Tile Installation: >> Deflection (Surface Prep)
When installing stone or ceramic tile over floors and walls, each have different requirements. However, they have one thing in common. That is the necessity that they have a firm stable surface on which to be applied. Tile, be it stone or ceramic is hard and will break or dislodge if the surface bends under load. This is called deflection. Volumes can be written on deflection. However, the basic rule is that if the finished tile surface span deflects more than the calculated maximum deflection amount, the tiled surface will shear, buckle, and fail.
How is deflection calculated? For ceramic tile the maximum allowable deflection is calculated at L/360. For stone tile, the MIA (Marble Institute of America/www.marble-institute.com) calculates maximum deflection at a rate of L/720. To calculate, convert the span from feet into inches then divide by 360 or 720. The end result gives you the maximum amount the floor can move under a or "expected" load.
Obviously, deflection is more of a problem when discussing floors than walls. However, walls can also be subject to loads. If the finished wall is not prepared properly and its deflection characteristics are ignored, this condition can be disastrous. Commonly, walls are leaned on or furniture is pushed against the surface. You get the picture.
How do we check for deflection? First of all, building codes are established with deflection in mind. This is the very reason that building codes mandate specific building materials and methods. When considering deflection, one must start at the framing level of construction. When considering floors, building codes mandate certain joist sizes and spacing. These codes take deflection into account for every minimum they mandate. The same is true for walls.
Does this mean that if our homes are built "to code," deflection will be eliminated? Emphatically, No! This simply means that a best effort was made for the floor to resist deflection. By the way, all floors using wood or steel framing members will deflect if enough load is applied. It becomes necessary to determine how many loads the floor needs to resist. This is common sense and a little mathematics. If the floor, for example, is in a kitchen, the weight of the stove, refrigerator, other appliances, and people in the room can be estimated. Then the possible load is known. Next we use that load in a somewhat scientific way.
We take a string and anchor it with nails across the longest span of the involved room. We anchor that string at either end with nails or in some way to allow the string to be tight. We carefully measure the distance from the bottom of the string to the surface of the floor. Next, we put the estimated load in the center of the room and re-measure the distance. If the distance exceeds the maximum requirements, we need to make some hard decisions.
This process seems very difficult and laborious to achieve. When should we go to this trouble? Well, a simpler test is also available and is much easier. If you stand in the middle of the room, jump up and down, and feel the floor move under your jumping weight, chances are there is a deflection problem. If you wish to proceed with stone or tile more testing may be necessary.
Dr's Tip: If the simple test is performed and there is obvious and considerable movement, there are two choices. First, rebuild or strengthen the sub floor or consider a material other than ceramic or stone tile.
Whether the sub floor is concrete slab or wood/metal framed, a od of installation must be chosen. Some ods are designed to increase the resistance of the surface to deflection especially when dealing with floor surfaces. Please refer to the specific area in The Tile doctor dealing with the intended project.