Securing a Shed

I had an interesting question posed via email about how to hold down a shed, particularly in high weather affected areas.

To qualify, I am not a structural, or civil engineer, so these are just my thoughts on the issue, so take them in that regard.

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As I see it, there is one primary failure method for a structure subjected to high wind, and that is for the roof to lift (whether or not it also lifts the walls with it!)  A roof, like any flat or convex curved surface causes a pressure differential when air blows over it, creating lift.  When the lift exceeds the strength of the fasteners (either locally or in total), the roof fails, and as does the structure.

The local building codes will hopefully have some guidelines, particularly on wind strengths to consider.

If you can secure the roof down sufficiently (and consider that the heavier the roof, the stronger a wind would be required to lift it), then the whole structure can be considered one element, and the problem becomes a lot easier.  Then you need enough bolts (I typically use dynabolts from Ramset into concrete, which have a high pull-out load (ie how much force needed to rip them out of the slab)).  If you REALLY want to get technical, this pdf product sheet gives you the calculations for working out just what a bolt can take, based on what material it is afixed into.

As a basic rule-of-thumb, I’d place a dynabolt in every 1.5m or so, with a minimum of 3 per wall (one in each corner, one in the centre).  Probably overkill, but the bolts are cheap, and a lot cheaper than watching your shed become the next door neighbour’s shed curtesy of a wind gust!

So the shed is secured down, and anything you add to the weight of the structure will also help – a full woodrack works wonders (yes, speaking from experience here!).

So back to the roof then – the real source of weakness.  A heavily constructed roof will survive a lot better than one that clips (or worse, rests) together.  So if you have one made from sections of tin that simply rest one on the next, and clip in top and bottom, you are destined for some heartache.  If you improve the structure so the roof is one unit (rather than a bunch of panels), things rapidly improve.  It can be as simple as steel rivets, so you end up with one large panel for a roof.  That way, if an individual joint or connection fails (such as round a doorway), the entire roof still has to be lifted, and not the individual panel. (Did I mention I really don’t like clip-together sheds?  At least not without some aftermarket modifications!)  If you want examples of what I mean, just watch any video footage of a hurricane.  What do you see blowing around?  Lengths of corrugated iron.  If you see an entire roof going, then we are dealing with something else (tornado!)  Corrugated iron is normally screwed to the structure individually, so if the few screws holding one panel down fail, that section blows off, exposing the rest of the structure to the full force of the wind, and more panels will often follow.  If the corrugated iron was joined together, separately to being fixed down it would be a lot stronger.

The final thing I’ll touch on is banding.  Guyropes (if you are into camping), which are used to hold a tent down.  Where wind is a real factor, throwing a rope over the top of the structure and securing it down significantly improves its survivability, and the same holds true for a shed.  A metal band or two over the top of the structure will significantly improve the connection between the roof and the walls, and effectively increases the overall weight of the shed.  These can be dynabolted directly to the slab.

So those are my thoughts on the subject.  I’m sure there are plenty of books on the subject somewhere!  Hopefully, they don’t contradict the-above thoughts!

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