DANA WILSON HOME INSPECTION SERVICES LLC
MASSACHUSETTS
How do I know if there’s too much snow on my roof? Get to know your roof!
This blog is not intended to be an engineering study or analysis of snow
loads or roofs. It is however, intended to explain the different types of roof
coverings, roof structures, snow and wind loads and how together they affect
your roof.
How much does the snow weigh? Well that’s easy, but snow doesn’t stay snow
– it turns to water and ice, then moves around on your roof!
The calculations made, based on the weight of snow, in my opinion cannot be broadcast to multiple homes. You have to
know your roof! Here’s why!
If we compare two houses, each will have a variance
in heat loss through the roof and rafter fatigue. So, say for example we get light fluffy snow. The snow on the house that is better insulated and ventilated will stay
in a lighter and fluffier state, which will be evenly distributed across the roof versus the
house that has heat loss through the roof and fatiguing rafters. The heat loss
will cause melting; as a result the snow will start to turn to water and slide
towards the drip edge of the roof. Now, with freezing conditions you have
water/ice concentrated in less square foot area than when it was evenly
distributed across the roof as fluffy snow. Water/ice results in a heavier build up within
less square footage than light fluffy snow that is distributed evenly across the
roof. Now, in the real world, the house with better insulation starts to lose
snow through evaporation and blowing wind. If we were to get another
snow storm there will be a phenomenal amount of weight on the roof with
water/ice buildup and an incredible amount of uneven weight load distribution.
You may have a house that is built in an area where building standards call for 30 or 70 pounds per square foot; however, your older house might not meet that
code. Get to know your roof!
Why are there exposed shingles at the peak?
The blue arrow is pointing at the area where the snow slid and accumulated near the edge of the roof. Note the extreme ice build up at the drip edge!
A roof might be able to
support this kind of load for days or even weeks, but you should have it shoveled off as soon as possible for safety sake.
A one-inch layer of water or ice weighs about five pounds
per square foot. A roof that is designed for 20 pounds per square foot of snow
load could hold up four inches of water/ice.
So, what does that mean to
you? Some meteorologists estimate that
12 inches of snow is equivalent to one inch of water.
Example taken from above: 1 inch of water/ice = 5 pounds
per/sqft. Then, 4 inches of water/ice = 20 pounds per/sqft. If 12 inches of
snow is equal to 1 inch of water/ice then 4 feet of snow is the maximum you’d
want on a roof that’s rated for 20 pounds per/sqft. In my opinion, to be safe –
cut the snow number in half (2 feet of snow on a roof rated for 20 pounds
per/sqft). You can research online your local building standards for snow load ratings
in your area and for the year your house was built. You might find most
load ratings are about 25 to 35 pounds per square foot and can be anywhere from 35 to 70
pounds per square foot in the snow belt areas of the country.
Don't be deceived by inches of snow, there could be ice underneath!
Over days and weeks the snow can melt and form ice which isn't as thick as as the snow. For example, lets say 2 inches. Now if its snows 12 inches on top of the 2 inches of ice you might be looking at the roof thinking you have about 14 inches - your right! But the weight of the ice and snow combine could be twice the weight of 14 inches of snow.
Don't be deceived by inches of snow, there could be ice underneath!
Over days and weeks the snow can melt and form ice which isn't as thick as as the snow. For example, lets say 2 inches. Now if its snows 12 inches on top of the 2 inches of ice you might be looking at the roof thinking you have about 14 inches - your right! But the weight of the ice and snow combine could be twice the weight of 14 inches of snow.
What about the wind-load? What zone are you in?
You can research this online by searching wind loads in your town, county
and/or state. Some states are broken into zones. Why should you know what the wind-load
zone is? You may have a small amount of snow built up on your roof, but if you
have a low sloped roof and if you’re located in a high wind-load zone your roof could be at
risk because low sloped roofs tend to have a higher build-up of
snow.
Ingress / Egress:
The doors leading to the exterior (typically one at the
front and one at the rear of your house) need to be free and clear. Ice and
snow blocking a door from swinging open will have to be removed.
-Hot roof: A roof structure situated over a
heated area of the house.
Concerns; heat loss causes warmer air to rise to the peak which creates snow
slide. The sliding snow typically accumulates at the edge of the roof near the
gutter increasing the chances of ice damming leaving the shingles at the peak
of the roof exposed.
-Cold roof: A roof without a heated room below
(porches and garages).
Concerns; no snow
slide, therefore snow tends to build-up which increases weight load.
Exposed shingles at the peak (heated room under the roof). Snow built up on the garage roof (the garage is not heated). The snow melts from up near the peak and slides to the drip edge and/or the roof below. This creates an unbalanced load at areas of the roof.
Here's an other picture of snow sliding.
Six or Seven things come to mind for me. Snow sliding creates unbalanced load on the roof, ice build up at edges of the roof creates potential for ice damming and damage to gutters, soffit and fascia. The sliding snow also ends up on top of the garage/porch roof, because these areas are unheated the snow builds up to astronomical weight loads. Now, try to factor in the weight of the the snow load if it rains (the weight could be more than six times the current load).
In addition, look at the picture below. The red arrows point at roof vents that were covered with snow! The ridge vent at the peak of the roof is part of attic ventilation system, it is crucial to keep it clear of snow because it provides air flow which helps to prevent ice dams. The hood vent accommodates the bathroom exhaust, when covered with snow it won't function. The waste water sewer vent pipe has to be able to breath in order for the sewer pipes and drains to function correctly. If you here gurgling when a sink drains or a commode is flushed you may have a blocked sewer vent pipe.
Here's an other picture of snow sliding.
In addition, look at the picture below. The red arrows point at roof vents that were covered with snow! The ridge vent at the peak of the roof is part of attic ventilation system, it is crucial to keep it clear of snow because it provides air flow which helps to prevent ice dams. The hood vent accommodates the bathroom exhaust, when covered with snow it won't function. The waste water sewer vent pipe has to be able to breath in order for the sewer pipes and drains to function correctly. If you here gurgling when a sink drains or a commode is flushed you may have a blocked sewer vent pipe.
LET’S GET TO KNOW OUR ROOFS!
Type of structure: wood frame rafters or wood
frame trusses:
What are collar and rafter ties?
Collar ties and rafter ties prevent movement in the rafters. With out them snow loads become much more of a concern!
The blue rafters are supported with a collar tie and a rafter tie. The yellow rafters depict movement if these ties are missing. The pictures above show roof sag and outward movement at exterior walls.
What does the age of your roof mean in terms of snow load:
Concerns:
- Old rafters, improperly built and fatigued (middle picture above).
- Missing cross ties / missing rafter ties (Left picture, separation at top of rafters can be an indication of missing cross ties or rafter ties).
- Undersized, weak, rotted decking boards (right picture, sagging between rafters)
The older the roof the more fatigue it has suffered over the years! Notice that there is only about 12 to 14 inches of snow on the roof in the above picture. No matter what the roof was rated for, no mater what the wind zone is or the type of structure. An older roof with deteriorated wood members and fatigue might not be able to handle 12 to 14 inches. Get to know your roof!
- Asphalt fiberglass: Not as heavy as other types, can be walked on.
- Metal: Not as heavy as others, should not be walked on.
- Slate: Heavy and should not walk on it.
- Wood shingle: Not as heavy as others, should not be walked on.
- Tile; Heavy and should not walk on it.
- Cement: Heavy and should not walk on it.
- Asbestos: Not as heavy as others, should not be walked on.
- The best option for roofs that can't be walked is raking the snow off.
Dangers – For people walking / shoveling roofs:
People who walk the roof are added weight. Make sure you tell the person
shoveling if you have skylights. It’s always prudent to look at the underside
of the roof (from the attic) to determine if there’s signs of fatigue, rot and/or soft spots at
roof boards.
Be careful not to damage roof components: the roof
covering itself, flashing, skylights, gutters, satellite dish/dish cables chimney/chimney flashing, waste vent pipes,
flue pipes, gutters/heat/melt wires at gutters.
THE AFTER AFFECTS:
Things to look for after the snow and ice melt: water penetration through
siding, trim boards, window sills and lower portions of doors (this seeping
water could rear its head at places like finished floors, surface of walls near
doors and windows or at exposed framing typically visible from the basement). Look
for water seeping through areas of foundation. In additions check the ceiling
at the highest level in your home that is parallel to the roofs drip edge, which
is the area where you would typically find gutters. Water from ice damming
will typically appear as staining on the ceiling, close to exterior walls.
 | |||
| The snow is above the doors, windows and siding. When it melts the water will attempt to penetrate to the interior. Shoveling the snow off your roof is priority one. Clearing it from the sides of your house is priority two! By the way, breaking icicles of this size can do some serious damage. |
Vulnerable and typical areas of water penetration due to ice and snow melting.
Viewed from attic (bottom of rafters). Dinning room windows. Foundation (sill / band area).
In the picture below you can see icicles outside the window, if you look close you can see the water drop inside the red circle.
DID THE ROOF GET DAMAGED FROM SHOVELING?
Check the roof and all roof components in the spring for damaged caused
from shoveling.
 |
| SHINGLES TORN AND DAMAGED |
 |
| TILES TORN OFF. |
WHAT IS MY LEVEL OF RISK FOR A ROOF COLLAPSE?
Here's what you should be asking!
- What type of roof structure do I have? If its older you could be at a higher risk!
- How old is the structure - are there any signs of fatigue? If there is - you're at a higher risk!
- Are Cross ties and Rafter ties in place? If one or both are missing, you're at a higher risk!
- What type of roof covering do I have? Can it be walked on?
- How many pounds per square foot is my roof rated for? If its a low rated roof, you're at a higher risk!
- What type of wind zone am I in? High wind zones add to the risk!
- How many inches (including ice build up) is on my roof?
How do I get rid of ice damming?
Best of luck, hope this was helpful!
Dana Wilson Certified Master Home Inspector.
Located in Massachusetts.
