must be installed to this specification. If Ethical Flooring is not your installation contractor, please make sure whoever is follows this warranty requirement! If they don’t, and you need warranty support from the manufacturer, you may just be out of luck.

The second, but related, reason is that the majority of floor failures – such as cracking, chipping, excessive noise and joint failure – are a result of a sub-floor that is not flat. The new floor covering will follow the undulations in the sub-floor and if these undulations are excessive they will cause the floor covering to “move” vertically more than the floor is designed to handle. Over time this can cause a multitude of problems. Perhaps by now most people have seen a poorly installed laminate floor that has unsightly gaps. This is typically caused by a sub-floor that was not properly prepared, causing the laminate to move excessively and thereby breaking or separating the joints.

One way that we have found to explain the necessity of sub-floor repair work is to utilize painting as an example – something most people have undertaken at one time in their life. Imagine you have a wall which is full of holes, dings and scrapes. If you were to come in and just apply a new coat of paint overtop it wouldn’t look very good would it? No, the actual tough work, and sometimes what takes the longest, is getting the wall prepared to accept the new paint. The same general principle is at work with flooring; if you want a good result, you have to put in the hard work before hand.

You may be with me at this point and understand sub-floor preparation and why it is needed – but here comes the hard part: it can be expensive to do! Often times on Ethical Flooring’s projects we spend as much time and expense preparing the sub-floor as we do installing the floor covering. As I mentioned above, we are doing this to maintain your warranty and to ensure you have a long-lasting and high-performing floor. Of course, people often ask for a budget on their particular project and unfortunately this can be near impossible to do. Every home is so very different and the amount and type of work required to fix a sub-floor varies considerably. What we ask clients is to please put away an additional budget for sub-floor preparation in the range of $1 to $6 per square foot (yes this is a big range). As work begins we will keep you up to date with how work is progressing and how much it is costing. You will be only charged for the work required to fix your particular home – hence why we cannot do a “blanket” or general charge for this work.

One of the key things to look out for when searching for a flooring provider is that their contract outlines exactly how sub-floor preparation is charged and at what rates. This type of work can open up a grey-area between the client and the flooring provider, and unfortunately there are instances of people being taken advantage of in these type of situations. To protect yourself, you of course want to be comfortable with who you are working with; but it is also always wise to have all the flooring providers’ costs and procedures for dealing with eventualities in a written contract. Having all of the “rules of the game” – so to speak – available for both parties to see and sign off on before any work begins is the best way to avoid hurt feelings later on.

There is so much more that could be said about sub-floor preparation, but this has hopefully been a helpful introduction to the topic. If you have further questions about it your Ethical Flooring sales and project manager is always happy to walk you through the process. To begin the conversation please visit our showroom or book an appointment here.

Kevin K.

President; Ethical Flooring Ltd.

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