De-Icing Calculations


Follow along through the tabs to determine the length of cable you will need. If you only intend to install the PRIME Roof & Gutter Kit in the gutters, skip to the “Gutter Measurements” tab.

For an example of a full roof layout, refer to Fig. 1.
For an example of a gutters only layout, refer to Fig. 2.

Every roof will vary slightly. The following worksheet will help determine the cable length needed to cover the areas entered into the worksheet. For problem areas that deviate from the specified calculations, remember to add or subtract as needed and follow the instruction manual for how to install properly. For questions or concerns contact PRIME customer support at 1-888-445-9955.

Enter in a value or check for each area under each tab. As you go through each tab,
enter 0 if your roof does not include a portion of a measurement. For example: if
the length of your roof edge is 24 feet 0 inches, enter in 24 and then 0, do not leave
the inches box blank. If your roof does not have a valley, in the valley tab enter
0 into each box.

Roof Measurements

Is a gutter present at the roof’s edge?

Length of Roof Edge (Fig. 1)

Overhang Width (Fig. 2)

Tracing Height: 0

Number of exposed 5.5 (in) shingles: 0


At no point should the cable be crossed over itself or touching itself when energized. Doing so may cause the cable to overheat and may damage the cable or cause injury by electric shock.

Valley Measurements

Number of Valleys

Total Combined Length of Valleys


Installation Tip: We recommend installing the cable 2/3rds up each valley (minimum 3feet/91cm).

Dormer Measurements

A dormer is a window that projects vertically from a sloping roof.
Use the images to understand the requested measurements if a dormer
exists on your roof to be covered, otherwise enter “0” in each box
before hitting submit.

Total Distance From Top of Tracing Height to Dormer (Fig. 1)

(Will typically be 0 as it will most likely be at the top of the
tracing height)

Total Length Around Perimeter
of Dormers (Fig. 2)


Installation Tip: Add clips every 3 feet (91cm) around the dormer to help hold the cable in place.” “We recommend you install around the dormer as shown in the images above. However, if you feel your only problem area is at the bottom of the dormer, you may treat it as a “Special Roof Area” and you may enter 0 on this page and use the “Special Roof Measurements” tab to cover this area.

Gutter Measurements

Use this tab if you are measuring gutters only. If you are using the
"Roof Measurements" tab, the gutter measurement has already
been added in.

Is the gutter width greater than 6 inches?

Total Length of the Gutters


Downspout Measurements

Downspouts may appear at several points on a roof. It is important
to install the cable inside the downspout as the water needs a place to
drain. If your downspout is not at an appropriate location on the roof
to finish the cable in the downspout you will need the cable to go down
into the downspout and back up using spacers every 6 inches to prevent
the cable from overlapping or touching itself.

Will the cable be finishing in a downspout?

Number of Downspouts

Length of Downspouts


Installation Tip: A weighted string can come in handy to help measure the length of the downspouts. Allow the weighted string to go through the downspout and mark or pinch the string at the top when the weight reaches the bottom. Measure the length of string that entered the downspout and double it for cable that does not finish in the measured downspout. This will prevent you from placing the cable in and out of the downspout too many times, helping to prevent wear on the cable during installation. Remember, add spacers every 6 inches for cable that is being doubled up in the downspout.

Special Roof Area

Special Roof Areas are spots on the roof not defined in other tabs in this work book. The “width of ice dams that form along special roof area” will be different depending on what your special roof area looks like. With a skylight, typical ice dam formation happens along the base of the skylight or the side facing the roof edge. In this case the width of ice dams would be the width of the bottom of the skylight.

Is a gutter present under the special roof area?

Number of Special Roof Areas

Distance From Roof Edge to Bottom of Special Roof Area

Width of Ice Dams that Form Along Special Roof Area (Fig. 1)



Roof Cable:
Valley Cable:
Dormer Cable:
Downspout Cable:
Skylight Cable:


Downspout Cable:
Gutter Cable:

Suggested Cable:

US Standard Scale
Metric Scale