A NOTE FROM GROTON ENGINEERING, LLC

Snow loads on your roof are and should be of concern. I have received a number of calls from concerned people.

FROM WIKIPEDIA: “New snow commonly has a density of around 8% of water. This means that 33 centimeters (13 in) of snow melts down to 2.5 centimeters (1 in) of water.[39] Cloud temperatures and physical processes in the cloud affect the shape of individual snow crystals. Highly branched or dendritic crystals tend to have more space between the arms of ice that form the snowflake and this snow will therefore have a lower density, often referred to as “dry” snow. Conditions that create columnar or plate-like crystals will have much less air space within the crystal and will therefore be denser and feel “wetter”.[40]

Once the snow is on the ground, it will settle under its own weight (largely due to differential evaporation) until its density is approximately 30% of water. Increases in density above this initial compression occur primarily by melting and refreezing, caused by temperatures above freezing or by direct solar radiation. In colder climates, snow lies on the ground all winter. By late spring, snow densities typically reach a maximum of 50% of water.”

OKAY. What does that mean to you? Snow is a form of water. Ice is another. Water weighs 62.4 pounds per cubic foot (pcf). Therefore 1″ of water weighs 62.4 pcf/12″ = 5.2 pounds per square foot (psf).

Based on the Wikipedia info above:

1. New snow, the light fluffy stuff, is 1″ of water for 13″ of snow or 5.2 psf.
2. Settled snow being 30% water = 3.6″ of water = 18.72 psf.
3. Late spring snow (wet snow) being 50% water = 6″ of water = 31.4 psf.
4. 1″ of ice + settled snow = (approximately) 4.6″ of water = 24 psf

Roofs of most homes and business buildings in the eastern half of Massachusetts have been designed for 30 to 40 pounds per square foot (psf). Let’s use an average of 35 psf. Some factories may have higher capacities if their roofs were also designed to suspend equipment used for manufacturing or controlling the building’s environment. Roof framing has a factor of safety built in of about 1.5 to 2 depending on materials and quality of construction. I give a caution below for all-metal buildings.

Freshly fallen light fluffy snow at 5.2 psf per foot of snow depth on a roof of 35 psf capacity, could get to 6.7′ of snow. After it sets awhile and compacts down (18.72 psf) it is only 1.87′ (1′-10″). Later, once the bottom few inches begins to melt and re-freeze, the weight increases to 24 psf. For a 35 psf capacity roof that is only about 18″ of this kind of ice+snow. If you get late spring type snow in February and March then 1.0 foot of that snow is equivalent to the 35 psf capacity of your roof.

So, if snow has gathered on your roof for a few weeks to a depth of 18″ with an ice layer on the bottom its weight has likely reached the design capacity of your roof. Considering all of the constructed conditions, including possibilities of roof-framing deterioration, of your home or business building and the type of snow/ice setting on top, once snow depth exceeds 16″ you would be prudent to remove it. This is an absolute requirement when it comes to metal buildings. They do not have as much reserve capacity. Just watch the news and see how many of the collapsed buildings are the all-metal buildings versus wood-framed or masonry & steel or concrete & steel framed buildings.

I hope this has helped you make a decision about removing snow from your roof.

–
Groton Engineering, LLC
Lynwood V. Prest, P.E., S.E.
11 Highland Road
Groton, MA 01450

It is prudent to determine in the early stages of your new buidling purchase what your total exposure is prior to making any commitments, whether purchasing land for new construction of a building or purchasing an existing building and making changes to suit your business operation, here’s why;

Based on what I have recently observed, the banks have the money and want to lend, but the appraisers are running scared, rightfully so. Their fear is a result of the highly leveraged lending zeal of this most recent financial debacle!  Appraisals are coming in at about 2/3 actual cost and the banks are willing to lend in some cases up to 90% of appraised value, in essence lending 60% of actual cost and asking the Owner to put up 40% cash.  I have seen more construction projects die on the vine because the required equity. These high contributions by our prospective clients would cripple their operating cash and put their business in jeopardy.  So they are leasing existing space or making do with what they have right now.  Interestingly many industries are reporting record profits and maximum production, but are skeptical as to whether this can continue.  Comparative building sales analyses in many cases are using existing buildings that are over 20 years old without consideration for depreciation of building systems and building envelope, and disregarding necessary upgrade costs to comply with today’s storm water management and building code requirements which add significant expenditures to enable the occupancy of the buildings on the market.  The income approach analysis is taking regional base rental rates and not factoring in the amortization of tenant improvements over the period of the lease therefore not using a realistic rental rate when calculating property values using the income approach.  The only accurate appraisal approach being used these days is the replacement costs analysis which becomes null and void since many of the lending institutions require that the lowest of three approaches be used to determine the projected value of the property.  This may continue to deter new construction until Regulators re-evaluate current appraisal standards and adjust them to consider the improvement costs of old buildings to meet today’s standards, and adjust regional rental rates to reflect the cost regulatory upgrades necessary.