How To Cut A Drainage Channel In Your Flat Roof To Eliminate Water Pooling

30 July 2015
 Categories: Construction & Contractors, Articles


Flat roofs are subject to several problems not ordinarily seen in angled roofs. One such problem is the pooling of water in various spots around the roof. A frequently-seen trouble-spot is along the edges of roofs; slight humps are prone to develop in this area, and they can form dams that trap water rather than allowing it to run-off. These humps are particularly prevalent in roofs that have been relaid several times with prior layers left intact.

Regardless of cause, the solution is the same: removing the humps and forming a shallow drainage channel. Below is how you can correct this problem and facilitate proper drainage:

Tools and materials needed

  • Asphalt roof primer
  • Trowel
  • Bitumen roofing membrane
  • Propane torch and tank
  • Utility knife
  • Gloves
  • Fire extinguisher
  • Claw hammer
  • Roofing nails
  • Whisk broom

Step-by-step procedure

1. Work safely - any kind of rooftop work involves risk, but the added dimension of using a torch and flammable substance adds to the potential hazards. You can minimize the danger by observing a couple of simple safety principles:

  • Protect yourself - be sure to wear appropriate clothing such as long-sleeve cotton or denim shirts, blue jeans, gloves and steel-toed boots. These articles of clothing will help keep you from being burned by the torch, hot asphalt and other heated items.

  • Protect the property - when using a propane torch on a roof, carry a small fire extinguisher with you to put out any unforeseen fires. Be sure the fire extinguisher is rated for multi-purpose use and is fully-charged.

2. Locate the highest humps in your roof edges - most rooftops will not be perfectly flat, and you will be able to locate the edge humps with simple observation. However, some humps will be higher than others, and it is to your roof's benefit to remove the "tallest" ones. Even if you aren't able to address all of them at one time, removing the worst-offending humps will aid in drainage.

3. Cut away the material inside the humped area - once you locate the first hump you wish to eliminate, use your utility knife to cut a perimeter around the hump and form a channel that leads to the roof's edge.

Make your cuts shallow and do not attempt to cut through all the layers of roof material at one time. As you complete cutting through each layer, pull it up and set it aside for disposal. Depending on how often the roof has been replaced, you may need to cut through several layers to reach the underlayment.

4. Prepare the drainage channel - once you have cut through all the layers and reach the plywood underlayment, you are ready to form a drainage channel. The bottom of your channel consists of the plywood as well as edge flashing attached to the plywood and that extends over the edge of the roof.

If the flashing is not firmly secured, hammer several roofing nails through the flashing into the plywood. After that, apply a ¼-inch layer of roofing primer to the edge of the flashing and blend it in with the plywood to protect from water incursion. Sweep away dirt, gravel, or any other debris that may be inside the channel.

5. Install the bitumen roofing membrane - from a roll of bitumen roofing membrane, cut out a piece of membrane that is equal to length and width of the drainage channel you just made. Add an extra 12 inches to the cut piece of membrane.

Line up the membrane so it covers the drainage channel, then tack down the edge of the membrane closest to you. Pull back the membrane and apply the flame of a propane torch to the back side of the membrane in a sweeping pattern. Continue heating until the bitumen backing begins to bubble and melt. If the membrane catches fire at any point, keep calm and pat out the flames with the trowel; a small fire will not harm the membrane's effectiveness.

As you heat the membrane, simultaneously unroll it over the underlayment and stomp it firmly with your boots. Whenever you come to the edge of the roof and flashing, apply another layer of primer on top of the flashing so it can adhere to the membrane.

6. Seal the edges of the membrane - apply roofing primer to the seam where the membrane crosses the existing roofing materials. Do not use nails or any other type of fastener to join the two seams, or you may cause a leak.

Click for more info or for help finding a professional if this seems like something out of your DIY abilities.