Take the 2-minute tour ×
Home Improvement Stack Exchange is a question and answer site for contractors and serious DIYers. It's 100% free, no registration required.

I want to build a small 10x10 shed in my backyard. My city code doesn't require building permits for structures under 120 sq. ft. and doesn't require footings that run down below the frost line for those structures. However, the code does require that the structure be built to withstand 90 mph winds.

I was planning to just put the shed on concrete block footings on a layer of gravel. I'm pretty sure the structural design will be over-built, but does the wind resistance requirement mean I need to anchor the building to footings buried below the frost line?

share|improve this question
1  
Wind + no ties to solid foundation = moving box. Having seen 90MPH winds hit and knock over a tractor trailer rig (Montague Notch), you want to take into consideration the sail area when the wind hits the side of the structure flat on. –  Fiasco Labs May 6 at 17:35
2  
I had a shed in my backyard that was simply nailed into a concrete pad. The cheaper aluminum kind but still a decent shed. Huge huge storm came... maybe 90 mph winds. Shed flew into neighbor's pool. The only left was a half empty bottle of chlorine, that was exactly where I left it in the shed floor. Good luck figuring out what rated for 90 mph is. Maybe the chlorine bottles were rated for it but not my shed... Just ask the inspector what he wants. –  DMoore May 6 at 21:01

1 Answer 1

up vote 1 down vote accepted

To resist wind forces, you don't necessarily need footings below frost line, but you do need a substantial mass to resist the overturning forces from wind. Buried footings work well because the the wind must also pick up a substantial amount of earth in addition to the foundation weight. The deeper the footing, the more earth being picked up, so it is a good solution that does not require excessive amounts of concrete or masonry to provide dead weight.

The structural portion of the IBC specifies how much pressure 90 mph wind exerts on various surfaces. While too complex to get into here, if you understand overturning moments, determining the required dead weight to resist the wind is not all that complicated.

There are too many unknowns to say for sure, but I doubt a course of blocks, even with the cells filled, will be adequate to resist wind forces. In order to not build more foundation than you need to, it's likely worth it to find someone who can calculate the minimum foundation to do the job. An engineer is an obvious choice, but since no permit is required, it's likely a licensed engineer is not required. You just need someone who understands the physics involved and can do the math.

share|improve this answer
    
Thanks for pointing me in the right direction! Do you know of some resources that would give me just enough of a crash course to be able to do the math myself? –  rob May 7 at 21:43
    
Sorry, no. I tried some basic online searches but didn't see anything I would recommend, though there is probably something to be found by digging deeper. I'd be more inclined to suggest a textbook intended for undergraduate architecture or engineering students, but without being able to scan the contents, again, I couldn't make any recommendations. Part of the problem is separating what you need to know from extraneous information, it seems most online references I saw went into much unrelated detail. –  bcworkz May 8 at 21:01

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.