Experience digging fence posts?

In My Humble Opinion
1

About 12-14 years ago I put up a fence across the back of our property (this was where I was glad that lots here are only 25’ across). This solution lasted maybe 10-11 years, and now it is sagging badly enough that I have to do it again. This time I would like to make it last longer, 20 years if possible (because by then I won’t be here any more).

Here is what I did: I bought pre-made fence sections 6’ high by 8’ across, cedar. I used 7 fence posts, 4x4 pressure treated lumber. I dug the holes about 2.5 feet, and put gravel across the bottom. I made sure each post was plumb before pouring in quick-set concrete, mixing it up well and trying to make sure there were no air pockets (when I pull the old posts I might be able to tell how well this worked). I think the holes were about twice as thick in each direction as the posts. When the cement had cured, I attached the sections to the posts. Each section was attached to a post at each end and one in the middle. That left 6" of space on each end, but that was fine, it wasn’t supposed to keep anything out, it was so I didn’t have to look at the back neighbor’s ugly overgrown back yard.

Changes I have considered: bigger hole, maybe wider at the bottom to get a solid base; maybe using just lumber instead of pre-made sections, redwood should do, and it would also allow me to cover the entire 25’; maybe use larger posts, such as 6x6 instead of 4x4, so if they end up rotting (I don’t know yet if the old ones did) that process will take longer.

Any suggestions from anyone who has done this kind of thing before? Cheap and easy solutions are good, if it can end up looking decent.

2

I’ve put up three privacy fences, but it’s been a while.

I don’t think your holes are deep enough. They should be 1/2 to 1/3 the size of the above-ground post (not including 6 inches or so for the gravel). I don’t know your location, but you are also supposed to go below the frost line. Finally, the holes should be three times the width of the post, so it sounds like you are ok there.

To anyone considering tackling a manual post hole digger, learn from my mistake. I dug in, struck a large rock, and the post bounced up and hit me in the eye giving me quite the shiner. I had to put up with a lot of comedians the next few days at work.

mmm

3

For your consideration: Foam post anchors

4

That’s very intriguing. It says it’s for load-bearing applications, which this isn’t really. The forces on a fence don’t go down, they go sideways, like wind blowing against it. Still, I will definitely look into that further. eta: Oh, I see it mentions fences on the package.

Hmm, 2’ below ground, 6’ above ground is 1/3. I thought about going to 1/2, but it means getting 10’ posts instead of 8’ posts, putting 3’ below ground, 6’ above ground and sawing off the extra. That is something I thought about changing this time around, but I forgot to mention it. Also, no frost here (San Francisco) so thankfully I don’t have that to worry about.

5

Do you mean the posts are sagging front or back, or the fencing sections in between sagging like a loose volleyball net on straight posts? I assume you mean the posts are no longer straight.

I don’t know where you are, but I’ve installed a few fences in southern Alberta where there’s not a lot of topsoil, mineral soil is a mix of clay/silt/sand/gravel, there’s not a ton of rain, and cold winters. I’m assuming soil composition is a factor.

I used an old hand auger, the type you twist down, about 8" diameter. I’d dig most of the length of the tool, close to 3 feet down. My posts were 8-foot 4x4 treated wood, with a finished height around 5 feet above ground. Our soil is dry enough that I didn’t add gravel at the bottom, but just refilled around the posts with the soil I’d dug out.

What I did a little different though was only add 1-2" of dirt at a time, then thoroughly compact with either a 2x4 or the rounded shovel handle point at the corners. It takes a long time (maybe 15 min or more per post), but I found really compacting the soil hard in small layers made the posts very firm by the time I got up to 3 feet of fill (Make sure to check how level they are as you go because it’s really hard to adjust them at the end and you mess up the compaction job you’ve done if you do). If you’re not sick and tired after pounding in 6-8 posts you’ve not done it right; it’s a lot of manual labour.

Not saying this is the best or only way, especially in a wetter climate, but my posts felt as though they’d been set in cement. The benefit of all the compaction is that I know there are no air pockets or missed spots. The last fence I did this way was in 2010 and I don’t live there anymore, so can’t verify the life expectancy.

Err, no, you’re thinking a ratio of 1:3 (2 feet below 6 feet above, total 8 foot post). A fraction of 1/2 to 1/3 buried for an 8 foot post is a 2.7 - 4 foot deep hole.

6

If the fence is just sagging can you drive T-posts to hold it up? I had a fence at my old house that was floating on t-posts after the wooden posts had rotted away. They make t-post to wood brackets.

7

The OP says the current fence is “sagging badly”. That sounds like the problem is weak horizontal cross beams. The pre-built fence sections I’ve seen are made with weak-looking cross beams — are there other points of failure on the existing fence?

The wood privacy fences I’ve made or owned have all been pressure treated, and are holding up fine —some for over 25 years. If you went with PT, maybe you could reinforce the cross beams with 2x4s or a length of deck board (maybe right underneath the cross beams).

I’ve built privacy fences from 2x4s and 6” pickets, and the strength sure looks better than the “bottom dollar” pre-made sections from Home Depot. But it does take time to put them together drilling screws. I’ve never used a nail gun or a screw gun, but imagine that could save a day or two.

8

Get 10 foot posts, put 4 feet below ground and 6 above. That’s slightly better than 1:2, and they’ll be solid as hell until they fall apart.

9

Well, I was pretty sure that all the references I looked at were comparing below ground to above ground, not the percentage of the total post. That’s the way MMM’s post reads, “1/2 to 1/3 the size of the above-ground post” (emphasis added). But I’ll check again.

The fence sections are still firmly attached to the posts, it is the posts that are now loose in the ground, and tending to sag back away from the house. Adding support in the back of the fence means potentially putting stuff in the neighbor’s yard. I’m not sure precisely where the property line is there, somewhere between my fence and their old fence (which they have finally torn down).

I did compacted dirt for a heavy-duty plant arbor that I built with 4x4’s about 3-4 years ago, and it’s still quite sturdy. Do you think that is better than concrete for a fence post?

I’ve considered switching to pickets. I thought the premade fence sections were better looking, but they really didn’t last very well. And there were no cross-beams, just the sections attached directly to the posts. Perhaps that was a serious flaw in the design.

10

Can I sneak in a semi-related question?

Is it normal with a chain-link fence to place the fabric on the outside of the posts (away from the owner’s house)? We have chain-link fences on both sides of our back yard, and I’m trying to figure out who they belong to.

11

For any common type of fence, the “smooth” side faces outward, and the side with the posts/poles faces inward. Thus (in theory) anybody outside the fence will not be able to see the posts or other supporting structures.

12

I always thought that was pretty universal, but I coincidentally learned just a few days ago about a neighboring city that does not have such a requirement. I’m sure there must be others, so check with your municipality.

mmm

13

Not familiar with “T-posts” but the local equivalent are “star droppers” which are reasonably cheap and readily available.

Driving in a dropper beats digging a hole and then backfilling.

Put two droppers down on the diagonal of the post, fix in a couple of wire twitches and the job will be done for decades, though not as aesthetic as putting in a new wooden post.

14

There are places in the uppermost northwest were that can be more than 80" though. It’s 60ish here in the northeast.

15

I installed my pressure treated 4x4 fence posts 40 years ago and they remain solid and I’ve replaced the stockade fence sections 2 times using the existing posts. The post holes were between 2’ – 2.5’ deep and I used 8’ posts with the in ground post sections coated with Cuprinol preservative. I poured 4 to 6” of crushed stone in the hole, set the post and filled with concrete 1 to 2” above ground level and beveled the above ground concrete to slope away from 4 sides of the post.

Actually, I’m amazed they lasted this long.

16

Standard industry practice for a 6 foot fence is a 3’ deep post hole. Thats why 9’ is a common size for fence posts. Thats 1/3 the length of the post, despite the terrible math up thread. 6x6 posts are way, way stronger and look great.

I have installed lots of posts by packing them in with native soil and/or gravel. Tends to drain better so more rot ressistant. A leaning post can usually be easily plumbed again by compressing the dirt using a digging bar on the side of the lean. Disadvantage is it is more labour intensive and loses the counter weight effect of concrete.

Concrete is a counter weight; not something to ‘glue’ the post into the ground. It helps keep the post upright. Think of the post like a buoy floating in water. It does not need to go to the top of the post hole. Often posts rot right at the level of the concrete if it is exposed as it traps water in the post. The foam stuff doesn’t make much sense to me, you lose the counter weight effect.

As a carpenter, I think wooden fence panels are garbage. They make little sense for me, but they do save labour. The joints are going to be weakened by transport, and are unlikely to be fastened as well as a site built fence.

Fence rails need to be installed with the long axis of the lumber vertical, never on the flat. Rails installed on the flat will always sag. Just look around the neighbourhood. Its common sense, I just don’t understand how this isn’t obvious, but people do it wrong all the time. Rails need to be fastened to the posts with at least 3" screws or nails. If you need to you can support the rail with a clamp while you fasten it. I do not think those metal brackets are particularly useful, but if you insist on them make sure the rail is still fastened to the post with fasteners angled through the 2x4 into the post (‘toenailed’). Three screws or nails is about right for a 2x4; two is not enough, more may split.

17

Are you saying you haven’t examined the existing posts, or are you wondering if they may be rotting deep below grade?

18

Yep. The rails are not built from full sized standard dimensional lumber and are usually stapled to boot. We (son’s construction company) built our fences with 2x4s for the top and center rails but used 2x6s for the bottom rail. It gives better protection from bumping. Also - stagger the rail joints so you don’t end up with all the rails joining on a single post. We used 16’ rails with 8’ as needed to make the stagger. As mentioned - long screws for rail to post connections. Fence board to rail can be 2" screws. Use Torx screws (star drive). Two screws on each 6" wide fence board at all rails.

You did mention you have a post in the center of each pre-made rail section and that will help a lot. They don’t span 8 feet without eventually sagging.

19

I’m saying I haven’t pulled the posts yet to see what happened.

@FluffyBob and @Dereknocue67 and others, thank you for your helpful and informative posts. Here are a few points that this discussion has helped me to reach:

No more pre-built fence sections. Looks like I’ll be cutting these up and taking them to the dump, I don’t know what else to do with them, can’t burn them, and I don’t want to try to muscle whole ones back up a flight of stairs, through the house, and into the front, which is the only way out.

I will use 2x4 pressure-treated fence rails, installed flat against our side of the fence, leveled and staggered. 3 rails, top middle and bottom. I don’t know what “bumping” refers to as a reason to have a 2x6 on the bottom rail, so feel free to elaborate if it’s important

I will install vertical pickets across the fence rails. I’m thinking about redwood. I haven’t looked around at what’s available yet. If I can find 6’ pickets that are 4" to 6" wide, that’s probably what I’ll go for. Anything I should watch out for here?

As for the depth of the post, here is what everything I find online says:

Dig post hole so diameter of the hole is 3 times the width of the post (i.e., the hole for a 4” wood post should be about 12 inches wide). The depth of the hole should be 1/3-1/2 the post height above ground (i.e., a 6-foot tall fence would require a hole depth of at least 2 feet).

Also this isn’t farm country, I’ve never seen pressure-treated lumber in lengths of 9’ (but I will check to be sure). I don’t mind going down 3’ if it will help keep the post straight and help it last longer, but I think I will have to buy 10’ lumber to do it.

4x4 or 6x6, my only concern is durability not looks. Once the fence is built we won’t see them, and I don’t care what they neighbors see (as, apparently, neither do they). A lot will depend on what I find when I pull the existing posts. 10’ long 6x6 pressure treated lumber is mighty heavy, I might have to hire help to deal with those.

That’s where I am now, if you see any red flags in what I have said, please point them out. Thanks again to all the experienced and professional people who have weighed in so far.

20

I myself did a couple of hundred feet of this type of stockade fencing about 25 years ago, and have replaced some of the fence sections, and a post or two, over the years. (Now when I replace a fence section I use a PT fence, so will hopefully need to replace fewer of them going forward.)

I did not bother with any concrete or gravel or anything like this. Just just holes, put the PT 4x4s in, tamped down the soil, and that was it. My understanding is that putting in concrete cuts both ways, because if the soil around a 4x4 shrinks other soil will drop into the crack, while the concrete will also contract a bit over time but is less likely to be filled. When I’ve had to replace a 4x4 it’s been because the 4x4 rotted underground, which I don’t think concrete would help for anyway.

I put my posts at a height that the top of the post would be level with the top cross-rail of the fence section. Spaced 8 feet apart (no middle post).