10 Things To Consider Before Replacing Your Flat Roof

When the time comes to replace your flat roofing system there are several things to think about that can improve the performance of your next roof. There are plenty of factors that can contribute to the failure of a flat roof, including improper slope, poor drainage, and structural problems. The location and use of the building will bring up other considerations such as R-Value, wind uplift, and fire resistance.

The guidelines in part one and part two of this post will help ensure that your next roofing system lasts longer than the one you’re replacing.


The slope of your roof system is the key player in how well your roof sheds water. Ponding water, the biggest problem among commercial or flat roofing systems, is caused by insufficient roof slope. When a poorly sloped roof starts to leak, the leaks will be much more severe for the fact that the water has nowhere else to go. There are older flat roofs out there that are performing satisfactorily with slopes of just 1/8″ per foot, however, it is generally recommended that the slope be a minimum of 1/4″ per foot in order to minimize ponding water on the roof surface and prevent subsequent leaks.

Roof Drainage

As your roof’s slope sheds water, your roof’s drainage system disposes of it. And, an inadequate drainage system will cause many of the same problems that improper slope will. Whether you’re using internal drains, scuppers or gutters and downspouts, the roof drainage system needs to be matched to the size and slope of your roof. As the slope of your roof increases, the volume of water that is routed to the drainage system will increase. Stop and ask yourself if there are enough drains, if the scupper openings are large enough, or if your gutter system is large enough handle the volume of water expected to hit your roof.


Your insulation’s ability to resist heat transfer is determined by its R value. The higher the R value, the better. Consider your roof’s existing R-value and how it affects your heating & cooling costs. Not only will adding insulation improve your R value, but if your roof’s slope is inadequate, using tapered insulation can be a more cost-effective solution than structurally altering the roof. It is the best way to insulate a flat roof and by adding thickness to the taper, it increases your R-value.


Another important thing to think about is the weight your roofing system can support, which is typically expressed in pounds per square foot (PSF). Roofs are generally engineered to handle projected wind and snow loads based on regional, historical data.

Let’s say your original built-up roof system was installed with a structural load of 2 PSF. This value would have also dictated the framing that was required to accomplish the desired structural strength. Now, let’s say you’re considering a modern single-ply EPDM membrane roof system as a replacement, and your prefer a ballasted system since it’s typically the least expensive. Ballasted systems usually have a structural load of around 10-12 PSF. The additional weight load placed on the roof structure (even if the old roofing system was removed) could easily cause the roof to collapse. Therefore, it is crucial to compare the weight of the new roof system to the limits of the original roof system that the building was designed around. Consult with an engineer if you aren’t sure about how much weight your roof structure can safely handle.


If your roof deck has structural problems this is another important element to contemplate when replacing your roof system. Wood-framed roofs often have joists that have bowed from years of constant load. This can cause water to pond in the middle of the roof. Maybe your roof decking has weird elevation changes from previous instances of construction and remodeling. Whatever decking issue you may be facing, it’s critical that these aspects are factored into your roof replacement project. If your roof structure has deficiencies and you decide to install a new roof system over the existing one, the new roof will have the same problems. You will have the best opportunity to address structural issues is the existing roof system(s) are removed down to the decking.

Find the last 5 flat roofing considerations in Part 2 of this post!

If it’s time to repair or replace your flat roof, check out our commercial roofing page and contact us today.




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10 Things To Consider Before Replacing Your Flat Roof, Part 2

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