dead load, Dead loads, live load, predicting snow load needs, psf, residential construction, roof live load, snow, snow load
This morning was amazingly cold and the snow that had accumulated for the past several days was conversation worthy. When people get together and tell stories, we can tell the story of our snow this week and of learning the ways of our house.
I have learned a lot about building this past 18 months but I go for the simple understandings since I cannot keep all this in my head. Ignoring things like deflection (e.g. how much a piece of wood flexes when you press on it), there are two key structural load types, live and dead loads. Dead loads are relatively easy to calculate. Live loads are not. Live load refers to anything that can move in or out of the structure. This includes people, dogs, furniture, toys, food and stuff like that. Live load is flexible and it is not expected to be at all places at any given time. Dead load is that which is not movable or not often moved. It is the stuff of the structure itself. Dead loads include beams, flooring, floor joists, bath tubs and the roof.
In private residences universal building codes specify live loads range from 30 to 60 pounds per square foot. A deck (40 psf) is higher than an attic (20 pfs). A balcony is higher than a deck (60 psf). A good old living room is is 40 psf and the bedroom is 30 psf. I get that a deck does not need to bear as much weight as a balcony to be safe. A balcony seems to imply dangling from something and it is important that the thing that you are standing on that is dangling off the side of the building is not going to fail structurally. What I have read the engineering explanations for but just don’t get is why the common areas of a house are 40 psf and the sleeping areas are 30 psf. The only place I can go for an analogy on the bedroom psf is the old waterbed one. “Spread out across the mattress, the water does not weigh that much.” Never really bought into that one but I suppose sleeping ares have fewer people wandering around and we know that supine your weight is distributed across a larger area than if you are vertical.
In general, roof loads are predictable and can be incorporated into the dead load of a house. But, there are loads of short duration that qualify as a live load (Roof Live Load). Our roof live load includes the weight of the people who will crawl around on it 50 years from now when our metal roof needs to be replaced. My spouse, our dog Sophie and I will be passed on to our greater reward but we do need to plan ahead.
Planning ahead is an understatement when it comes to snow load. Unlike live load which is not assumed to be present at everywhere at the same time, snow load is. You have to build the structure so that it can hold up the maximum snow load as if it were there all the time, across the whole roof. It is notoriously difficult to predict the variations of how much snow will occur from one year to the next or even one storm to the next. Tonight, it is even difficult to predict the load from a single storm.
We have had a few days in the deep freeze again. This storm has lasted about four days we got around 15 to 18 inches of snow from it. When my spouse drove the Subaru to town today after being “holed up” since Friday, the space he left when the car moved out looked like a white swiming pool. The snow that had piled up stayed right where it was making a U with two foot high snow walls. Like the trees and cars, our roof was perfectly camouflaged by smooth, evenly distributed snow. This afternoon temperatures started rising. By five pm the snow softened and started its inevitable downward slide on our slippery metal 8 in 12 roof. An 8 in 12 roof is not as steep as it could be but for every foot across the width of the house the roof rises 8 inches so it gets up there pretty fast.
Between the metal material and the pitch of our roof, this evening was a crashing event. At first we went to check to make sure things were OK. After a bit even the dog did not budget when we heard another KBOOM. That is until we heard the one that went KBOOM (crack). That one we traced down. Turns out the (crack) was the sound of the window well at our basement apartment window.
We had quite a time sorting out the snow load on the roof we had. Construction came to a near standstill for a few weeks while an engineer fussed with calculating the snow load and what we needed to keep the roof from falling down. We have a high snow load. It is right on up there with a balcony at 55 to 60 psf. In case you need to know the snow load requirements should you be thinking to build a house in Montana, you can go to the Montana Ground Snow Load Finder. Right off the top count on a minimum of 30 psf, that is State Code. To find your precise required snow load, you enter any geographical latitude and longitude stated in decimal form and it returns a snow load value. Of course, not every point in the State of Montana is measured. The values are derived from reading at official stations around the State. As the Snow Load Finder states, “[the] value is the result of an interpolation of station values (normalized to elevation) that is multiplied by the actual elevation at the location of interest.”
What is scary to me is that I understand the statistical statement more than I understand the snow load. It should not be that way. I am retired and my statistical career is behind me. I am building a new house in snow country and finishing and living in my house is in front of me–hopefully not on top of me.
What I do understand is that this house is a live one. It has its creaks and groans. It has the way the wind sounds as it moves across the side porch. It whistles softly from the deck door where the green electrical cord is threaded. It’s heat pumps breath like a giant peacefully sleeping in the next room. Tonight it spoke to us with its authoritative voice of live snow load at 60 psf and falling.