This may sound like a strange question, but I don't mean leak-proof in the typical sense, but actually water-tight.

Part of our house has a flat roof (virtually new) that connects to a pitched roof. Water rolls off the pitched roof to the flat roof (actually slightly pitched), and then to the ground. However, in heavy (very heavy!) rain, there is a leak. I suspect that the water builds up when it hits the flat roof and backs up a few inches under the pitched roof shingles and gets in that way.

Is this plausible? It's only happened once, recently here in Houston when we had about 15 inches of rain in 6 hours.

Thanks.

 

Yes, that is very plausable in that situation.
There should be a strip of flashing under the shingles where the two roofs meet, and it might need to be put down onto a thin bed of plastic roofing cement to seal it tight.
Good Luck!

 

Thanks for the response. My guess is that whatever flashing is there is insufficient, but I will check.

 

My new sun room is attached to my kitchen. The sun room roof has a very slight pitch and is attached to the pitched roof of my kitchen. After rain w/some degree of wind, I noticed water coming in from my new ceiling. After several attempts to correct this, my contractors called in a professional roofer who said that the contractors had used the wrong type of materials. I needed a 'torch-down' material applied. Because the pitch was so slight, it didn't take much wind to drive water up the shingled material. The new roof came off, the torch-down went on, and no more problems. An expensive learning experience for my contractors. Pitch, apparently, is a major factor when determining roofing materials.
Good luck.

 

i am a maint mech in a large (4.7 acre)facility .90% of the roof drainage runs off in one direction,plus there are large ventilators & many sky lights .there is an expansion step down about 1/2 down and it leaks terribly when there is a heavy rain. i went up on the roof to see the problem during a storm & the water actually was running backward or uphill if you will about 6" under the step down which is a 10-12" overhang.