One of the weakest parts of a roof is the dome or cupola. This is because the dome and cupola set on part of the roof structure and is an extension of that structure. Although a dome or cupola is constructed in an architecturally sound form these structures are prone to leaking because of the stresses to which they are subjected.
Usually there is little more than a brick structure built. The brick is carefully placed to form the dome or the cupola but there is often little else added to hold the structure in place. This is especially true with cupolas. It is common practice to simply plaster over the brick work once the dome or cupola is formed.
With some domes, concrete beam “ribs” are added. These are made the same as the “dala” in walls. There is a type of rebar tied in place then concrete is poured over the steel to form the rib. The concrete adds strength to the structure as does the steel but this practice is not wide spread except for very large domes.
Because of the stress which is place on a dome or cupola from minute movement in the soil under the foundation and columns which support the house, it is common for these structures to develop leaks. There are two ways of attending the problem of a leaking dome or cupola.
The first is to place a band-aid over the leak. A band-aid in this case could be a new plaster job, or a painted-on sealer. In many cases the band-aid will add years of life to the structure and serve well. When you re-plaster, special care should be given to the joints of the structure. That is the base of the structure where it joins the roof and the top of the structure where the lantern joins, if the structure has a lantern. At these two joints, extra plaster must be applied so that the water can sheet off easily.
If the structure is so badly damaged or is so prone to movement that band-aids are not an option, then the entire structure needs to be covered in concrete. This is usually accomplished by placing about three to five inches of concrete over the dome or cupola. In areas where there are windows you do not cover with concrete, obviously. However, you may have to replace the windows.
When concrete is placed over a dome or cupola it must be accomplished in a continuous pour. That means that all old plaster needs to be removed before beginning and enough people need to be on hand so that the concrete can be mixed and lifted into place all in one day. Some masons like to have a rubber hose tacked or wired in place to use as a form and some masons prefer to simply place the concrete. Either way works as well as the other.
Once the concrete is dried you may wish to cover the dome or the cupola with tile. It is quicker and less expensive to use broken tile for this job. Since domes are not conducive to maintaining straight lines with tile, much time can be saved by using broken tile. It looks great and you can usually find a grout or mix a concrete color in white cement to obtain a matching color to your tile.
This method is not a guaranteed fix for all time but concrete will not leak unless it is broken. Besides, concrete will take a lot more stress than just brick and plaster. If you opt for the concrete solution you may find that it will pay for itself over several years, however.
If you have question about this subject, or any other subject concerning construction, please feel free to drop me an e-mail. I’ll try to get back to you the same day.
