How many bags of postcrete per post

[Steve111111]

I'm putting up a fence which will have 3 galvanised steel H posts. Posts are 1850mm long. Supplier site says to install posts 600mm deep.

Post dimensions are 125mm x 105mm. I'm wondering how many bags of postcrete I'll need. Would it be 1 bag per post? Bags are 20kg. Instructions only say to half fill hole with water. Google says holes should be 3 times the width of the posts so I was planning to big them out that size.

Any advice on whether I could place the posts flush against the concrete so that I could get away with using less postcrete (obviously I don't want to reduce the post support too much by doing this)?
Likewise putting the post at the top of the pic against the concrete and boundary wall? Would this be an issue?
BTW concrete path is about 150mm deep at the edges.

 

[Ivor Windybottom]

Just dig a spade width, it will be about right.

Just use one bag per post, this will hold anything, initially at least.

If you want to bring it higher then you could add more an hour later, after the first has set well. Don't use more than one bag at once or you risk the water not getting through it all.

But I'd recommend using conventional concrete on top of that first bag of postcrete. Mixed with a mixer or by hand, it's much better as it's consistent. You don't need the fast-setting once that first bag is pinning it in place.

 

[Deluks]

Everyone's got their pref.
I still prefer to prop mine and use regular concrete, unless in a hurry.

Prop by clamping a bit of 4x2 or whatever to the post, and then sitting a bag of sand on the other end, adjust to suit.

It's cheaper in materials you've got more working time to correct errors, and you don't have to limit yourself to digging a hole smaller than a gnats chuff. Hit a rock that needs digging out and suddenly, a 1 bag hole becomes a 3 bagger!

 

[23vc]

As above, pointless using postcrete, unnecessary cost, just buy bags of ballast and cement, as it’s cheaper you can be more liberal with how much you get too, often end up with bigger holes to fill than planned due to adjustments etc

 

[Ivor Windybottom]

It's waaay faster and you'll get them much straighter if you buy one bag of postcrete per post, then add proper concrete after. It's amazing stuff. Stand there with the spirit level on the edges of the post, hold it dead straight and get your assistant (the missus) to pour the stuff in then the watering can of water.

It takes about two minutes. You just keep checking it's plumb, making any tiny corrections as needed. Then you'll feel it take hold of it from you.

This is probably good enough, and is frankly all that would be done by most fencing companies. But I find it reassuring to then bring it up to just below ground level with wet-mixed standard concrete. Then you get the best of both worlds - the precision of postmix plus the added strength of full-depth concrete.

I doubt that any post jammed upright with bits of scrap wood will be as precisely vertical as you can get it holding it with your hands while staring at the bubble on a level.

 

[Deluks]

I doubt that any post jammed upright with bits of scrap wood will be as precisely vertical as you can get it holding it with your hands while staring at the bubble on a level

Of course it is, you hold a level on it and adjust it until it's where you want it. The weight of a bag of sand on the end of a length of wood stops it from going anywhere. Plus you can jam bricks and rubble down the side as a wedge.

 

[Ivor Windybottom]

That all sounds a lot more involved than just standing there for 2 minutes while it sets!

There's a good reason why fencing companies buy the stuff, it costs a bit more but saves a load of time and hassle.

You need to prop in two directions, you don't rely on the prior fence panel being plumb.

One human hand on top of the thing can apply infinite tiny adjustments in any direction, no woodworking required at all.

You do need to check the height though, and put bits of tile or whatever in the bottom of the hole under the post. Sometimes it takes popping the post in and out a few times if you're a fussy sod like me.