Measurement of Snow Loads on Roofs

1. Introduction
2. Things you will need
3. Steps
4. Indirect measurement
5. Conclusion

1. Introduction.

Every now and then just about anywhere will get more than normal amounts of snow. When it happens, concerns arise as to whether it is `too much’ snow for our roofs and decks and other structures to handle. How much is too much? More snow that what the structure is designed to handle would be too much (designed to handle plus some factor of safety). How much the roof is `designed for’ will be handled separately. Determining the amount of snow on the roof or other structure at the time of concern is the subject of this article. And it is really pretty simple.

Snow loads are generally considered in terms of pounds of snow per square foot of surface area. The `surface area’ used is generally the surface area of the flat projection of the surface in question, so if you are measuring loads on a sloped surface, an adjustment from slope to flat is also involved. But the adjustment is relatively simple and will be provided later. For the purposes of this article we will start with snow load on a flat roof (or deck) surface, and then deal with adjustments.

2. Things you will need.

Things you will need for measuring the snow weight (load) are:

1. Shovel (straight blade preferred)
2. Tape measure (ruler, yardstick, whatever)
3. Heavy duty garbage bag(s) or can(s)
4. Scale (bathroom type will suffice)
5. Pencil and Paper (something to write on)
6. Calculator (the one in the `tools’ part of your cell phone will probably do)

This article assumes that you can access the roof or deck or other surface relatively safely. Thus, I have not included safety equipment. Nor, however, should I be taken to suggest you don’t need any (safety measures), even if the roof is accessible and access looks safe.

Basically what you will be doing is getting on the surface in question and shoveling out a (vertical) `column’ (pillar) of snow over a determined amount of area of surface, weighing it, and dividing it by the surface area, to obtain the in-place present snow load.

3. Steps

The steps in this procedure follow.

1. Access the area in question and decide the size of the `area’ you want to measure. Depending on the depth of snow a 1 ft x 1 ft square might be sufficient, but I have found that a 2 ft x 2 ft square is more practical. A 1 ft x 1 ft square is convenient for math’s sake, as the `load’ from a column of snow above a 1 ft x 1 ft square (and however many inches deep) will simply be the weight divided by 1 (sq. ft.). But any area will work – it just involves more math. If you decide to dig out and weigh the column of snow above a 2 ft by 2 ft section, then the load will be the total weight divided by 2 ft x 2 ft or 4 sq. ft. If you want to be fancy, dig out and weigh the snow over a square that is 17 in. by 17 in. This square will be almost exactly 2 sq. ft. total, so your load will be the weight divided by two. Obviously the snow is covering the actual surface area of roof, or deck, so your `square’ will have to be marked on the surface of the snow, and then, …
2. Dig down through the snow (vertically) and put it in a stout bag or garbage can or whatever, and take it and weigh it. If your hole gets bigger than the area intended (as you go down), don’t weigh the excess. If there is a lot of snow, it may take several bags, or cans. A bathroom scale is sufficient. Obviously, if you are carrying the bag and standing on the scale, you need to subtract out your weight, and the weight of the bag (if significant), or can, or whatever, to get only the weight of the snow. If you bring the scale to the roof, just make sure it is on a hard enough surface to `work right’. I recommend doing a number of measurements (tests) to get consistencies in numbers so you are confident you are doing it right.

   So, here’s an equation,

   ÏÂ? = W / A …

   Snow load (in lb/sq ft, or `psf’) = weight of snow (lb) / area (sq ft) …

   Example:

   Let’s say you dig out the snow covering a 2 ft x 2 ft area, and it weights 75 lb.

   The snow load is …

   � = W / A = 75 lb / 4 sq ft = 18.8 psf

   3. Adjust for slope if needed by dividing the weight per square foot of roof surface by the cosine of the roof slope angle. Since your (cell phone) calculator might not readily do the cosine function, I provide the adjustment for several roof slopes below.

   Roof Slope (rise over run) of 1 on 12 Adjust by Multiplying by 1.00 (no adjustment)

   2 on 12 multiply by 1.01

   3 on 12 multiply by 1.03

   4 on 12 multiply by 1.05

   5 on 12 multiply by 1.08

   From the above it should be obvious that the adjustment for mild slopes is small, just a few percent, and probably smaller (way smaller) than any and all the `imprecision’ in using this shovel-and-garbage-bags-and-bathroom-scale method. I have purposely omitted steeper roof slopes for the `sheer’ reason that I don’t want to be responsible for you being on a steep roof messing with snow and falling or sliding off. But, if you must (get on a steeper roof), the calculation process has been set before you.

   Example:

   Let’s say the 18.8 psf from above was measured on a 2 ft x 2 ft portion of roof sloped at 3/12. The adjustment for the `plan’ area load would be,

   18.8 psf x 1.03 = 19.4 psf, or so … not that much difference.

4. Indirect Measurement of Snow Load.

If the roof in question is not accessible, or the slope or other conditions are too precarious, or if you feel that getting on the roof presents a safety hazard, consider the following.

All other things equal, a flat roof of an unheated structure in a sheltered area will carry `about as much’ snow as is on the adjacent ground. By saying `all other things equal’ I have left out a lot of things, such as, drifting, differences in ground temperature and shelter temperature, differences in activities or other conditions on the ground and roof, etc. Hence, the simplest measurement of the roof snow load would be to measure adjacent ground snow load.

A flat roof with some exposure to weather and wind, and that is heated to a point where the roof is not always below freezing temperature in freezing weather, might be expected to hold about 70% of the ground amount of snow. And sloped roofs will tend to hold less than flat roofs. There are enough factors involved at this point that it would be a disservice to provide actual numbers, as they would be mis-used. But, as a very `first-order’ estimate, we could say that a mildly sloped roof with some exposure and normally heated could be expected to have (at least) 70% of the adjacent ground snow cover (weight). (See Snow Loads on Roofs and Decks, Associated Content.)

The measurement procedure of the `ground’ snow load is the same as that of the roof snow load described above (though a lot easier since you’re on the ground and not on the roof).

But, what about valleys, lower roofs, decks, etc.? `All other things equal’ also left out accumulations in snow that result from drifting and sliding. Valleys on roofs may easily accumulate nearly twice the ground snow amount by simple drifting. A lower roof may accumulate three times or more snow than the roofs (and ground) around by drifting and sliding of snow. Hence, straight comparison with ground snow cover with some adjustment will not be adequate, even applicable, in these cases. In such cases, one might use local snow densities (another discussion) to estimate snow loads where they cannot be directly measured (and they will only be estimates).

5. Conclusion.

The above procedure is a simple one and can be used to measure the weight (load) of snow on a roof or other structure. Once this value is known, judgment (or some more calculations) can be made as to whether or not the amount of snow presents a safety hazard. Certainly if the roof in question is suspected of being loaded to near collapse by the amount of snow already present, it may not be at all wise to `get on the roof’ to make the measurement, unless the roof (at least in the area of measurement) can be off-loaded of snow before getting onto it. (In other words, you may have to shovel your way out there and get rid of the snow along the way.)

Note: snow density measurement and calculation is illustrated in another article (here).

References

Snow Loads on Roofs and Decks, Jeff Filler, Associated Content.

Measuring Snow Density, Associated Content.