How to Fix Poor Roof Drainage
If you have ponding water or algae on the roof, you have poor drainage. A roof inspection and evaluation are necessary to determine the root cause. Only after the problem is defined can a solution be applied. It could be as simple as doing a little maintenance or, unfortunately, as expensive as a whole new roof.
Roof Maintenance for Better Drainage
Make sure that drainage isn’t obstructed. Perform visual inspections after storms to check that environmental debris isn’t blocking drains. If drains are clogged, clear the drain as soon as possible. An especially bad clog may require that a plumber be called to snake the drain. If drain baskets are missing, replace them; they prevent large vegetation and debris from getting into drain pipes and cost less than $100 each.
To keep drainage paths open, be proactive. Trim nearby trees to reduce the amount of leaves and pine needles that could clog drains and scuppers.
When rooftop workers (HVAC technicians, in-house staff, etc.) use the roof as a work surface, provide a friendly reminder to remove all debris from the roof during each visit. Spare parts, rags, soda bottles, etc. can easily end up in a roof drain.
At least annually, have your roofing contractor perform general preventive maintenance that inspects and cleans drains and waterways. The cause of clogged drains, scuppers, gutters, and downspouts is typically carelessness and lack of maintenance.
Modifying the Roof for Better Drainage
While it may not make sense on every roof, there are some possible modifications that can be made to improve drainage.
- Consider adding a cricket. These diamond- or inverted v-shaped ridges direct water to (and prevent ponding between) drains. They’re often placed in front of large curbs and at parapet walls to divert water to a drain or through-wall scupper.
- Build a small trough. A small section of insulation can be removed to create a trough out of membrane to divert water to the drain.
- Lower or enlarge gutters. If gutters are built too high or sized too small, water cannot drain off the roof effectively.
- Change a metal-type gravel stop to nosing so water can flow freely over it.
- Add drains or scuppers. Have your roofing contractor and plumber work together – possibly with an engineer – to help determine the optimum placement and feasibility.
Installing a New Roof for Better Drainage
Investigating the cause of ponding water on the existing roof will help you make determinations about the new roof design. An engineer will help determine the placement of new roof drains; there are some locations where a drain isn’t feasible. Make sure there are no structural problems. If the roof trusses are too far apart or the decking has corroded, deck deflection is probably to blame for poor drainage. Your reroofing project must remedy this to avoid the same problem in the future.
Slope is critical on commercial and industrial buildings because it facilitates proper water drainage. Unless the existing roof already has an adequate structural slope, then slope should be added. This is typically accomplished with the addition of tapered insulation.
If a drain is too high (e.g. if they sit on an I-beam), it might be necessary to sump the drain. This can be achieved by adding thicker insulation, so the drain is lower than the field of the roof, and water flows more easily to the drain.
No solution is arrived at through observation alone. Consider involving professionals – your roofing contractor, a roof consultant, structural engineer, and/or plumber – to help you determine why water isn’t draining and how best to address it.
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