Flat and Low-Slope Cottage Roofs in Lake-Effect Snow Country

A flat or low-slope cottage roof holds every snowfall it gets. Lake-effect snow off Lake Huron and Georgian Bay is heavy, wet, and arrives in bands, so the pack builds, turns to ice at the bottom, freezes the drains, and drifts deep against any taller wall. A pitched roof sheds some of this on its own. A low-slope roof needs a person to clear it.

If you own a cottage with a flat main roof, a shed-roof addition, a sunroom, a porch roof or a boathouse, and you will not see it between Thanksgiving and the May long weekend, this post is for you.

Why does snow stay on a flat cottage roof?

A pitched roof has gravity on its side. Snow creeps and slides on the steep side, the sun works on the south face, and wind scours the ridge. Some of every storm leaves on its own.

A low-slope roof has none of that help. The pitch is too shallow for snow to slide, so the first band of the season lands and stays. The next band lands on top of it. A mild day softens the top, a cold night hardens it, and the layers bond into a single pack that is heavier and denser than the fresh snow that made it.

Then the water problem starts. Meltwater on a low-slope roof is supposed to leave through drains, scuppers (the openings cut through the roof edge to let water out) or over a low edge. In winter the edges and scuppers are among the coldest spots on the roof, so they are usually the first to freeze, and once one outlet blocks, water backs up to the next. Meltwater then ponds behind the parapet (the short wall that runs around the edge of many flat roofs) or against the house wall, freezes, and adds its own weight to the pack.

The last piece is drift. Wherever a low roof sits below a taller wall, wind drops snow into the sheltered pocket. The open part of the roof might look manageable while the drift against the house is far deeper. Nobody sees that wedge from the road.

Which cottages have a low-slope roof problem?

You do not need a modern flat-roof build to have this issue. Across Grey-Bruce, the Bruce Peninsula and the Georgian Bay shore, these are the usual suspects:

• Older flat-roof cottages and camps. Plenty of older lakefront places have flat or nearly flat roofs, and a new membrane (the rubber or asphalt sheet that is the actual roof surface) does not change the pitch.
• Shed-roof additions and sunrooms. The classic cottage pattern is an older core with a single-slope addition tacked onto one side. The addition roof is lower and shallower than the original, and it meets the old wall at a junction.
• Porch and carport roofs. Wide, flat, usually simpler than the main roof, and usually attached to a taller wall on one side.
• Boathouse roofs. Low pitch, wide span, close to the water, and the last building anyone walks out to in January.
• Any low section under a taller wall. Wherever a roof steps down, the drift line forms against the higher part.

If your place has any of these, the pitched main roof is not the part of the building that carries the most winter. The low section is.

Why is lake-effect snow the worst case for a low-slope roof?

Grey-Bruce and the Collingwood area sit in Ontario’s snow belt, and our post on the snow belt in Grey-Bruce and Collingwood explains why the geography does what it does. For a low-slope roof, four things about lake-effect snow matter more than the total:

1. It is wet and dense. Snow that picks up moisture over open lake water lands heavier than the dry, fluffy snow you get further inland. A low-slope roof holds all of it.
2. It comes in bands over days. A lake-effect event is not one storm. It is a band that sets up, dumps, shifts along the shore, and dumps again. Between bands the pack settles and compacts, so the next band lands on a firmer, heavier base.
3. Thaw and refreeze. Shoreline winters swing above and below freezing. Each swing melts the top of the pack, sends water down through it, and freezes it at the bottom. Over a few weeks the base of the pack on a flat roof can turn into a sheet of ice that a rake cannot move.
4. Rain on snow. A January or February rain soaks the whole pack. The snow holds the water like a sponge, then the temperature drops and locks it in. The roof is now carrying snow, ice and trapped water at once.

The weight question is covered in how much snow a cottage roof can hold. We will not restate the numbers here.

What goes wrong on a flat roof over a winter?

Here is what tends to go wrong on low-slope roofs by mid-winter when nobody has been up.

• Ponding and ice at the drains. The drain or scupper freezes, meltwater backs up behind it, and a shallow pond forms and refreezes. Every thaw adds another layer.
• Ice dams at the edge. On a low roof the edge is one of the coldest spots, so meltwater running toward it can refreeze into a ridge, and water pools behind the ridge. Our post on ice dams at the cottage covers how they form and what to do once you have one.
• Water backing up at the wall junction. Where the addition roof meets the old cottage wall, the junction is meant to shed running water, not hold a frozen pond. Backed-up meltwater can find its way under the membrane and into the wall.
• Sag you can see from below. Stand inside the addition and look up. A ceiling that has taken on a dip, or a line of cracks along the junction with the old cottage, is the roof telling you what it is carrying.
• Doors and windows that stick. When the low section takes on weight, the framing can move. Patio doors in the sunroom and windows in the addition are often the first things to bind.
• The drift wedge against the upper wall. It sits right over the most vulnerable junction on the building, and it is usually the deepest snow on the roof.

What does a roof snow removal visit do on a low-slope roof?

A roof snow removal visit is a person, a roof rake, and where the roof can be walked safely, careful on-roof clearing. No sensors, no gadgets. Here is what happens on a low-slope roof, in order:

1. Check the drift and where it has piled. Before anything is moved, we walk the perimeter, measure the drift against the taller wall, and note where meltwater has ponded and where ice has formed.
2. Rake from the ground where the roof can be reached. Porch roofs, carports, low additions and the edges of flat main roofs can usually be reached with a roof rake from the ground. That takes the bulk off the edges and the lower drift without anyone stepping on the membrane.
3. Clear the walkable roof in a pattern. Where a roof can be walked safely, we clear it in a pattern that spreads the work evenly rather than piling snow in one spot, and we leave a thin layer on the membrane so the tools stay off the surface.
4. Open the drains, scuppers and edge. Frozen outlets get cleared so the next thaw has somewhere to drain. The edge is opened up so water can leave instead of building a dam.
5. Never chip ice. A layer of ice bonded to a membrane stays. Chipping it off can tear the membrane. We leave it to release on its own once the drains are open and a thaw comes through.
6. Photograph before and after, with the date. You get pictures of the drift line, the drains and the cleared roof, plus a note of anything else we saw while we were there: a stuck door, a stained ceiling, a branch down on the porch roof.

The roof is one part of a bigger walkthrough. Our post on what we check on a winter cottage visit lists the rest.

Roof section	What winter does to it	What gets done on a visit
Flat main roof	Holds every band; drains freeze; ponding and ice at the low points; drift against the parapet or taller wall	Drift checked and photographed; walkable areas cleared with a thin layer left; drains and edge opened; no ice chipped
Shed-roof addition or sunroom	Drift wedge against the old cottage wall; meltwater backs up at the junction; sag and sticking doors inside	Drift line measured; raked from the ground where reachable; junction photographed; inside checked for sag and binding doors where we have access on a checkup
Porch or carport	Takes the full pack plus whatever slides off the main roof above it	Raked from the ground; edge cleared; posts looked at from below; before-and-after photos
Boathouse	Low pitch, exposed to lake wind; often the last building anyone checks	Drift and ice noted; raked from the ground where safe to reach; photographed and reported

How often does a flat roof need to be cleared?

The honest answer is that it depends on the storms. A winter with three big bands and a long January thaw is a different winter from one that snows a little every day. A flat roof on the Huron shore south of Southampton sees different weather from one on the Peninsula or in the hills behind Collingwood.

The general cadence question, and how to pick a schedule for your place, is in weekly vs monthly cottage checkups. For low-slope roofs specifically, two rules of thumb:

• After a big band, an extra visit is the sensible call. When the band sits over your stretch of shore for two or three days, the low roof took all of it. Our post on after-the-storm emergency snow response covers what an unscheduled visit looks like.
• After a thaw-and-refreeze, same thing. A thaw followed by a hard freeze is when drains lock up and ponds turn to ice. Getting the outlets open before the next band arrives is worth a visit on its own.

If you are on a monthly schedule and your cottage has a shed-roof addition or a wide porch roof, it is worth telling us. The low section is the reason to move to a shorter interval, not the pitched main roof.

What about trees over a low roof?

A branch loaded with wet snow over a flat porch roof or a sunroom drops that load on the roof when it lets go, and it can drop the branch too. Low roofs are more likely to sit under branches than the main roof is, so they take more of this. We look up as well as down on every visit, note branches that are carrying snow over the low sections, and photograph anything that has come down. What can be done about those branches in the off-season is a topic for another post.

How to get a low-slope cottage roof looked after

If your cottage has a flat main roof, a shed-roof addition, a sunroom, a porch or a boathouse, that low section is the one that needs a person on it between storms, and the one you cannot check from home.

Roof snow removal covers what a clearing visit does: the drift check, the raking, the on-roof clearing where it is safe, the drains and scuppers, and the dated photos you get afterward. Cottage inspections covers the winter checkups that put someone at your place on a schedule, so the low roof gets checked and photographed through the winter instead of only after something shows up inside.

We look after cottages across Grey-Bruce, the Bruce Peninsula, the Lake Huron shore from Southampton to Grand Bend, and Georgian Bay from Collingwood through Thornbury and Meaford. Call 519-783-1575, answered seven days a week, and we will walk you through a clear quote up front.