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jjohnston 02-05-2013 10:07 AM

Fence keeps blowing over
I built a fence a couple years ago. I used 4x4 posts which I planted 2-3 feet deep and cemented in. The fence panels I got are 6' high and 8' long. they were pre-built panels and it is a wood privacy fence. The fence uses 3 1x4 boards that the pickets attach to. I have 2 3" long screws in each 1x4 to hold it to the post. Every year in the winter we get a few times where winds get up to 50mph and it never fails, at least 1 or 2 panes get ripped out of the screws and land in the neighbors yard. the posts seem to be fine, its just the panels blow right off the posts.

Is there anything I can do to help keep this from happening? I thought about adding 1 washer on the top and bottom 1x4 to give it a bit more surface area to hold... but I don't really want to do that, because I think that would look a little bad...

Any help would be greatly appreciated!

mudmixer 02-05-2013 10:24 AM

Washers will help as well as larger diameter and longer screws are the obvious things to do as you replace panels.

Once the panels are strong and attached, the next problem you may have is the posts and anchorage into the soil since they will see 100% of the wind.


Wuzzat? 02-05-2013 11:19 AM

One or two panels out of how many? This info is more useful than if the whole fence fell or was not damaged at all by the wind.

Use this to figure the force on the panels
but you may have trouble finding the pull-through force for hardwood/softwood boards for some screw head or washer diameters.
In that case, run an experiment using one or more people to simulate the wind force by standing on the loose & damaged panels, and try different size washers. Each doubling of the washer diameter increases the pull-through force by 4x. You may have to prime & paint fender washers for this.

You want the fasteners to hold 4x or more of the max expected tensile force due to wind. For your area there may be max predicted wind speeds that are not exceeded 90% or 95% of the time, see below.

Once this is fixed you may find out what is the next weak point in the fence structure.


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nealtw 02-05-2013 05:33 PM

Wuzzat? 02-05-2013 07:50 PM

Fastener strength seems paramount here.

forces are in lbs.
50 >enter wind speed [WS] in MPH
PSF = 0.00256xWS^2
6.4 =calc'd PSF
48 >enter sq. ft.
307 =calc'd force
2 >enter 2 for flat, 1.2 for rounded
614 =calc'd total force
4 >enter # of fasteners
154 =calc'd force on each fastener
4 >enter safety factor
614 =calc'd ultimate strength for each fastener

For 75 MPH wind you need 1380 lbs/fastener. No wonder your fence is failing.

BridgeMan 02-10-2013 11:13 AM

I'd be strongly tempted to modify the panels, creating some gaps to let the wind through, instead of installing hundreds of washers. Pulling every 4th vertical plank would take a lot of pressure off each panel. If privacy is a concern, re-mount the removed planks (with spacers, if necessary) on the opposite side of the fence rails.

Wuzzat? 02-10-2013 01:11 PM


Originally Posted by BridgeMan (Post 83425)
I'd be strongly tempted to modify the panels, creating some gaps to let the wind through, instead of installing hundreds of washers. Pulling every 4th vertical plank would take a lot of pressure off each panel. If privacy is a concern, re-mount the removed planks (with spacers, if necessary) on the opposite side of the fence rails.


Our fence has overlapping vertical boards which must present less resistance to wind than would solid panels and still gives privacy.
Another way would be to have some vertical boards weakly fastened or hinged at the top, others more strongly fastened and some percentage really bolted in place.

I saw an ad by a fence post maker showing data on what wind force their posts and other types of posts could resist.
Problem was, they were galvanized steel and not so pretty.

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