I have a privacy fence at ground level, that gets a lot of
wind. It has started to lean. I have placed a heavy piece of outdoor
furniture next to it, but it gets moved over eventually, and leans
again. I have also wedged pieces of wood into the ground next to the
bottom, and it helps for a while, then the leaning starts again.
Fence is 6 feet tall. About 11 feet long. Three posts ( 3.5 in by
3.5 in) going down into ground, don't know how deep. Doubt there is
any concrete. 20 years old. Don't know what kind of wood it is. The
wood above ground is in good condition. No rot. Soil is often damp,
clay. Mostly in shade. No comparable neighbors' fences around to
By the age and conditions, they're probably about rotted off below ground.
Wet clay ground will move when force applied as well unless there's
sufficient depth to be to really solid dirt to resist.
The wind load of a solid fence is pretty good when look at the
cross-sectional are times the velocity-squared term to convert from
speed to pressure and the area multiplier to force so takes a pretty
good counterforce on just the area of the posts to counteract that.
You're taking 66 sq-ft and counter-balancing that w/
<4"/12"-ft*(say)3-ft (probably optimistic) depth --> 66:1 ratio w/o the
moment arm difference of 6ft/2 vs 3-ft/2 as midpoints of another factor
of 2 besides. That's all quite approximate to an actual force/moment
diagram but gives the overall picture of what's going on.
That 132:1 ratio/#-posts is the actual multiplier so if it's got the two
ends and a center for a 5'-6" spacing, it's "only" 44X the wind load at
The fix probably means replacing the posts if I had to guess because
they're gone or near so and either add depth or backfill w/ concrete or
other means to be more stout. It's possible, of course, that it was
good enough originally and it is just the posts rotted w/ time.
You can listen to all these numbers / ratio BS like this guy dbp (aka
pointdexter) is spewing or you can listen to me - dig up and set new
posts in cement. Bury posts 1/3 the height of the fence. My privacy
fence withstands winds of 60-70mph. Does not move / will not move.
You be wrong there, good buddy, whatever you think... :(
Well, if you would read what I wrote, in essence I'm agreeing w/ ya'
that in all likelihood his posts are gone...and needs new. The "ratio
BS" as you call it simply serves to demonstrate to OP what he's
expecting a small cross-sectional area to do.
The concrete will, in fact, increase the cross sectional area and hence
the resistance force by that factor. The down side to setting a post in
concrete in a wet area is that it traps water around the post and
thereby may promote the rate of decay.
Also, in heavy frost areas, the concrete 'plug' surrounding the post
can be 'heaved' by frost action.
Here (cold climate next to the North Atlantic), the recommendation is
that concrete be applied to the base of the post, well below ground
level, but not all the way up to ground level. Thus avoiding something
that frost can exert an upwards pressure on.
BTW: One formula I 'seem' to remember for wind pressure was;
Force in pounds = 0.003 x wind speed (in mph.) squared
Hence a fence 11 feet by 6 feet; i.e. 66 sq.feet. in direct wind
against one side ....................
At 1 mph = 0.003 x 66 = 0.2 lbs
At 2 mph = 0.012 x 66 = 0.8 lbs
At 10 mph = 0.3 x 66 = 20 lbs
At 20 mph = 1.2 x 66 = 80 lbs
At 40 mph = 4.8 x 66 = 320 lbs
At 80 mph = 19 x 66 = 1270 lbs
At100 mph = 30 x 66 = 1980 lbs
Noticing that as wind speed doubles the force just about quadruples!
So any fence whose posts are already deflecting at all won't stand
much chance in a 'good blow'.
(snip) or you can listen to me - dig up and set new
Nobody else said it, so I will - never set wood in a completely enclosed
concrete pocket. Dig the hole extra deep, and put gravel in the bottom.
Put in the post, and about 6-8 inches of tamped gravel up the sides.
THEN pour your concrete in the hole. As others have noted, frost can
make the ground spit out the concrete, so if you can make the hole a bit
of an upside down cone, it will better resist heaving. (not hard with a
decent post hole digger, unless soil is sandy. (for deck and basement
pole footers, a lot of people like the leave-in-place plastic forms
shaped that way, but that is way overkill for a fence.)
As to if you should bring the concrete all the way up to daylight- if
you do so, make sure to shape the top so it doesn't puddle water against
the wood. IMHO, stopping it it 3-4 inches below grade makes it easier,
and reduces how much water will run down the post into the concrete.
On Thu, 18 Mar 2010 09:45:11 -0700 (PDT), utilitarian
Take a post hole digger and dig down next to the posts. See if they
are rotted. If not, dig all the way around and fill with concrete.
If they are rotted, replace the posts and use concrete. You can
likely salvage the rest of the fence if it's still good. Just saw the
nails off the post with a sawsall. The posts may be too short too.
You wont know till you dig. They should be at least 30 inches in the
ground. If its a 6 foot fence, I bet the posts are 8 footers / 2 feet
in the ground.
If you just want a cheap and quick fix, drive some metal posts
(T-posts or pipe) into the ground next to the posts, and put lag bolts
thru the metal into the wood posts. Of course be sure the fence is
straight and level when you use concrete or steel posts, ot it will be
I saw a fix on one of the TV home shows where they used a metal post
reinforcement that got hammered in right against the post,then screwed into
the post. It seems that the post can rot under the surface and not be
seen,but the post breaks below ground,and you get lean.
They also did a post replacement,and used rock and gravel to provide
drainage around and under the post so the rot would not reoccur with the
Dicking around trying to shore up the fence and fretting about it,
you've wasted more time than it would have taken to completely rip the
fence out and reinstall it properly. Three posts, two panels... You
could have it done in an afternoon even if you hand-dig the holes by
yourself. Less if you have help.
My fence was put up in 1978. Stockade fence 6 feet tall. The first year
it started falling/leaning. My husband called the company back and had
them install galvantized posts. Problem solved and the fence still stands
Diminish the force of the wind (by cutting holes in the fence) or strengthen
the fence (deeper foundation) or external means (guy wires or diagonal
The remaining option, blowing back, is not practical.
You have to either dig deeper and use longer posts, put the post in
concrete, or add more posts.
You can add heft bracing behind the fence but that causes other problems as
they are in the way of grass cutting, tripping hazard, etc.
Since the fence is already leaning, the supports are loose in the ground and
you have to dig down to fix them properly, possibly set them in concrete.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.