{"id":583,"date":"2013-08-08T23:16:13","date_gmt":"2013-08-08T23:16:13","guid":{"rendered":"http:\/\/bathursthandyman.ca\/?p=583"},"modified":"2013-08-08T23:16:13","modified_gmt":"2013-08-08T23:16:13","slug":"ceramic-tile-floor-cracking-this-is-why-a-lath-and-scratch-coat-doesnt-work","status":"publish","type":"post","link":"https:\/\/buildingology.com\/?p=583","title":{"rendered":"Ceramic tile floor cracking: Why the scratch coat method doesn’t work well"},"content":{"rendered":"
\"An<\/a>
This is an uncoupling membrane sample. This one is made by Schluter called Ditra.\u00a0 Uncoupling membranes are best practice for installing ceramic tile installations and the best alternative to scratch coats and backer board installs.\u00a0 An example of a failed floor with a scratch coat is shown later in this article<\/figcaption><\/figure>\n

The scratch coat method<\/h2>\n

One of the most common and outdated ceramic floor practices used today is the scratch coat method on top of sub-floors and wood joists.\u00a0 Some people call it the “Jersey mud job”. \u00a0 This seems to be the most common practice in Toronto for ceramic floors, and maybe Ontario as well.\u00a0 Failure rates are relativity high within 5 to 10 years from what I have seen with some homes.\u00a0 Homes with a decent sub-floor do last longer.\u00a0 Why is this outdated method used?\u00a0 The simple answer is construction economics and the use awareness of the use of uncoupling membranes is still not known by homeowners and most contractors.\u00a0 The flooring trades in Toronto’s housing sector have a scratch coat culture that appears to be difficult to shake.<\/p>\n

\"scrach<\/a>
This mesh has been placed over plywood and the nails are bent over to hold it in place as an example of how a scratch coat is applied.\u00a0 Usually roofing nails or staples are used (I didn’t have any roofing nails for the picture, sorry). \u00a0 Afterwards, a layer of thinset mortar is troweled in place and allowed to set.\u00a0 It is usually thin, maybe 1\/8″ thick.\u00a0 A real scratch coat would be using the galvanized steel lath that looks slightly different than this plastic one.<\/figcaption><\/figure>\n

The scratch coat method secures the the ceramic tiles to the sub-floor and joists as best as possible.\u00a0 It is the philosophy of maximum control of your building components and structure as a whole.\u00a0 The firmer, the better.\u00a0 This is similar to the philosophy of maximum vapour control of using polyethylene vapour retarders (barriers) anywhere you think you are controlling the movement of water vapour, which not surprisingly also doesn’t work that well. \u00a0\u00a0 The scratch coat is installed by using a sheet of metal wire that sort of looks like chicken mesh and nailing it down to the plywood sub-floor.\u00a0 The nails are installed between the mesh and purposefully bent over top the mesh as to hold the mesh down tight to the sub-floor.\u00a0 Thin set mortar is forced between the mesh, and when it sets, ceramic tiles can be installed as they have something else ceramic to bond to as it can’t bond to the metal.\u00a0 The steel mesh was thought to control the expansive forces, removing the possibility of cracks.<\/p>\n

The reason for this type of practice from what I have heard from installers is that they believe it is similar to the function of re-bar in concrete.\u00a0 Many people assume that this is a good method because it “controls” the movement of ceramic tile, whatever that means.\u00a0 We have all seen ceramic tiles in various houses where the tile has de-laminated\u00a0 from the floor and has become loose at the grout or the tile has cracked in half.\u00a0 When I ask a homeowner why do you think this happened, the assumption I often hear is somehow the tiles were not tightly adhered to the floor with cheap materials and a lot of mostly dumbfounded looks.\u00a0 Although substandard material failures do happen, most often it not always the underlying reason why the floor tiles crack or de-laminate.\u00a0 \u00a0As I have mentioned in previous articles, your house moves for various reasons and ceramic products such as tile and grout don’t handle expansive forces well and must be taken into account when installing any new component in your home.<\/p>\n

Lets look at the physics on why this total control philosophy doesn’t really work a little later on.<\/p>\n

This is a cross section of a ceramic tile floor<\/h2>\n
\"ceramic<\/a>
There is a bonding substrate between the tiles and the sub-floor. The bonding substrate is either made of a scratch coat, cement backer board, or an uncoupling membrane. It’s purpose is to combine compatible materials<\/figcaption><\/figure>\n

 <\/p>\n

<\/p>\n

Accommodation of expansion in a scratch coat? Yes, but in the wrong places<\/h2>\n

Ceramics cant be adhered directly on the sub-floor itself as wood does not bond to cementious produces well, so a substrate between them is used.\u00a0 Today, the choices for that substrate is either a scratch coat, a backerboard (which is a layer of 1\/4″ cement or fibre cement board), or an uncoupling membrane.\u00a0 Since there is a substrate between the structure and the tiles, we basically have two different bonding planes (goals) for a ceramic tile install.<\/p>\n

1: From the ceramic tile to the bonding substrate.<\/p>\n

2: From the bonding substrate to the sub-floor<\/p>\n

\"scratch<\/a>
Differential movement in the structure can cause the scratch coat to delaminate. The nails that allow a little bit of movement are attached to the wire mesh which makes the situation worse.\u00a0 Keep in mind, that the differential movement can happen in the if all components move in the same direction, but at different rates and still crack.<\/figcaption><\/figure>\n

The steel mesh of the scratch coat handles expansive forces quite well in terms of the substrate adhering to the structure, however that is not the main goal of using a substrate. The goal is not for maximum adhesion, but rather good compatibility.\u00a0 If the structure moves in different directions (see figure above), then the steel mesh moves in tune with that structure (1).\u00a0 However the movement in the structure is not uniform, and the steel mesh has a mechanical bond through the nails into the plywood, which provides some movement accommodation which is good (2).\u00a0 Since the steel mesh is designed to handles expansive forces well, it is cohesive to itself and moves uniformly (3). To better illustrate this cohesiveness, the steel by itself will never fail by splitting in two, or bending.\u00a0 This is the common error that people make, thinking because the mesh can handle expansive forces, that it handles the expansive forces we need.\u00a0 We don’t really care how well the steel does, we care about how the steel affects the tile and mortar that is around it.\u00a0 The mesh is a double edged sword, as this cohesiveness allows the mesh to move differentially to the tile and to the plywood (4). \u00a0 Delamination occurs on bonding goal #1 and\/or #2.<\/p>\n

The nails in this scenario are what allow only a slight amount of movement.\u00a0 One may think that more<\/em> nails are needed to fix this problem of tiles cracking, but more nails do not improve compatibility.\u00a0 Reducing the bending moment (flex) of the floor by adding another sheet of plywood is what helps improve compatibility.<\/p>\n

In a perfect world, the physical properties of the house will never change, but we don’t live in such a world.\u00a0 Whatever movement can’t be accommodated by bonding goal #2, the result will have to be transferred to the next layer, which is the ceramic tile and the grout.\u00a0 As we know, ceramics can’t accommodate movement well and shatter or delaminate.\u00a0 This is why cracks do happen.<\/p>\n

Cement backer board: more of the same<\/h2>\n

I don’t have proof (yet) of this therory, but this is why I believe a floor would fair no better with backerboard.<\/p>\n

Cement backer board installs are really more of the same thing.\u00a0 It is difficult to say whether the result will be slightly better or worse than a scratch coat as the composition is different, but the basic mechanics are the same.\u00a0 The 1\/4″ cement board or fibre-cement board has the same physical limitations as the scratch coat.\u00a0 It is essentially cement that is mechanically attached to the joists with no accommodation for movement.\u00a0 I personally would not be surprised if this is worse than scratch coat install as nails that are not driven in the floor completely tend to bend which is a good thing, where as a screwed down fibre-cement board looses the slight advantages of the movement of that nail.<\/p>\n

This is the preferred method<\/h2>\n
\"uncouple<\/a>
The uncoupling membrane allows movement for both bonding goals #1& #2. The uniformity and rigidity of the membrane makes it work well.<\/figcaption><\/figure>\n

An uncoupling membrane has an opposite philosophy than the scratch coat.\u00a0 Rather than making the bonding substrate (uncoupling membrane) firm to the plywood, the bond is intentionally very weak to encourage movement where it is wanted rather than discouraging it altogether.\u00a0 The uncoupling membrane can slide over the plywood.\u00a0 Bonding goal #2 intentionally allows movement.<\/p>\n

For the connection to the ceramic tile, the thin set mortar is not so much bonded but friction fitted in place (thinset does not bond to polyethylene plastic).\u00a0 A good analogy I can think of is how your socks bond to your leg.\u00a0 The elastic allows a tight fit, but those socks never pierce your skin, or you have bigger problems than the composition of your floor.\u00a0 This is a good type of fit because if your leg were to get slightly thicker because you have increased muscle mass, the sock’s elastic movement would still accommodate that larger leg and still fit. \u00a0 But most important, the shape of the ceramic remains uniform, so the ceramic floor “sheet” never changes and any movement is transferred to the uncoupling membrane to deform intentionally.\u00a0 This system is essentially a floating floor.<\/p>\n

The failed scenario, a scratch coat victim<\/h2>\n
\"cracked<\/a>
The lower red box shows a crack that is about 1\/32″ thick through the centre of the floor. The above red box shows secondary effects of the movement of the structure and why you shouldn’t grout the wall to floor transitions<\/figcaption><\/figure>\n

Granted, the age of this ceramic floor is older than me, but it is not really an excuse.\u00a0 I couldn’t tell you how old the crack is, but it shouldn’t really matter.\u00a0 The bathroom is not used much luckily, or water would have definitely seeped through the cracks and rotted the floor boards long ago.\u00a0 The extent of the damage at this time is unknown, but is structurally sound.<\/p>\n

\"Buildingology<\/a><\/p>\n