You have probably seen poorly-installed heat wire for roof ice before: No rhyme or reason to the layout, odd or irregular angles, and just plain unsightly. We’re here to tell you that your DIY project doesn’t have to look like your third-grader was up on your roof. With only a few extra steps, your heat cable can have that desired professional look at a DIY price.
However, before we get to that part, let’s talk about your project. Laying heat trace cable yourself involves a little bit of risk in that you have to be on your roof to do the job. Professionals who install ice dam prevention products for a living generally agree that the best time to launch your DIY venture is when the weather is at least partially sunny and dry, and the roof is free of snow and ice. If the forecast is especially wintry for the foreseeable future, wait or call in an expert. Risking life and limb over a potential roof leak just isn’t worth it.
Where You’ll Need Your Cable
Now, onto the fun part. What type of roof do you have? For instance, an A-frame or bonnet roof only requires you lay cable along the bottom edge and in the gutter system, while a gabled roof will need additional feet of line to extend into the valleys and around any dormers. You may also want to protect sneaky spots like skylights and chimneys from freezing. Remember, any point on your roof that allows snow to melt and refreeze without draining off the roof represents a potential hazard.
Time to Measure
Once you have all your areas picked out, it’s time to break out the tape measure. You can begin by measuring from the edge of the roof gable to the valley point. Next, measure the valley. Typically, you’ll want your heat trace cable to go up the roof valley about six feet and back down again, so about 12 feet. Don’t feel like you have to be exact down to the quarter-inch. Rounding up to the nearest whole inch works just fine.
Next, measure your downspout to where the water exits away from your home. Sometimes, this may be under a concrete sidewalk, so it’s essential to measure the entire length of the drainage spout so you can thread the heat cable through to the end. This way, the water flowing off your housetop won’t have a chance to refreeze anywhere in your gutter system and cause issues.
Finally, you need to know how many inches you want to take your zig-zag pattern up the roof from the edge of the eave. The underside of your eave is called the soffit. The top of the triangle in your zig-zag design should end within the six-inch zone directly above where the inside of the soffit meets the exterior wall. That’s 4 to 5 shingle exposures (rows of shingles) up from the edge.
Calculate Your ‘Zig-Zags’
Remember the time you scoffed when your high school teacher told you all those math classes would be useful one day? Well, today is that day. Getting your measurements is only half the battle. The other half is figuring out how much heat trace cable you will need.
Let’s say the edge length of your roof measures 20 feet, and your downspout is about 15 feet. According to most cable manufacturer instructions, the standard method for determining your cable length across the edge of your roof is to multiply by four. So a 20-foot roof edge will need 80 feet of cable. But don’t forget your downspout. Adding that extra 15 feet gives you a total of 95 feet. Of course, you’ll need to figure in all your measurements for chimneys, dormers, gutters, and valleys as well, but we’ll leave those off for now.
The problem many DIY-ers encounter at this point is that most of the packages of pre-cut roof heat wire you find at big-box hardware stores don’t come in odd lengths of 95 feet or 115 feet, for example. You either have to go with the smaller 80-foot cable or the larger 100-foot cable. Getting your cable from a professional vendor like HeatCable.com, however, somewhat resolves that particular issue. We sell our cable in 5-foot increments and can customize lengths if the job requires it.
Back to our calculations. Let’s suppose you pick the 100-foot cable from the local hardware store. Here’s where the math gets a little tricky. First, subtract the 15 feet of cable you’re using for the downspout. That leaves you with 85 feet. Then, divide 85 by the 20-foot length of the roof edge. The number you end up with is 4.25. If you look at the roof edge as the base of each zig-zag triangle, you want to keep the length of that base between 18 inches to 24 inches: the wider the base, the less efficient your heat cable. Another good rule of thumb to remember is that the sides of your triangles should be longer than your base.
Let’s use an 18-inch base for our calculations. Take your 4.25 and times it by 18. Your answer should be 76.5. Using a stiff tape measure, place your thumb on the 18-inch mark, and then continue to pull the tape until you reach 76.5 inches. Bend the excess tape measure to form a triangle with your thumb in place. This is your zig-zag. It’s a good idea to take your makeshift triangle up to the roof to double-check that the top tip falls somewhere in the 6-inch zone we mentioned earlier. Don’t worry if it’s a little long. You just don’t want it too short.
Stay tuned for Part II of this series, where we will give you some tips on how to mark the spots where your cable clips will go and how to attach the heat trace wire to your roof without doing any damage. In the meantime, check out our How-To video on heat cable basics. See you soon!