Q: Last winter, the interior walls of a corner bedroom occasionally bubbled out as if water was gathering on the other side of the paint. This usually happened after a good downpour.

I just checked that corner of my flat roof and discovered that a neighbor’s overhanging tree had clogged the corner drain and downspout with leaves and soil-like detritus. I asked the neighbor to trim her tree and she did. But it seems the branches grew back more quickly than we both thought they would.

I can clear out the corner drain and downspout as a spring project. Next fall, I’ll be sure to trim the overhanging branches myself if her gardener fails to do so. But it seems as if there’s still some water lodged behind the bedroom paint. How do I get that out or will it dry on its own? Also, should I be worried about water damage or dry rot? If so, what would you advise I do?

Interestingly, there are no water bubbles breaking out on the outside paint.

A: Flat roofs are notorious for leaks at drain spouts, especially when coupled with parapet walls.

Drains get plugged with leaves and other debris, the water puddles and finds its way through cracks in the built-up asphalt roofing and down the walls, sometimes all the way to the basement or crawl space.

Back in the 1980s when Kevin was selling real estate in Northern California, he listed and sold a home with a flat roof and parapet walls in San Leandro. The owners had just finished a complete remodel and were looking to trade their two-bedroom home for a larger one. They had even replaced the brick foundation with a new poured reinforced concrete foundation.

Everything looked great and the deal moved right along — until the structural and pest inspection. At first, the inspector found a few minor problems, but recommended making some exploratory holes in the stucco to determine if any infestation had occurred underneath.

The buyers asked that the inspection be made, and boy, were there problems. Two entire corners of the house were infested with dry rot. It was so bad that there were no corner studs, only dust. Cost of the repair was around $16,000.

In those days this represented about 10 percent of the home’s value. The homeowners bit the bullet, did the repairs and closed the deal. We learned to be very leery of homes with flat roofs.

So far you’ve done everything right. But to answer your questions: The water may drain out on its own, but the leak may leave some residual damage, such as dry rot or mold.

You should be concerned and you should take a few more steps to determine if you have a problem.

The fact that there still seems to be some water in the wall tells us that the problem may not be solved. Water on the inside of a stud wall will generally show itself on the interior because plaster or Sheetrock absorbs water more readily than stucco.

First, we’d recommend you have your roof inspected by a licensed roofing contractor. Hopefully it’s watertight.

Next we’d recommend that you drill several exploratory holes in the inside corner where you suspect moisture is present. Use a 1/2-inch drill bit and drill a hole at the bottom, the middle and at the top of the wall. Just drill through the plaster or Sheetrock until you hit wood.

In each hole, probe the stud with a screwdriver. If the studs are solid, there is probably limited damage, if any. Repair the holes with patching plaster and repaint.

If the screwdriver goes right through the stud, the corner is rotten and needs to be repaired.

If you don’t want to tackle the job yourself, a licensed and bonded structural and pest control company is your best bet for repairs. They handle this type of repair every day and can do it in an efficient and workmanlike way. They are also licensed to apply any fungicide that might be necessary.

In any event, keep the tree trimmed and the drain spout free of leaves and other debris.

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