How Much Ponding Is Acceptable On A Flat Roof?

How Much Ponding Is Acceptable On A Flat Roof?

How Much Ponding Is Acceptable On A Flat Roof?

Ponding that drains or evaporates within 24hr to 48 hours is acceptable and not cause for alarm, but ponding that occurs on a regular basis should be addressed.

If the ponding persist after 48 hours, you should contact a professional roofer, since this indicates that the roof drainage system is malfunctioning or that there is an issue with the roof slope, roofing material, or roof deck.


When ponding water on flat roofs, the National Roofing Contractors Association (NRCA) guides what is acceptable. According to the NRCA, standing water on a flat roof for 12-36 hours after a rain storm is normal.

However, if water is found ponding on a flat roof for longer than 2 full days, it is advisable to have a professional commercial roofing contractor take a look. There are several reasons why water may pond on a flat roof.

Poor drainage is the most common reason, as flat roofs are not as pitched as traditional roofs and therefore do not allow water to run off as easily. In addition, debris or leaves accumulated on the roof can contribute to ponding water.

The NRCA recommends that flat roofs be designed with a minimum slope of ¼ inch per foot to help ensure proper drainage.

How Do You Finish The Edges Of A Flat Roof?

There are a few different ways to finish the edges of a flat roof. One way is simply to install a flashing layer over the roof’s edge. This will create a water-tight seal and help protect the roof from potential damage. Another way to finish the edges of a flat roof is to install a drip edge.

This is a metal strip installed along the edges of the roof. The metal drip edge protects the roof’s edge from water damage and provides a place for the water to drip off the roof. Another way to finish the edges of a flat roof is with a parapet.

A parapet is a wall that extends above the edge of the roof. Parapets can be made of various materials, such as concrete, stone, or metal.

Another way to finish the edges of a flat roof is with a fascia. A fascia is a board that is attached to the edge of the roof. Fascia can be made of various materials, such as wood, plastic, or metal.

What Size Joists Do I Need For A Flat Roof?

The most common flat roof joist sizes are 150 x 50mm, 175 x 50mm, and 200 x 50mm. The spacing will depend on the span and loads, but usually, we can see flat roof joist spacing between 400mm and 600mm centers.

It is fairly accepted that the largest joists should be placed at the edges of a flat roof, and smaller joists should be placed at the center of a roof.  The longer the span, the deeper the joist will be. The loads on the flat roof will also dictate the size of the joist.

A live load of 40 pounds per square foot will require a stronger joist than a dead load of 20 pounds per square foot.

Can I Put A Flat Roof On A Pergola?

A flat roof pergola is a great way to add shade and protection from the elements to your outdoor space. Whether installed over an existing patio or as a freestanding structure in your backyard, a flat roof pergola offers numerous advantages to homeowners.

One of the biggest advantages of a flat roof pergola is the amount of sun and rain protection it can provide. A well-designed pergola will keep you cool and dry even on the hottest days and provide a comfortable spot to relax or entertain guests during a rain shower.

Another advantage of a flat roof pergola is that it can be built to any size or shape to suit your specific needs. Whether you need a small pergola that can provide some shade or large outdoor dining space, you can build it according to your specifications.

You may also need to consider the stability of the structure. For example, if your flat roof pergola is built into a wall, it will be more stable than one on top of a patio. However, if your platform is not supported by beams or another structure underneath, it will not be as stable.

Lastly, a flat roof pergola will add character and value to any outdoor space in your home. Whether building an existing pergola or constructing a new one from scratch, the finished product will be more unique than any other outdoor structures previously used.

How Can I Make My Flat Roof Look Better?

There are a few ways to make a flat roof look better and complement your home’s character. One way is to use lead roll finishes. Lead roll finishes are a great way to make a flat roof more attractive and can also help it better match and complement your home’s character.

By installing lead roll finishes, you can give your flat roof a more finished and polished look that will make it stand out and look great. Lead roll finishes are also durable and long-lasting, so you can be sure that your roof will look great for years to come.

Another way to improve the look of a flat roof is to use different colors or finishes for the trim and gutters. This can help to add some visual interest to the roof and make it more attractive.

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