What size earth auger bit do you use for a 4x4 fence post?

[Learning2Tractor]

I've seen 3x the size online but nobody seems to do that in real life. What size bit would you use for a 4x4 post? I will use either concrete or the foam stuff, probably the foam, so maybe the hole should be smaller diameter?

 

[M59]

Learning2Tractor,

Diagonal on 4x4" post is 5.65" so add a min of 1 inch to each side of diagonal for 7.65". Bore a minimium hole size of 8". Augers come in 8" & 10"

Never heard of foam. What is foam?

 

[MapleLeafFarmer]

depends on how deep you want the hole AND how good you are at making the hole straight.

regularly I use a 8" bit for a 5-6" fence post (rnd) set 4' in depth and I know with my PHD i creep the hst a little forward half way through digging the hole so it stays vertical. No problems with hole size. Creeping to keep vertical creates an oblong hole.

last summer i set 24' long laminated posts that were 7 1/2" x 7 1/2" (5 laminated 2*8's) 7 1/2 feet in depth and borrowed a neighbours 12" auger and this worked fine. Also had to creep forward a significant amount to keep hole vertical.

 

[Rockbadchild]

Learning2Tractor,

Diagonal on 4x4" post if 5.65" so add a min of 1 inch to each side of diagonal for 7.65". Bore a minimium hole size of 8". Augers come in 8" & 10"

Never heard of foam. What is foam?

expanding foam

 

[M59]

expanding foam


View attachment 782731

Yeah, polyurethane foam. Thats what I thought but I wanted to see what Learning2tractor responded with. That crap don't last since polyurethane breaks don't with moisture and UV and becomes brittle in about 2 years. Then someone taps your fence post and it falls over.

I'm a compacted soil or mixed concrete into the hole kind of guy. If I'm going to do the work, I will do it once NOT many times.

Amended response. When using wood post. I coat pressure and non pressure treated wood with "roofing cement" for the entire length that the wood will be exposed to earth plus extra 3ish inches above ground level. Posts don't rot off at ground level because water and bacteria can't get at the wood to rot it.

 

[RjCorazza]

I use a 9" auger to put in standard split rail line posts (6"x3") and corner / end posts (4"x4"). For non tensioned fence types I only pack the soil, and don't use concrete. I would not use a foam product on a permanent fence of any type.

 

[Streetcar]

Yeah, polyurethane foam. Thats what I thought but I wanted to see what Learning2tractor responded with. That crap don't last since polyurethane breaks don't with moisture and UV and becomes brittle in about 2 years. Then someone taps your fence post and it falls over.

I'm a compacted soil or mixed concrete into the hole kind of guy. If I'm going to do the work, I will do it once NOT many times.

For the past five years local utility is using foam when setting poles. They are not having poles fall over

 

[M59]

For the past five years local utility is using foam when setting poles. They are not having poles fall over

Streetcar,

Several huge differences in what the power company is probably using for urethane foam and what guys walking into the local hardware are going to be able to purchase. I spent 20+ years in advanced elastomers and there are many grades of urethane for many different applications. I could bore ( pun intended ) your with the details but finished cured urethane can range from glass like to foamy elastic stretchy sponge just by changing the polymerization components/ ratios.
Power company is also sinking that power pole a min of 10 feet and probably closer to 15-18 feet into the ground and they are probably using a very small overbore tolerance which all produces a tight fit and very little foam gets used.

 

[Wolfy]

You surely do not want concrete on fence posts. It will trap moisture and either rot the post, freeze and split the concrete, or both.

 

[LS Tractor Owner]

I use a 9" for 4x4 posts, but jump up to a 12" auger for 4x6" posts. Usually pack the hole with 1/2" minus gravel. Never had one fall over / lean or rot out with the gravel.