Often we have to fight it to get to the scene, then work against it to pull lines and consider it as another issue in strategy and tactics to take into consideration in our decisions.
I am hoping that this blog will be somewhat informative on the aspects of snow pertaining to the fire scene. A few of the things I hope to discuss here are the affects snow has during the response, inital scence sizeup/360, hose and ladder deployment and particularly on the building itself.
In this first of my blog series, I hope to discuss how the weight of snow may have an effect on a fire building and how it affects building construction and fire conditons. Afterwards I hope everyone would be willing to give some feedback on your experiences from incidents where the snow may have impacted conditions on a fire building.
Recently the Northeast has been hammered by the snow and now we are looking at most of the Midwest and East at the same time affecting close to 100 million people. While snow in America is a common occurance and can be found in all of the 50 states at one time or another, it does in many ways affect our capabilities as firefighters. One of those is snowload and while not all the states have a heavy snow fall, many do receive a fair amount of accumulation and depending on where you live, the moisture content can be quite high adding to the weight the snow applies to structural members in the roof and it's supports.
Here are just a few collapses I found on the internet, Senior Center garage roof collapses trapping two men. The roof of a warehouse failed in West Bridgewater, Mass. Other roof and wall failures are being reported all over the upper U.S.. Remember seeing the video where the plow truck went through the car garage. Probably the most memorable for 2010 is watching the Metrodome roof collapse. While none of these collapses happened under fire conditons, we can see how the snow affects the weight the roof can hold under ideal conditions without the structural members underneath being impinged upon by flames. Recently a new situation has developed that we have to be aware of. Recently some businesses have taken up Snow removal. This not a new job but because of their economic situation and hurting for money some companies and individuals are trying to drum up new business removing snow from roofs. This could lead us to responding to more falls or even collapses with the added weight to the already stressed structure.
Now lets try to get into the meat of the blog. Some might say the greatest danger to firefighters is that of a possible roof and or wall collapse due to a heavy snow load on a building. This is a potential hazard but many other factors must also be considered. We must take into account the conditions of accessing the roof and trying to sound it and accessing possible escape routes which can make it even more difficult to do our job on top. For those working inside, the danger is just as great in that of a need to be aware of what is above and how the fire could be impacting the structrual members during periods of heavy snow accumulation and looking for event changing conditions. One indicator of roof conditions can include snow melting or snow missing in an area that fire and heat are below.
So now lets look at some types of roof starting with flat roofs. Many thoughts center on buildings with these types of roofs as the greatest danger because they do not allow the snow to slide off. During periods of melting, the surface does not drain well because the roof drains are often frozen, and as a result, the snow retains the water like a sponge adding to the amount of weight on a roof because it can turn to ice which increases the weight. The common misconception is in thinking the physical properties of a flat roof are such that the structure cannot hold as much weight as a structure with a pitched roof and could eventually lead to buckling walls causing the roof to collapse. The statement above in my opinion is not entirely true because often flat roofs are constructed of heavy timber, heavy laminate boards or steel trusses and designed to cover great spans. These types of roof often have large service equipment on the top for building services or businesses but the potential for collapse is always there is evidenced late last year in two firefighter fatalities even with no significant snow load.
When we talk about pitched roofs, we firefighters often think of ordinary light weight construction. Some might think that the greater the pitch on a roof, the more weight it can hold and also the greater tendency for the snow to slide off on its own or allow some of the melt water to run off. Here in lies the danger of the peaked roof to firefighters believing it can hold more weight. Peaked roofs are designed to shed snow and are designed with lightweight wood trusses because the snow is supposed to slide off it. Often pitched roofs are covered in shingles or metal which impairs footing and the snow may cause difficulties in traversing the roof while concealing potential hidden objects below like vents or skylights and why it is so important to sound the roof every few feet all around you while staying close to a ladder. In heavy snow areas, many metal roofs incoporate avalanche bars. These bars are designed to subsequently hold the snow back allowing for moderate run off as it thaws preventing an avalanche effect to those below. While these bars do serve a purpose, enough snow can and will come sliding off without any warning. The freezing and thawing of layers and the accumulation of newer snow causes weaking in the layers which causes a weight shift breaking the layers and causing a subsequent mini avalanche causing those on the peaked roof to come sliding down on those below possibly causing injuries. This is why the handy roof ladder comes into play for our roof operations in situations where there is no truck in your area.
I did some research on peaked vs floot roof and in an article from Chief Dunn of FDNY (Ret.) his thoughts are that pitched roofs are more dangerous to us than a flat one. Here are a few reasons he stated why.
A peaked roof is more dangerous than a flat roof because:
1. There is no stairway, fixed ladder or adjoining building that can provide safe access to a peaked roof. Climbing fire department ladders is always required to get to a peaked roof.
2. There is no parapet around the edge of a peaked roof to keep a firefighter from walking or falling off at night.
3. The sloping surface of a sloping peaked roof can become too slippery to walk on when wet, icy, or coated with wet leaves or moss.
4 A peaked roof is built to support less weight than a flat roof because it has been designed to shed snow loads off its sloping surfaces.
On older buildings, peaked roofs may not comply with codes. They may have flimsy two-by-four-inch support roof beams, or a roof deck consisting of furring strips nailed to the joists. This type of roof deck serves only to fasten shingles - it will not support the weight of a firefighter.
His considerations are under average environments and no where is there mention of any snow being in the equation. With that being said I hope you go ahead and read the rest of his article and I'll wait right here. Did you learn anything...I sure hope you did or got a refresher!
There is something else Chief Dunn mentioned, that was in knowing your buildings and being familiar with their construction. Not many of us can keep up with every building type being built but we should do all we can to know if something out of the ordinary were being built. Some might ask what if we have a combined peak roof over a flat roof? It's really not anymore of an issue than the above roofs. If possible, find an access to get in whether from a gable end or an access hatch and take a look at the situation. If the snow weight seems unstable then look at alternate ways of performing ventilation. While we are here, we should look into preengineered roofs in a peaked roof configuration. These roofs are said to have a high load and heavy snow load rating again we see under ideal conditions. With the materials these roofs are made of, I would look at not having much longer to work on them than a light weight truss because you are walking on a few pieces of of glued wood chips with foam in the middle of the panels (video) and it's even scarier how they are joined together.
If I haven't lost your attention yet, lets look at the weight of average snow. The key to a snow's weight is how much water is locked up within the snow. This season, only a relatively small amount of that water has been able to drain off, blow off or evaporate from natural thaw. With this situation, you can literally have tons of extra weight on a roof, because of snow and the water it contains. Imagine this weight and then think about the already added weight of mechanical devices such as HVAC units and hoods on the flat roofs. Lets look at snow dynamics, for example, Boston has received about 40 inches of snow this month. A significant amount of that snow and more importantly the water content, 3.50 inches worth, weighs about 18 pounds per square foot, or 1,800 pounds, or close to a ton, per 10-by-10-foot area of roof which would be equal on a flat roof and could vary on a peaked roof depending on the pitch ast o how much has ben able to slide off already. Because of weather patterns and historical data we generally view snow as being a heavier wet snow in the Mid Atlantic the North East and Upper Great Lakes areas and usually the Northwest. While snow in the Midwest and Rockies and South we assume that it is generally a drier mix however the weight is still a factor but often not as significant because the water content will usually be lower. This is only a gneralized theory of maoisture content and none of us can say that you won't get heavy wet snow in Alabama and dry snow in New York.
Here is another basic calculation – (for a quick analysis only) This calculation uses a 25% moisture density which may be conservative for our current snow fall. As a rule of thumb, saturated snow weighs about 20 pounds per cubic foot. The moisture content of snow can range from about 1% to about 33% so snow can weigh from about 1 pound per cubic foot to over 21 pounds per cubic foot.S=Inches of snow on the roof1.25= Weight of 1 sq ft of snow for each 1” of depthP= Pounds per square footExample:If my roof has 20” of snow, what would that equate to?(S)(1.25)=P(20”)(1.25 lbs/sq ft) = 25 lbs/ sq ft. Any ice build-up on the roof needs to be added to this formula. Use 5.2 lbs for every inch in thickness.
While it is true that all roof operations are a necessary part of our job, we must take into account the ability to due it safely just as much as inside the structure. Whether it is from climbing a ground ladder or operating from a stick or basket adding inclement weather and added weight to the already stressed roof is not helping our fight. This might be a good time to consider carrying a grain shovel or even a leaf blower for light snow so we can remove it from the work area. Any snow removal should be coordinated with command and relayed to ground crews to avoid the hazard of snow coming down on them from above. Adding slip protection on our boots and strapping in is always a good idea too. On the next part we will talk about ground operations pertaining to snow.
Stay safe and remember why we do what we do because having the inclement weather is just a bonus!