Flat Roofs: The Wicking Witch of the West

Posted by jondavis on Friday, 3 of August , 2007 at 12:57 pm

Most people who decide to build a new custom home begin by choosing a home style that they like. Most of the homes that I have designed have had pitched roofs, but a substantial number of our projects have had flat roofs; usually either Southwest pueblo or contemporary styles.

In the past, flat roof homes were thought to be lower quality because a flat roof is cheaper to build. In many cases cost has been a factor in the choice, but not always. In some cases zoning height issues don’t allow a pitched roof on a two story home. With the new popularity of modern and pre-fab homes touted in high style magazines such as Dwell, flat roofs are gaining status. There are a lot of people in the Southwest who love pueblo style homes and for these people, a flat roof is a must.

So what are the issues in a house with a flat roof? The most important one is that flat roofs have more leaks than pitched roofs. Water that sits on a roof or runs off very slowly is certainly more prone to leakage than a roof that drains quickly.

Let me introduce you to the monster that is the cause of most of the flat roof horror stories but before I get into that keep in mind that you pretty much get what you pay for. The most popular finish roofing for residential flat roofs is tar and gravel. The reason it is the most popular is that it meets code and is cheap to install! Most homeowners are not roofing experts so they simply do not know what kind of roof they are getting or even that they have options and, in the case of homes that are built on spec, the roof is already in place when the new owners purchase it.

In my state of New Mexico, roofers offer what they call a 20 or 30 year roof but is only warranted for two years. There is a glaring contradiction with those two numbers. If the roof is really a 20 year product, shouldn’t it have a warranty of more than 10% of its so-called life?

Here is the reason that a hot tar and gravel roof does not last very long. First, when a home is framed, a wooden decking is laid down for the roof. Then the tar and gravel roofing is applied by laying down several thin sheets of tar paper with hot tar mopped in between. This is also called a “built-up roof” because it is built of layers of tar paper and tar. Gravel is put on top as the final layer to act as a barrier to the sun and to add a hard, walkable surface. As the roof goes through the daily cycles of hot and cold the materials expand and contract. The tar on a new roof has the elasticity to deal with this movement, but over time the solvents in the oil-based tar evaporate and the tar gets brittle. Eventually that brittleness causes the membrane of tar and paper to crack and you now have a way for the water to get through the roof. If you have a house with a tar and gravel roof, the best preventative maintenance is to put a white coating on the roof using a product like Snow Roof. This material is flexible and is less prone to drying and cracking. Another advantage is that it keeps the roof much cooler during the heat of the day which reduces the movement. A white roof that reflects the light means less unwanted summer heat migrating from the roof into the home, which keeps the home cooler too.

There is another type of roof that will last longer and save you a lot of grief over the long term. Commercial buildings with flat roofs use a “membrane” roof which is a fairly thick, rubberized material. The membrane is a single layer and the seams are melted together with a torch. This roof costs more that the tar and gravel roof but it lasts many times longer. It is the best investement in the long term and it is the roof that we specify on all of our flat roof homes.

One more issue about a flat roof has to do with architectural style. Pueblo style homes (such as the one pictured above) typically have a small (18″) wall (parapet) around the perimeter of the roof. The parapet, in effect, creates a shallow bath tub on the roof and the water is drained through openings in the parapet. Along the bottom of the openings is an gutter extension called a scupper or, in the Southwest, a canale. The roof must be built to direct the water toward the opening in the parapet so that it drains properly. Unfortunately, flat roof never seem to drain perfectly, especially over time. Flat roofs with parapets are prone to leaks where the scupper penetrations go through the walls. In commercial buildings it is common to have drains in the middle of the roof so that water drains through a pipe and gets taken down through the building and out to a drain area. It is rare to see these on homes (because of the cost) but they are a good solution. On many of the contemporary style homes, the flat roof will only have a few parapets, or none at all. In this case it is easier for the water to run off the roof.

I have probably told you more about flat roofs than you ever wanted to know, but if you are thinking about building a custom home, it’s a good idea to be knowledge about roofs so that you make the best decision for your home. Don’t simply allow your builder pick the roof for you.

A membrane flat roof will last about as long as a pitched roof but the key is picking the right roofing material and making sure it is correctly installed and maintained as per manufacturer’s instructions so that you don’t ever have a run-in with the Wicking Witch of the West.

Comments (1)

Category: Custom Design

• Add this post to
• Del.icio.us