Snow Guards and Snow Retention: Protecting People, Gutters, and Lower Roofs

You need snow guards where a sliding sheet of roof snow would land on something that matters: a doorway, walkway, driveway, deck, lower roof, gutter, gas meter or vent. FEMA’s Snow Load Safety Guide puts the weight of a foot of fresh snow at anywhere from 3 pounds per square foot for light, dry snow to about 21 for wet, heavy snow, so a slide off one roof plane can land hard.

The roofing material changes how readily snow slides and how guards attach, but the need for retention comes from what sits below the eave. Map the landing zones first, then let the guard maker’s layout method decide how many guards go where.

Why Snow Slides Off a Roof All at Once

Snow sticks to a roof until the bond at the roof surface lets go. Sun on the roof, heat leaking up from the house or a warm spell can melt a thin layer where snow meets roofing, and once that layer forms across a wide area, gravity pulls the whole sheet down the slope together. Smooth surfaces such as metal panels release snow more readily than textured shingles, and steeper slopes pull harder.

The weight is what makes a slide dangerous. Besides the fresh-snow range above, FEMA’s Snow Load Safety Guide (FEMA P-957) states that “1 foot of ice weighs approximately 57 pounds per square foot.” The same guide warns that “One foot of snow on the ground does not necessarily equal 1 foot of snow on a roof,” so depth seen from the yard says little about the weight that can slide.

Hypothetical example: a slab 10 feet wide, 20 feet long and 1 foot deep covers 200 square feet. At FEMA’s wet, heavy snow figure of about 21 pounds per square foot, that slab weighs about 4,200 pounds before it picks up speed down the slope. The figure shows why a slide is dangerous; it is not a calculation of how many guards to install or where, which the guard maker works out for your roof.

Mapping Where Sliding Snow Would Land

On a day when the roof holds no snow, walk the house on the ground and look up at each roof plane in turn. Every eave is a launch point, and the question for each one is what sits in the strip of ground or roof below it. Photograph each spot with the eave in frame so you and the roofer are looking at the same thing.

Landing zones that call for retention:

• Entry doors, steps, walkways and the path to the car.
• Driveways and parking spots under an eave.
• Decks, patios and play areas.
• Lower roofs, porch roofs and additions, along with any skylights or vents on them.
• Gutters on the shedding plane, which a slide can bend or tear off; see gutters under a shedding roof.
• Gas meters, electric meters, outdoor air conditioning units and exhaust terminations on the wall below.
• Plumbing vents, chimneys or other items on the roof itself, downslope of a large snow field.

Also note where two planes meet. A valley gathers snow from both sides, and an upper roof that sheds onto a lower one adds its snow to the lower roof’s load in a single event.

Ground rules for keeping people clear of a shedding roof during a thaw, with or without guards:

• Do not stand, park or let children play under an eave that has not shed.
• Do not climb onto the roof or a ladder to clear snow from an eave.
• After a slide, check gutters and meters from the ground before anyone walks under that eave again.

Pad, Rail, and Fence Guards: What Each Is For

Snow guards come in two broad shapes. Individual guards, sometimes called pads or cleats, are small units spread across the roof in a pattern, each holding a share of the snow so it melts in place or sheds in small amounts. Continuous systems, rails or fences, run along the roof on brackets and form a barrier the whole snow sheet bears against.

Treat any layout as something the guard maker calculates for your roof. That calculation depends on the roof’s slope, its length from eave to ridge, the roofing material and the snow your area gets. One row near the eave may or may not be enough; a row count that is not tied to the maker’s method for your roof is a guess.

What you can control is the request. Ask for a written layout that names the guard type, the number of rows and their positions, the spacing, and the snow and slope assumptions behind it. Whether any local rule requires retention over entries or walkways is a question for your building department, so ask it before you decide a plane can go without guards.

Metal, Tile, and Shingle Roofs Need Different Attachments

The attachment method has to suit the roofing, and each method carries its own leak and warranty questions. Treat the points below as questions to put to the roofer and the guard maker, not as specifications.

On standing seam metal, guards can clamp to the raised seams instead of being screwed through the panels. Ask how the clamp grips the seam and whether the panel maker approves it. On exposed-fastener metal panels, guards are fastened through the panel into the structure below, so each fastener is a hole that has to be sealed and maintained. The guide to standing seam and exposed-fastener panels explains how those two systems differ.

Tile and slate are brittle, and guards there are fastened to the deck or structure beneath the roofing. That work belongs to a contractor experienced with that roofing. On asphalt shingles, guards are fastened through the shingles into the deck, which is simplest when the roof is being installed; on an existing roof, ask how the roofer will keep the sealed shingle courses intact.

For every roof type, ask in writing whether the roofing maker allows this attachment and whether the guard maker’s warranty depends on a specific fastener or installer.

What Holding Snow on the Roof Changes

Retention trades one risk for another. Snow that would have slid off now stays on the roof until it melts, so the roof carries that weight for longer. Whether the existing roof can carry its local snow load is a separate question; see how heavy roof snow gets for the warning signs of overload.

Retained snow also sits over the eaves, where ice dams form. The University of Minnesota Extension’s guide to dealing with and preventing ice dams states that “nonuniform roof surface temperatures lead to ice dams.” Snow guards do nothing to even out those temperatures, so they do not prevent ice dams; if your house already gets ice dams, read why ice forms at eaves before you add retention.

When snow does need to come off, keep everyone on the ground. FEMA’s guide notes that “Snow removal can be conducted from the ground, removing the risk to people on a hazardous roof,” and that roof snow removal should be done by “a licensed, insured professional roofing contractor.” UMN Extension cautions that a roof rake and push broom can remove snow “but may damage the roofing materials,” so work gently with a non-metallic rake, which FEMA’s guide advises for steep slopes, keep the pole clear of overhead power lines, and stay out of the landing zone while snow is moving.

Where Should Snow Guards Go on Your Roof?

If a roof plane sheds onto a doorway, walkway, driveway, deck, lower roof, meter or gutter you want to keep, that plane gets snow retention laid out by the guard maker for your slope, roofing and snow. If a plane sheds onto open ground with nothing below, retention is optional, and keeping people clear during thaws may be enough. If the house already has ice dams or a roof showing overload signs, deal with those first, because retention keeps more snow at the eaves.

Walk the perimeter on a day when the roof holds no snow, photograph each landing spot with the eave in frame, and send those photos with a request for a written layout, the attachment method and its effect on your roofing warranty. Leave the installation to a roofing crew working under fall protection for roof work. If the layout names roof parts you don’t recognize, the plain-language list of roof parts defines them.

This guide is general information for U.S. homeowners, not legal, insurance, tax, or engineering advice. Codes, laws, and policy terms vary by location and change over time; confirm details with your local building department, your insurer, or a licensed professional.

Sources

• Snow Load Safety Guide, FEMA P-957, Federal Emergency Management Agency
• Dealing with and preventing ice dams, University of Minnesota Extension