Why a flat commercial roof is not really flat

Most commercial buildings have what people call a flat roof. From the ground, and even from the roof itself, it looks level. In fact, a well-built low-slope roof is tilted on purpose, just enough to move water toward drains, scuppers or gutters.

This post explains why that small slope is there, how it gets built in, and what happens to a roof when water has nowhere to go.

Flat is a nickname

Roofers sort roofs into two families. Low-slope roofs are membrane systems installed at a slope of 3:12 or less. Steep-slope roofs use overlapping pieces such as shingles on slopes greater than 3:12. Industry writers note that low-slope roofs are often called flat roofs.

The nickname can be misleading. A low-slope roof covers the building with a continuous membrane that keeps water out even when water sits on it for a short time. But the roof is still designed so that water keeps moving toward a drain. It is not meant to hold water.

The code minimum is 1/4 inch per foot

For new buildings, the building code sets a minimum slope for membrane roofs. NRCA's Mark Graham explains that the 2018 International Building Code requires a 1/4:12 minimum slope for built-up, modified bitumen, single-ply and liquid-applied membranes, and for spray foam roofs.

A slope of 1/4:12 means the roof drops a quarter inch for every 12 inches it runs. That is a 2 percent slope. Over a 40 foot run, the roof drops 10 inches. You would not notice that by eye, but water does.

When an existing roof is replaced or re-covered, the code works a little differently. Graham notes that the codes waive the 1/4:12 minimum for those projects and require positive roof drainage instead.

The 48 hour rule

Positive roof drainage has a specific meaning. The building code defines it as a drainage condition where the roof deck's deflection under load has been considered and enough slope has been added to drain the roof within 48 hours of rain.

NRCA uses the same test. In a Professional Roofing article, NRCA's Jason Wilen writes that ponding water for short periods is unavoidable and acceptable to NRCA and membrane manufacturers. Ponding water that stays longer than 48 hours can harm the roof assembly.

So a puddle the morning after a storm is normal. A puddle that is still there two days later, in weather dry enough for it to evaporate, is a sign the roof is not draining the way it should.

How the slope gets built in

Wilen lists several ways to give a low-slope roof its slope:

• Slope the structural framing or deck itself.
• Install tapered insulation, which is thicker at the high points and thinner at the drains. This can also add to the roof's insulation value.
• Use an insulating fill, such as lightweight insulating concrete or spray foam, that can be shaped to drain.
• Place roof drains, scuppers and gutters in the right locations.
• Combine these methods.

Smaller sloped pieces called crickets and saddles help too. A cricket sits on the high side of a curb or rooftop unit and turns water around it. A saddle sits in the valley where two slopes meet and sends water toward the interior drains. Wilen notes that some ponding along the valleys of crickets and saddles will occur and should be expected.

What happens when water cannot drain

Standing water is heavy. A WoodWorks guide, quoting the American Institute of Timber Construction, notes that one inch of water weighs 5.2 pounds per square foot. Two inches of water across a 1,000 square foot area adds more than 10,000 pounds to the roof.

That weight can make the problem grow. The same guide explains that ponding can cause extra loads and additional progressive deflection. When a roof bends under the weight of water, the low spot gets deeper, and a deeper low spot holds more water. The guide notes this is usually a bigger problem on roofs with long structural spans.

Water that sits on a roof also has more time to find a way in. The Buildings article, citing NRCA, notes that when water gets underneath roofing materials, it can work its way to the roof deck and cause the roof structure to rot. Wilen also notes that equipment, snow and foot traffic on the roof can create low areas where water collects.

Drains, scuppers and backup drainage

The slope only helps if the water has a way off the roof. Drains, scuppers and gutters carry it away, and they have to stay open.

Wilen explains that the International Building Code requires secondary drainage, through overflow drains or scuppers, where the roof edge would trap water if the main drains backed up. The WoodWorks guide adds that drains should be kept clear of debris and opened if they freeze over, since packed snow or ice can block the drain path.

What an owner can watch for

You do not need to be a roofer to spot drainage trouble. A few things are worth noting:

• Water that is still on the roof two days after rain in dry weather.
• Leaves, trash or gravel sitting in or around drains and scuppers.
• Water collecting next to rooftop units, curbs or walls.
• Low spots that seem to grow deeper from year to year.

If you see any of these, have the roof looked at before the next season of rain and snow.

What to take away

A flat commercial roof is not flat. For new construction, the building code calls for at least 1/4 inch of slope per foot on membrane roofs, and the industry standard is that a roof should drain within 48 hours of rain. The slope can come from the structure, tapered insulation, fill, or crickets and saddles, and drains and scuppers carry the water away. When water cannot drain, its weight can make low spots deeper, and water that stays on a roof has more time to get underneath it.