How to Cover a Monolithic Dome with Tile or Rock

Covering a dome with tile can be both practical and beautiful.

People worldwide have been tiling their domes for centuries. Many stunning, multicolored, tiled domes are found in Mexico and Spain. Some of the earliest examples of tile decoration can be seen on Islamic Mosques and palaces around the world.

Tile on a Monolithic Dome can help protect the Airform. If the Airform has been pulled off, then covering the Monolithic Dome is absolutely essential and can halt the erosion of the polyurethane foam. There are many different coatings, but tiling a dome is not only a practical protection solution but a very attractive one.

We use five essential steps in tiling a Monolithic Dome:

Step 1: Decide what size of tile to use.

The smallest tile is 2" square. On small domes we have used 4" square tiles. With a diameter of 280 feet, Faith Chapel’s Christian Center in Birmingham, Alabama is the largest Monolithic Dome encased in tile to date. Tiles 20" square cover Faith Chapel’s lower half, and 13" square tiles cover its top.

Deciding the tile size is very important because placing a flat, inflexible tile on a dome’s curved surface creates a gap between the tile and the dome. You will want that gap to be as small as possible. A dome’s radius of curvature (Rc) helps determine the size of tile to use. If you lay a 12" square tile on a dome with a 15’ diameter, the tile’s edges will stick out and away from the dome’s surface. Choosing the correct size of tile for the dome portion it will cover minimizes the gaps. The calculations can be a little complex, but with modern calculators, it is actually fairly easy to figure how much of a tile will jut out — or one can simply lay a piece of tile on the dome and measure the gap.

First, the radius of curvature must be determined. Generally, a dome’s radius of curvature differs vastly from the radius of the dome’s floor. Most dome homes are elliptical in shape. (See more about Monolithic Dome Shapes. Consequently, the dome’s Rc changes from its bottom to its top. The bottom of the dome will have a smaller Rc, increasing to the largest Rc at the top of the dome. We suggest using the smaller Rc when calculating tile size. (See Radius of Curvature.)

The radius of curvature can be calculated by hand or you can use our Rc calculator. To determine tile size, make sure the radius of curvature is in inches. If the Rc measurement is in feet, simply multiply by twelve to obtain the Rc in inches.

Now you have the radius of curvature in inches, preliminarily “guess” what size of tile you will use. If you plan to use 4" square tiles, divide the size of the tile in half. Now, square the Rc (in inches) and square the half-size tile. Subtract the half-size square from the square of the Rc. You will end up with another big number. That’s okay.

Next, take the square root of that big number and subtract it from your original Rc (in inches). You should now have a number that is really quite small — probably something like 0.02 or 0.03. We really like to have this number stay below 0.03. This number indicates the one-hundredth of an inch that a tile will gap away from the dome’s surface.

If this number is larger than 0.03, you will probably want to try a smaller tile size. If you multiply this number by 64, you will get the number of sixty fourths of an inch that the edges of the tile will stick out, if you lay the tile with its center touching the dome. In most cases, we really like to have it less than two with a maximum of three sixty fourths of an inch – but two is better.

As a rule-of-thumb, check, or for your initial tile size guess, in your calculations, if the Rc is 99", as it is in our little space cabin with its 16.5’ diameter, you will use 4-inch square tiles. If the Rc is up around 20 feet, you can probably use 6" square tiles. If it is up around 100 or more, you can usually use 12"" square tiles.

Step 2: Select the tile and adhesive.

The best tile to use on the outside of a building is called porcelainized tile or plastproof tile. Both names essentially mean the same. Any ceramic tile designed for the outdoors may be used.

You can also use actual cut rock, travertine, marble, or other natural stones, including river rock. And I suppose there is nothing to keep us from using stepping stones, such as they use for sidewalks. That would make quite a building! Imagine one with little Texas stones like you see around flower beds.

You will find that the tile or rock will be the cheapest part of the project. We don’t want to tile the dome twice, so we use Dow Silicone 795 adhesive. This is available in multiple colors, in large and small caulking tubes, and in buckets. You must decide which to use.

The caulking tubes take a lot of squeezing — unless you have an electric or air driven tube. The bucket works, but it’s a little messier. There are other brands that work but be very careful with your selection. You want this job to last for centuries.

Caution: Do not use just any old caulking or glue. If you use the silicone caulk from the local hardware store, I can promise that you will feel really bad as it lets go. It does not adhere to the Airform. It will adhere only while it is still gooey. That is called adhesion; but when it’s cured it becomes glue, and it won’t stay as a glue.

Step 3: Decide how and where to put the tile.

For the jobs we have done so far, we start by drawing a line all around the building, using a laser level that is absolutely level. Placement of the line is as low as is possible considering the line needs to be continuous. Sometimes conditions of the ground or the dome make it so the line must be located high enough that in places you end up putting tile below the line as well as above the line. If the line is positioned low enough, start tiling at that line so that gravity can be used to hold subsequent rows of tile in place.

Step 4: Install the tile.

We have taught ourselves to install the tile by running a bead of glue around the perimeter of the tile. On larger tiles, we put two rows of bead, one in a little ways from the outside edge as well. Experts advise that one-twelfth of the tile needs to be covered with adhesive. That means, if you have a tile that is one square foot, or 144 square inches, a fourth of that is 12 square inches that needs to be covered with adhesive. Common sense tells us that the adhesive should be evenly distributed around the perimeter of the tile.

We then fit the tile against the Airform and hold it long enough to make sure it will stay. Big tiles may have to be braced until they set up. Repeat this process with each tile. Squeeze caulking in between the tiles to produce a type of order or to fill in the joints. You really want to minimize the joints; always push the tile as tight against the dome as you can. Remember to always keep it dead level on the line made around the building.

As we go around the dome, we don’t want the tiles to line up on top of each other vertically. On a dome, it is not possible to achieve an exact vertical positioning, so don’t even try. Instead, stagger the joints. Put the first tile on as a full tile and the next as a half tile. Keep staggering the joints around the dome. This makes the tiles uniformly different.

As we install tile around-and-around using square tile on a round building, we find that the gap between tiles gets much larger at the bottom of the dome than at the top. The gap widens as you go up the dome and eventually something has to be done about it. What do we do about it? Obviously, to begin with, we just put down a little more adhesive in that area. It is when we hit a little higher up that the blank spots draw more attention.

Our suggestion right now is to fill the voids with caulking. If they are small, ignore them. Again, remember you cannot fill all of the cracks. Be sure you leave channels for water to get out from under the tile. Do the best you can to keep the sun from reaching the Airform fabric. The fabric is your reinforcing and sunshine is about all that can hurt it.

What happens to the fabric if you get water between the tile and the Airform? Nothing. The Airform fabric is very stretchy so even if there is some frost that gets under the tile, it can’t lift it off.

Step 5: Clean the tile.

After the tile has achieved its initial set, use a damp cloth and wipe off any of the adhesive left on the tile. You now have a practical and beautiful exterior for your Monolithic Dome.

Note: Remember, we are just a page ahead of you in this book. If you have something that works better than what we are doing, we would like to hear about it. Please send us pictures and experiences you have had tiling your domes. Your help is greatly appreciated.

April 25, 2008