Where To Find Info On How Much Weight (Snow) A Residential Roof Will Hold
jimmiem
| Posted in General Discussion on
As I’m sure all have seen and heard we have had a lot of snow in Massachusetts. The weather people give a rough idea of how much the snow weighs depending on depth and moisture content. I realize there are lots of variables to how much weight each roof can hold. Is there any reference available that would allow the weight calculation to be figured out by plugging in the variables? Contacting an architect or engineer is impractical. The standard answer is ‘it depends’. I’m just trying to get a rough estimate.
Replies
Put a ladder up to your roof. Then remove one cubic foot of snow. This sample should be representitive of the entire roof. You'll have to do this carefully to get an accurate sample. Take the snow into the house and weigh it. Now you know how much the snow weighs per cubic foot. Measure or guess at the depth. Multiply by the unit weight and presto! You know the pounds per square foot load of the snow.
I suspect that it will be far below critical.
The more sample you measure, the more accurate you will be.
Don't fall off the ladder.
Did The Snow Sample(s)
Ok. I got the samples as you suggested. I took several samples as the sunny side of the roof had more water content to the snow due to solar melt than the shady side. I did a Google earth check of the sun trajectory so that I could get a weighted average as the sun melt was not uniform across the roof. So now I know how much snow weight is on the roof. Now all I have to do is calculate how much weight the roof can hold. You've been spot on so far....can you help with the next part? Thank You.
If the roof is conventional framing ( stick built, not trusses) you can check the allowable loading for the size, span, and slope of the rafters in the building codes.
You may also have to check that the proper nailing schedule had been followed for the rafter to ceiling joist and rafter to collar ties.
Do you know the roof construction details?
Roof Is Conventional
Roof is Stick built. I can easily get all all the details. Does the code give the loading info?
I live in Canada, rules may be slightly different, but the lumber is the same.
Building code lists max spans of various size & spacing vs various snow loads. Just need to scan the tables for the correct combination of rafter size and spacing, to see which snow loads give a span equal to or slightly longer than what is in your house. Note that span is the horizontal distance that the rafter spans, not the length.
Span tables can be found in Appendix A of the CMHC book Canadian Wood Fram House Construction, available for free download here: http://www.cmhc-schl.gc.ca/en/inpr/bude/bude_001.cfm
It is a 12MB file, but gives you a lot of information on how to build a house.
However, when it comes to withstanding something like a snow load, the devil is in the details. Rafters are supposed to be secured properly at the ends, eg, but if someone didn't follow code because "it nevers snows that much here" then you could be in trouble.
Catmandeux
Thank You. I just downloaded it.
Right Answer Needs Right Question
If you Google "roof load design," your first link will be to Chapter 3 of a government publication, "Design Loads For Residential Buildings." That's your first step- understand how roofs are designed.
The link has a masp that lets you make a pretty good guess as to what load was used to design your roof. Yourlocalbuilding department can give a more specific answer - assuming that there was any codeenforced at the time the house was built.
The idea of climbing on the roof and removing a snow sample is scary. Not only are results of such a sample misleading - snow deposits vary greatly over the same roof - but the process of collecting the sample is dangerous. Worrying about the roof after snow has fallen is like the Titanic's Captain counting lifeboats after the ship left port; it's too late to do anything about it.
If you're worried, here are some things you CAN do:
1) You can make sure your gutters are clear and there's a path for snowmelt to drain away;
2) You can watch for any sign of ice dams or water backing up the roof;
3) You can enter your attic and inspect for signs of leaks or damage.
That last point is telling; you're far more likely to have a rotten section of roof deck spring a leak than to have a complete roof failure.
I said it that way for a reason. Good roofs never fail in part; loads are distributed and the entire roof usually fails at once.
Yet, a flaw in the roof covering CAN lead to a small area of the roof deck getting wet and rotting over time. Add some weight, and before you know it there's a leak drip, drip, dripping into your attic .... soon to be followed by a large puddle of soaked insulation and mushy drywall crashing into the room below.
Get up on the roof and shovel off excess snow? Probably a very bad idea. It's rarely necessary and never safe. If you MUST, stay on your ladder and pull snow towards you with a rake or broom. It's the snow at the eaves that poses the risk of making ice dams.
renosteinke
I did not climb a ladder to get a snow sample. I thought that sapwood was kidding when he made the suggestion so I was too. I had the roof re-shingled last summer and supposedly have 6 feet of ice and water barrier. Have some icicles but no serious looking dams. 14 inches of insulation in the attic so roof should be quite cold. Plywood decking was in good shape when it was stripped during the reshingling. The house is 2 story colonial with 1 story porch abutting main house and attached garage abutting the porch. The garage and porch seemed to catch more snow than the main house. I used a snow rake to remove a lot of snow from garage and porch....I never had to leave the ground. I made some snow shoes from info I found on the internet and they actually worked....made 'em out of plywood (old table saw cross cut sled). Snow is almost waist deep....couldn't have gotten to the back of the house without the snow shoes.
The only way to figure it out would be to check all the construction details and do some calcs. Not something the average guy can do.
As someone else mention, the size and spacing of the rafters is a start.
But the pitch of the roof and connection details also come in to play. Doesn't matter how big the rafters are if they didn't do a good job of connecting them to the ceiling joists.