Use of Heat Tapes on Roofs to Prevent Ice Dam Leaks in Buildings
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Guide to using de icing cables or heat tapes on roofs to prevent ice dam leaks into buildings
How to stop ice dams and roof leaks on buildings
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This article describes ice dams, attic condensation, attic mold, and inspection methods and clues to detect roof venting deficiencies, insulation defects, and attic condensation problems
in buildings. It describes proper roof ventilation placement, amounts, and other details.
Our page top photo shows roof edge heating tapes installed by an owner to attempt to melt channels through ice that may accumulate at the roof eaves in winter. This is an inexpensive band-aid that may be sufficient if ice dam formation on a roof is rare and/or it is difficult to install good under-roof venting. This roof has eaves and a ridge that could have been vented.
These recommendations for use of de icing cables and heat tapes on roofs (and other solutions to ice dams on building roofs) are based on many years of
building inspections, on the observation of the locations of moisture, mold, ice dams, condensation stains, and other clues in buildings,
and on the correlation of these clues with the roof venting conditions at those properties, and frequent literature review and professional discussion.
We have also measured changes in airflow, temperature,
and moisture before and after installing roof venting.
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Owner's guide to Use of Heat Tapes and De-Icing Cables on Roof Eaves
"Illinois: Electrical Flaw Suspect in Fire" reported the New York Times in February 2009. The Times reported that experts were focusing on a possible electrical malfunction in a roof ice buildup protection system on the Holy Name Cathedral in Chicago on February 4th. The resulting fire resulted in a blaze that "tore holes in the roof".
Photos of of Ice-damaged Roofs Indicate Where Roof Ice and Ice Dam Leaks are Likely
This glass solarium suffers from ice build-up during most winters. The ice action led to damage to the glass seals, resulting in leaks into the building.
If a roof shape prevents easy installation of under-roof venting, or if a home is located where building custom excludes under-roof venting, use of an ice-and-water shield product under the shingles at roof edges, or use of metal covering the lower roof slopes are other measures taken to prevent ice dam leaks into the building interior. See Roof Venting: Un-Vented Roof Solutions for a discussion of how we prevent leaks into roofs where venting is difficult to achieve.
How to Roof De-Icer Cables Work?
Roof de-icing cables melt a channel (above left) through the ice build-up at roof edges or wherever ice is forming and heating cables have been placed. The melted channels permit water that backs up behind the ice (over the warmer roof sections) to run off of the roof rather than backing up under the shingles where it leaks into the building.
Our photograph (above left) shows the typical installation pattern of roof de-icing cables or heat tapes installed on roofs in the Northeastern U.S. The second photo (above right) shows the roof de-icing tapes melting a drip-channel through snow at the roof edge. No ice had formed on this roof, just snowfall, when we took the photo, but in other weather conditions the owner had seen ice build-up along this roof edge. Inspect Building Exterior provides additional photos of clues that indicate a history of ice dam leaks even when the building is being inspected in warm, snow and ice-free weather.
In the photo the installation uses clips that attach the heating cable to the edges of individual shingle tabs. The installation looks a little sloppy as we can see that the roof de-icing cable has come loose from its shingle clip at the left side of our photo.
The cables might work better at avoiding ice damage to the roof and ice dam leaks to the building interior if they were carried all the way to the edge of the lowest shingle.
Where ice builds up in gutters or eaves troughs (photo at left), additional cabling may be needed in those systems as well.
Typically roof heating cables or de-icing heat tapes are installed along the lower roof eaves in a V-pattern or zig-zag pattern in the area where ice forms. As our page top photo illustrates, when the roof heating cables are operating they melt channels through the ice forming along the roof edge.
Guide to Placing Roof Heating Cables in Roof Trouble Spots
Some building roofs, by their shape and design, are just difficult to vent and are likely to have snow traps, ice build-up, and ice dam leaks into the interior unless special measures are taken.
Our photograph of heating cables on a roof slope (left) shows a combination of intersecting roof planes and a building sidewall that will naturally trap snow and perhaps lead to ice dam leaks on the building.
When we see a heating tape or de-icer cable on an older home in a location like this one we pose that the owner has already had reason to worry about snow and ice backup and leaks into the building at this location.
Installing the roof heater cables in this trouble spot is an easy, inexpensive, and quick step to take to avoid ongoing ice and leak problems here.
We'd have preferred that the architect avoid creating this problem intersection of roofs and walls. If the design demands creation of a snow and ice trap such as the one we show here, we'd have preferred that the builder install custom-built flashing or at a minimum, carefully fitted ice-and-water-shield membranes throughout the problem area, including running the membrane up the side-wall of the vertical building dormer against which these roofs spill.
Special products are available to move air up along roof valleys and hips, but they are so costly to retrofit to an existing roof that it makes sense to add them when re-roofing rather than while the roof shingles are still in good shape.
De Icer Cable & Heat Tape Safety Guide from the US CPSC
The U.S. CPSC has provided safety recommendations for homeowners using heat tapes to help prevent fires. While this advice was originally aimed at indoor use of heating tapes for freeze-proofing piping, this advice is helpful for outdoor de-icer cable use as well.
Some manufacturers of both roof de-icing cables and indoor heat tapes for plumbing produce electrical heat cables that can be crossed over or touch themselves without melting or burning or starting a fire. Be sure to read and follow the instructions on the de icer cable package.
Replace heat tapes that are older than three years
Use only heat tapes certified to meet recognized voluntary fire safety standards such as those provided by Underwriters Laboratories (UL), the Canadian Standards Association (CSA), and the Factory Mutual Research Corporation (FMRC).
Plug the grounded (3-prong) plug into a grounded GFCI (ground-fault circuit interrupter) protected electrical outlet
Don't cross the heat tape across itself unless the manufacturer says that's acceptable - crossed heat tapes can overheat and start a fire
Put the heating tape on the surface of the pipe to be protected from freezing, not on top of pipe insulation, and do not cover the heating tape with insulation unless the manufacturer says that's acceptable. Use only non-combustible insulation on pipes where heating tapes are installed (like fiberglass insulation). Foam or vinyl pipe insulation may catch on fire from a faulty heat tape or faulty heat tape installation.
Protect the sealed end-cap of the heat tape (see our photo above) from damage and water leaks which could cause a short circuit or fire.
Use heating tape only for the application approved for that particular heating tape, such as for use on piping. Some heat tapes are designed for use in gutters or driveways while others are designed for use on piping or fuel piping.
Be sure that the heat tape thermostat (if one is provided) is placed where the instructions say. For example some heat tape thermostats should be placed in contact with the pipe, others left hanging in air. (See our photo above)
Replace any heat tape that is discolored (a sign of overheating), cracked, melted, or damaged in any way.
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Additional technical contributors & reference sources for this article are listed below.
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"Illinois: Electrical Flaw Suspect in Fire", National Briefing, New York Times, P. A15, 6 February 2009.
Alan Carson Carson Dunlop Associates, Toronto, Ontario. Mr. Carson is a home inspection professional, educator, researcher, writer, and a principal of Carson Dunlop Associates, a Toronto home inspection and education firm. Mr. Carson is a past president of ASHI, the American Society of Home Inspectors
Some great illustrations of the proper under-roof ventilation pathways are offered by Carson Dunlop.
Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. (727) 595-4211 mark@BestTampaInspector.com 11/06 & 12/08
John Annunziata, P.E. - NY Metro ASHI during informal chapter discussions about roof and attic ventilation options (1986-1996).
More Information on Building Diagnostic Inspections and Repairs
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Be sure to review HOW TO FIND MOLD: How to Inspect Homes and Other Buildings for Mold - the Basics of How to Find Problem Mold Indoors
in our Mold Action Guide. Here are other articles that will be helpful in
evaluating attic mold presence, causes, and cures:
How to Clean Moldy Wood Framing & Sheathing How to clean/seal mold from/on exposed lumber or plywood subfloor or roof sheathing indoors - some suggestions based on our field and laboratory research
Lighting, proper use of: proper aiming of a good flashlight can disclose hard to see but toxic light or white mold colonies on walls.
Stuff that is not mold but is often mistaken for it - things you may not want to test. Also, not all "black mold" is toxic - here are examples of harmless black mold.
Humidity: What indoor humidity should we maintain in order to avoid a mold problem?
Mold-Resistant Building Practices, advice from an expert on how to prevent mold after a building flood and how to prevent mold growth in buildings by selection of building materials and by anti-mold construction details.
InspectAPedia® Home & Site Map - Building & Environmental Inspection, Testing, Diagnosis, Repair, & Problem Prevention Advice: In-depth research & advice on diagnosing, testing, correcting, & preventing building defects & indoor environmental hazards. Unbiased information, no conflicts of interest.
The Mold Information Center: What to Do About Mold in Buildings, When and How to Inspect for Mold, Clean Up Mold, or Avoid Mold Problems
Environmental Inspection, Testing, & Diagnosis On-Site IAQ, Gas, Air Testing, Mold Investigation, Sick Building Diagnosis, Lab Services, & Remediation Plan Preparation - indoor air quality testing, problem source determination, supporting lab work, written remediation plan addressing removal of environmental and other hazards and prevention of their recurrence.
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