Sacrificial Barn Roof

Question

So I am building a rather large (large for me anyway) barn/sheep shelter. It will measure 16x48 when complete. I have already started the framing, as you can see in the pics (the diagonal pieces are just temporary braces). When complete this will be a single slope structure, with all the walls enclosed except for a 8 foot section on the tall side.

So I might not have made the best decision with the footings. I think they are fine, and I am not worried about their ability to hold the weight. My concern is the uplift. I'm just not terribly confident with them, or rather, my use of them. The have hard-plastic 12" "plates" screwed onto the bottoms, although I had to cut some of them. I saw on the manufacturer's site that it will support 900 lbs of uplift, I think, but I just could not get them all the way down in this soil, even spending all day with a skid steer and big rock auger. So I got what I got. I'm not changing the footings.

I know that nothing will withstand a tornado. However, I'm worried about a strong t-storm coming along and carrying the thing off to Mexico in a crumpled-up ball. My first instinct, as always, was to overbuild ... big beefy rafters, 16" o/c, roof deck, then tar paper, and finally metal.

But then I thought today ... if I have a super-stout roof, like I planned, maybe it really will get carried off, and no joke. But if I just have some thin 1x4 cross members (purlins?) across light rafters at 24" o/c, and some r-panel screwed into those ... maybe the storm will simply peel the roof off, leaving the frame, rather than bolloxing up the whole smash. I can handle that.

Am I being silly thinking this way? Do I need to go ahead and build the tank roof?

Answer 1

Ahhh… the sacrificial roof concept.

My friend decided to buy an extra small car because if he got into a accident, he thought his car would just bounce off the big cars, trucks, etc. and he’d be safe. I don’t think it works that way.

You have a couple of issues: 1) uplift, and 2) blow down (shear).

1. Uplift comes in two forms: wind blowing on a wall and then deflected upwards sucking the building, roof, etc. with it, AND wind blowing in your 8’ open door and not having anywhere to go but up (taking the roof off) or blowing out another wall.

I don’t know where you live, but if you’re talking about tornadoes then you probably get some good winds each year. It’s complicated to compute, but on an average uplift calculates out to about 30 lbs. per square foot (psf).

So, take the weight (dead load) of the building and subtract it from 30 psf to see if it will fly away. Let’s start with the roof: 2x10’s at 16” oc = 2.5 psf plus sheathing = 3 psf plus membrane = 2.5 psf plus metal roofing = 2 psf will only be about 10 psf down load while the wind will create about 30 psf. So, you’ll need some hefty fasteners.

Likewise, the walls are susceptible to being blown away too.

2. Winds typically push on the side of buildings and want to twist, rotate, them until they collapse. Buildings stay in place by use of shear walls. You have none. Those shear loads transfer around the building by connecting the roof to the walls and then the walls to the foundation.

You’ll need to work on the connections to keep your building safe or plan on some major repairs every year. (I hate to mow my grass a few times a year so I could never do that kind of work.)

Answer 2

My great-uncle had a small shed/barn that had a flat roof and got torn apart by an extremely strong windstorm. When he rebuilt it, he made several adjustments to help it survive heavy winds. He left about two inches of open space between the top of the walls and the bottom of the roof. He also installed a couple of roof vents. His general idea was instead of the roof acting like a parachute and causing your shed to parasail away into the sunset, you'd put holes in the parachute to give the wind somewhere else to go (thus reducing drag and reducing the force applied to the structure). In other words, instead of excessively reinforcing the structure, reduce the magnitude of the uplift forces that you have to endure.

He was particularly proud of the "cowboy safety valve" he installed. He put a square window near the top of the wall opposite the door. The window had no glass, just a frame with a wooden shutter that swung outward. He drilled a small hole about an inch deep in the bottom edge of the frame (on the exterior side of the shutter when closed). A toothpick in the hole was enough to keep the window closed during normal weather. If the wind really picked up, the toothpick would snap, the window would blow open, and the wind would have a nice big hole to blow through instead of pressing on the underside of the roof.

That shed survived several decades of storms that were as bad or worse than the one that carried off the original shed. The wind eventually got its revenge, though (a storm knocked a huge dead limb off a tree that went right through the shed).