How much ponding is acceptable?
In general, the minimum slope for water to flow down is 1% (1/8″ per 1′). However, the minimum pitch for a flat roof required by building regulation in most areas in the United States is 2%. (1/4″ per 1′). If you have a lowly inclined slope, you will most likely require membrane roofing to prevent ponding.
Should there be pooling on a flat roof?
Standing water on a flat roof for 12-36 hours after a rain storm can be normal, but according to the National Roofing Contractors Association, any water found ponding on flat roofs for longer than 2 full days should be examined by a professional commercial roofing contractor.
How do you raise a low spot on a flat roof?
A variety of liquid patching materials are appropriate for fixing low spots on flat roofs. Roof patch putty can be applied, using a trowel, to fill low areas. You can also use a general epoxy concrete filler and apply mastic on top, using a trowel. Mastic is a flexible sealant that adheres to almost any material.
How much pooling is acceptable on a flat roof?
Opinions range from, “It’s a flat roof, it’s meant to hold water,” to, “No water should ever sit on a flat roof for more than 24 hours.” The truth is in the middle. As a general rule of thumb, most single-ply manufacturers require 80% of water to be off of a roof within 48 hours of a rain event.
Is ponding on a flat roof normal?
Ponding is when water collects in pools on a flat roof after a period of heavy rain and doesn’t drain off once the rain has stopped. Whilst brief and occasional ponding is acceptable on a flat roof, regular or long-lasting ponding could be an indication that there is an issue with the roof that needs to be addressed.
What happens if water pools on a flat roof?
Common problems caused by flat roof ponding:
Extra stress and damage to the load bearing structure due to the extra weight of the water. In winter freezing standing water will expand causing more damage. Where seams overlap the ponding water can cause the membrane to lose it’s seal, allowing water in underneath.
How do you slope a flat roof for drainage?
For drainage purposes, the current building codes require a minimum two percent slope or a one-fourth unit vertical for every 12 units horizontal. In other words, the roof must slope one-fourth of an inch for every 12 inches. The slope is barely noticeable to the naked eye.
