Build a concrete slab roof on your new house and save yourself the headaches of wood trusses

Photo by Arno Senoner from Unsplash

Today, as a Miami residential architect, I want to tell you why I think that a concrete slab roof is much superior to a wood truss roof. However, both of these systems are in common use. Of course, if you look around Miami-Dade County, you will see that the vast majority of roofs are wood truss roofs. Why is this, you may ask?

Miami’s traditional architectural systems are based on what was popular in the United States in the last century. Traditional residential architecture does not use flat roofs. Prior to 1960 wood trusses did not even exist. The roofs of houses were framed with mostly 2 x 8 roof rafters and ceiling joists. These rafters and joists had a span limit of about 12’. So, most houses were built with interior bearing partitions to support the roof. It wasn’t until the invention of roof trusses, which are made mostly of 2 x 4s in a triangular form, that we were even able to span from one side of the house to the other. Most houses were built with sloped roofs, either gable-end or hip roofs. This gave you a look that was popular then. In the 1950s and ‘60s, it was common in Miami to install tar and gravel roofs over the wood structure. Later, shingles became popular. This was a relatively cheap material, but it was more attractive than tar and gravel roofs.

Since our offices are in Coral Gables, we are particularly familiar with this area of Miami. Coral Gables has always insisted in its zoning code that the roof of any house had to be cement tile or clay tile. It could be Spanish curved tile or flat tile. However, in the rest of Miami-Dade County, there were options. You could use tiles or shingles. Tar and gravel roofs disappeared with the passage of time.

Typical Coral Gables Spanish style cement tile roof

People seemed to like these sloped roofs. They were cheap and easy to build. However, when Hurricane Andrew hit Miami, we realized the deficiencies of Miami's coastal construction. As Miami architects and experts on residential coastal construction, we can tell you that the flaws of this kind of construction became obvious. The typical sloped roof has joists or trusses, then plywood sheathing, then a roofing membrane, and finally, the finished roof which is cement tile or shingles.

The problem with this type of construction is that when you lose the tile or shingles and the waterproofing membrane, it rains inside your house. When you have a concrete roof, the concrete is much more forgiving. You may have small concrete cracks, and you may get moisture, and maybe even a little drip through there, but it will not rain inside your house.

So, you ask – well, but why can’t we just get rid of the trusses and build the roof with a sloped concrete slab? Yes, you can, but it’s an expensive process. First, you must form and pour a reinforced concrete slab on a slope. This means it must be a different kind of concrete. And you must make sure that the concrete, while still in liquid form, is not going to run away down the slope. Lastly, when you are done, you must pull the forms away from the sides and the bottom. Then, if you are going to put in a flat ceiling, you have to put in some kind of wood elements – joists or suspended furring strips to hang the drywall ceiling. All of this is a costly process and there is much resistance to this system by developers.

Traditional architecture is "out" and modern is "in"

The good thing is that now, flat roofs on new houses have become popular. Traditional architecture is out and modern is in, at least for the high-end market in South Florida. In actuality, modern has been around for years. In Cuba in the 1950s, there were many flat-roof houses. But in the US there has been a prejudice against modern architecture in housing, particularly in the northern states where most houses are made of wood framing. People thought modern architecture was cold.

It may be the influx of Venezuelans and other Latin Americans that now has made modern residential architecture popular in Miami. Whatever the reason, I think this is a good thing. Even though initially, a flat concrete slab roof is more expensive to build than a roof made out of wood trusses, the expense will pay for itself in the end. You won’t be having to deal with so many roof leaks, you don’t have to be as worried during a hurricane that your shingles or your cement tiles will fall off. If you have impact windows as well, you will have created a hurricane shelter for yourself and your family. You will feel more secure in your house during a hurricane!

Concrete roofs are stronger

Flat concrete roofs can have insulation on the outside and can be finished with a clean smooth surface on the inside. If planned ahead of time, they can also have recessed lights in the slab, although it might be better to have a small, dropped drywall ceiling for the recessed lights.

Concrete roof slabs are poured continuous with the concrete tie-beam at the top of the walls. This makes the structure very strong. Even if the roof membrane is torn off during a hurricane, it will not be raining inside your house. This is very important because during Hurricane Andrew, water intrusion caused an immense amount of damage inside the houses. In fact, I even saw where a part of the plywood sheathing was torn off and a hole was in the roof. Although cement tile and shingle roofing systems have improved since Hurricane Andrew, nothing can beat a concrete slab roof for strength.

Also, on your insurance, if 100% of your house is covered by a concrete roof, you can get certain deductions. But check with your insurance company because I don’t think you can get deductions for both impact windows as well as the concrete roof. Nevertheless, building your house with both a concrete roof and impact windows will make your house nearly impenetrable.

So next time you’re thinking about building a new house in Miami, consider these advantages to a concrete roof. You will not regret it!