RV roof snow load: How much snow can your roof hold?

There is no single number that applies to every RV, because roofs are built light to keep towing weight down and every maker draws the line differently. The figure that binds yours is the one in your own manual, and two makers publish one for a towable. Keystone's owner's manuals put their roofs at 30 pounds per square foot, and give the same limit as two feet of snow. The same passage adds that loads beyond it can damage the roof structure as well as the axles and suspension, and that the roof should be clear of snow before the RV is towed. Boxabl builds the Baby Box 120 as a factory-built towable recreational vehicle, and its spec sheet rates that flat roof at 20 pounds per square foot of live load.

Those two figures are both useful and they are not the same kind of number. Keystone's 30 is a maker's roof rating for the RV it is printed in. Boxabl's 20 is a live-load rating, the weight a flat roof is engineered to carry. Neither of them is your roof's rating, and where a manual publishes nothing there is no number to put in its place. Taking the lower of two figures published for two other RVs does not produce a rating for a third: a roof with no published limit may be built to carry less than either, and a manual that stays silent is not a manual that has promised you 20. Treat an unrated roof as a roof with no stated snow allowance, clear it earlier than the numbers below suggest, and do not stand on it.

The maker gives the limit two ways, as two feet of snow or as 30 pounds per square foot, and the arithmetic is worth doing once: whichever of the two a given snowfall reaches first is the one that binds, and in light snow it is the depth. 30 pounds per square foot is 5.8 inches of water, and 5.8 inches of water spread through two feet of snow is snow that is about a quarter water. That is denser than any snow is when it falls, and it is what snow on a roof becomes after it has sat there a while, on its way to the 40 percent or more a snowpack reaches by spring. So the depth that reaches the limit depends on the weather that dropped it: 30 pounds per square foot is about five feet of ordinary fresh snow, and about 14 inches of late-winter snow gets there first.

Snow load is simple arithmetic: total weight = roof area times snow weight per square foot. The area takes a tape measure. The weight of the snow takes one number, and there are two ways to get it: work out what kind of snow it is and look it up, or melt a sample and measure the water in it. The second is what the National Weather Service does, and it is the more reliable of the two.

Step 1: Measure Your Roof Area

Multiply the length of the roof by its width. A 30-foot travel trailer that is 8 feet wide has a roof of about 240 square feet. A 40-foot fifth wheel at 8.5 feet wide comes to about 340 square feet. Round up, because the corners and the air conditioner add a little, and write the number down, because every load in the rest of this guide is that figure multiplied by a weight.

Step 2: Know What Your Snow Weighs

A cubic foot of snow is a rough guide to its density, and density is everything. Snow is water with air in it, so its density is measured as water content: the share of the volume that would be liquid if the whole sample melted. The USDA's Natural Resources Conservation Service puts new snow between about 4 and 20 percent water, very light powder at the bottom of that range and heavy, wet snow at the top. Snowpack density climbs all winter as it settles, and by late March or early April the same service measures it at 40 percent or more.

The same depth of snow is a different load depending on what the weather did to it: the depth that reaches 30 pounds per square foot runs from about five feet of fresh snow down to about fourteen inches of the late-winter kind.

Step 3: A Shortcut That Is Easier Than It Looks

One inch of water weighs 5.2 pounds per square foot, whatever kind of snow it arrived as. Measure the water rather than guessing at the snow type, and the weight falls out: it does not matter how many times the snow has frozen and thawed since it fell.

The method the National Weather Service uses is a core sample. Push a long cylinder down through the snow to the roof, a capped piece of PVC pipe works well, turn it right side up and bring it inside to melt, then measure the depth of the water left in the cylinder. That is the water equivalent of the snow. Multiply the inches by 5.2 and you have pounds per square foot: two inches of water in the cylinder is 10.4 pounds per square foot sitting on the roof. Two inches of water is a lot of snow, which is the useful part. Powder that deep may be four feet, and late-winter snow may be only five inches.

Two distinct failure modes do the damage, and they are not the same problem.

The first is pure weight. Enough snow, dense enough, simply exceeds the roof's structural limit. The signs are a roof that sags between the rafters, or a drum-tight cover that starts to press down on the membrane. If the roof is visibly bowing, or the cover is tight where it used to be loose, the load is too high, remove it.

The second is ponding. As snow melts, the water runs toward the lowest point of the roof, and an RV roof has almost no fall to it, so the melt pools where the deck dips instead of running clear. A pool of water is denser than the snow that made it, so it presses harder where it collects, and it sits against the membrane and the seams for days. Ponding is often the real culprit behind spring leaks, not the snow itself. Park on a deliberate slope rather than dead level so the melt has somewhere to run, and keep the roof edge clear so it can get there. This is why clearing snow away from the low side matters so much.

Inspect the roof and its seals before the first snow

Do this on a dry day in the fall, because February is the wrong time to discover what winter has been hiding. The roof is thinner than it looks from underneath. Keystone describes its own as interior paneling, a truss system, insulation, and decking with the TPO membrane glued over the decking, and the same manual says plainly that these roofs are not designed for chairs, coolers or several people, only for one person doing routine maintenance. Whether yours can be walked on at all is a fact the maker states rather than a judgement call, and it is worth settling before you climb up.

Where the roof is walkable, work from the front to the back and look at the following:

• The membrane itself. Look for cracks, blisters, or seams that have opened. On a sheet seam, an edge that has lifted away is an entry point for water.
• The lap sealant. This is the flexible seal applied around every roof penetration, the vents, the air conditioner mount, the antenna. It is what keeps water out, and Keystone's own maintenance schedule is blunt about how long it lasts: attachments, seams and joints need attention every 90 days, and heavy sun, aggressive temperature swings and cold climates all speed up the deterioration.
• The fasteners. Screws around vent boots and AC gaskets can back out, and a lifted screw head is a puncture waiting to happen.
• Prior repairs. Any covered seam or patch from a previous owner is a weak point. Blisters or bubbles in a repaired area mean the patch is separating, and winter will pry it further.

Reseal what is failing. Use a roof-compatible lap sealant, the self-leveling type made for RV roofs, over the fasteners and around the flange of each fixture. Clean the area with a roof-safe cleaner first, let it dry completely, then run a continuous bead so the sealant bridges from the fixture onto the membrane. Two rules from the maker are worth following exactly: use the same sealant that is already on the roof, because there is no way to know whether two brands will bond to each other, and never put a silicone product on the membrane. If your roof carries the Alpha system, the sealant has to be Alpha's. The same manual calls the membrane Alpha TPO, says Alpha sealants must be used on an RV with the Alpha system, and has both supplied through your dealership, so your unit's paperwork is what tells you which membrane you have.

Clean the roof before winter

A clear roof is a dry roof, and a dry roof survives winter. Sweep off the leaves, needles and debris that collect in the fall. Debris holds water against the membrane for the whole season, and a trapped wet layer is one more thing working on a sealant that the maker already says is aged by sun, by aggressive temperature swings and by cold. In the same pass, check that the roof vents and the air conditioner returns are open so nothing is holding moisture inside the bays.

Read the load, not just the depth

Two numbers decide this. The first is the rating for your roof, from your manual. If the manual publishes nothing then you do not have that number, and the honest reading of it is a roof with no stated snow allowance rather than a roof rated at whatever another maker prints: clear it early, and do not stand on it. The second is what is sitting on the roof now, from the core sample or from the table in Step 2. Watch the forecast for what the snow will actually be rather than for the depth, because three feet of cold powder is a light load and six inches of rain-soaked slush is not. Where you do have a rating and the load is close to it, or more snow is coming, plan to take some off.

Ice dams target the eaves

An ice dam forms when the roof surface under the snow is above freezing while the edge of the roof is below it. The snow touching the warm deck melts, the water runs down the slope, and at the cold edge it freezes into a ridge that blocks everything behind it. The Building America Solution Center names two causes: heat escaping through a ceiling that is not stopping it, and sunlight warming an exposed dark surface.

The second cause needs an exposed dark roof, and a snow-covered RV roof is neither, because the snow is what the sun lands on. The first is the one that reaches an RV, and its roof build is why. The assembly Keystone describes is interior paneling, a truss system, insulation, and decking with the membrane over it, in that order. Between the air you are heating inside and the snow outside there is insulation and very little else, so the heat you are running inside reaches the snow touching the deck, and that snow melts first. Running the heat hard through a snowfall is what sets the dam up.

The dam matters for what the water does behind it, and that same source puts the worst case plainly: in severe cases, ice dams have caused roof collapses. For a roof you are sleeping under, that is the failure to plan for, and it comes from ice rather than from depth.

To manage a dam, pull snow away from the eave with a plastic roof rake so the melt has a path off, and never chop at the ice with anything metal, which will tear the membrane. On a warm afternoon near freezing, a stream of warm water along the eave can clear a modest dam without contact.

If you have to clear the roof

Start with whether you should be up there at all. Keystone's rule is that walkability is something the maker states rather than something you judge: units with a factory-installed ladder have walkable roofs, some units have a walkable roof without a ladder, and others have neither. The ladder is rated to 250 pounds. If yours is not a walkable roof, or you cannot find out, this is a job for a rake worked from the ground or from a ladder at the eave.

The safe tools are soft and long. A wide, soft-edged plastic roof rake, used from a ladder at the eaves, pulling snow down and away from the roof and never scraping the membrane. Work from the ground or a ladder rather than up on the ice, and stop a few inches short instead of scraping down to the surface: getting the roof from two feet down to three inches removes nearly all the weight, and the thin layer left behind protects the membrane from whatever tool you are using.

The manual's own warning about working up there is that the roofing material can be slippery when wet, and a winter roof is a hard place to test it. A fall onto a frozen membrane punctures it where you land, and the damage stays hidden until spring water finds the wall. Never climb onto an icy roof with a tool in your hand.

And clear it before you move. Keystone asks for the roof to be free of snow before the RV is transported, in the same passage that carries the 30 pound limit.