This is part plumbing and part roof issue. I have a flat decked roof with a 3" drain that goes down 4 levels (in interior walls) to the sewer. The drain always clogs at the top. I've tried different things (even tried a thread here ( Roof drain clogging from debrisl), but nothing really keeps it from blocking at the top.

Only a handful of leaves causes it to clog. We check it diligently, but I am worried if it clogs when we aren't around. Last night the water was 5" high until we removed some leaves. It could cause the whole townhouse to flood.

I have an idea to use a bilge pump around a foot from the drain. I would run a hose from the pump directly into the drain. I would power this with a motorcycle battery or a AC to DC transformer. This would be a backup and we'd obviously still try to keep the drain clear. Any thoughts?

I only have about 5" clearance under the decking.

 

Have you done a ring or screen further away from the drain? I've had good luck with a large screen further away so it has a lot more surface area making it more difficult to clog. Then have no grate or a grate with large openings at the drain. The no grate idea is so that anything that makes it past the outer screen will go down the drain and not form a clog at the inner barrier.

 

Yes. I was afraid it wouldn't catch everything though so I still have chicken wire covering the top of the drain. Unless specifically engineered to conform to the curves of the roof, nothing will catch everything. Maybe letting a few leaves go down the drain isn't so bad and I should get rid of the chicken wire?

 

Trying to understand how 5 inches of water can sit on a flat roof. Please explain or provide picture. Try a bottle brush or a duct brush stuck in the drain pipe standing vertically above the top of the roof. There is a product called Gutter Brush for laying in gutters that can be trimmed to fit your application.

 

sounds to me like you didn't try what was suggested the last time you asked... because that's exactly what we were telling you to make.... a large custom made box that conforms to the roof, covered by a course screen like hardware cloth.

 

If you want to catch debris and not clog you need more surface area. That means a bigger screen than the little one right over the drain.

 

There are space limitations. The access panel to the drain is only around 12" square. There are also runners (supports) holding up the deck (I believe every 16 inches). So I cannot really build something big without tearing the deck apart.

What I have now is a large metal colander inverted with a rock weighing it down. I also have 1/2" hardware cloth in the drain. The hardware cloth is where the problem is. When the water was backed up, that is where I had to remove the leaves from.

Obviously some leaves got past the colander,

 

I would loose the colander and get creative with hardware cloth. Make a screen as big as you can that will fit through the access opening. You want to catch the debris as far away from the drain as possible. A 12" square screen will require hundreds of leaves to block and even then the water will rise a bit and flow over the clogging leaves. But, at the drain it takes relatively few leaves to block it and cause a back-up.