size of hole for concrete fence post + costs for concrete

[David WE Roberts]

I need to dig and fill holes for 12 concrete fence posts - slotted ones to take fence panels.

General guidance is they should be 2' deep. Homebase 'how to' brochure suggests 1' * 1' by 2' hole for a 4" post. This seems generally O.K. apart from the issue that if the concrete post is on the boundary then the concrete filling will go 4" into the neighbouring garden. Please tell me if I am barking up the wrong tree.

That would be (allowing for the 4" post in the middle) about 8" * 8" * 2' ~ .8 cubic feet of concrete per hole. No! The hole is 2' * 1' * 1' = 2 cu ft. The post is .33' * .33' * 2' = .2 cu ft The remaining hole is therefore 1.8 cu ft. [Damn - spotted this part way through.] Mmm..

does not have a calculator for square holes with square posts within. Tried to frig it - 2 * 3 * 0.3 should equal 1.8. Used 2' * 3' * 4" but it rounds up to 2 cu ft (or down to 1.5 cu ft if you go down to 3"). Damn again. Suck it and see gets 2' * 1' * 11" to give me 1.83 cu feet

12 off it gives me 22 cu ft or 0.623 cu m. In built (dodgy) calculator says 12 * 1.8 = 21.6 cu ft Web converter says this is 0.648 cu m and 22 cu ft is 0.66 cu m. Conclusion - web concrete calculator is doing rounding all over the place and telling porkies (on a small scale). Also, my head hurts and I need a drink.

The calculator suggests that this volume is equal to 48.4 * 60lb bags (but of what?). Postfix seems to come in 20kg bags (44lb bags?).

This is all rough approximation, but this suggests that you would need more than one bag of postfix per hole (approximately 4). [Unless, of course, the quick setting concrete doubles its dry volume when mixed with water.]

Assuming 48 bags at £5 per bag this would be £240 for the pre mixed fast setting stuff. If I have under estimated the number of bags required each extra bag adds £5 to the total.

Surely I could mix 2/3 cubic metre of concrete, including hiring a mixer, for much less than this?

gives me, for 2/3 cu metre of 1.2.4 ratio:

211kgs cement 396kgs sand 729kgs gravel

From the Travis Perkins Trademate web site:

8 * 25kg bags cement at £6.50 a bag is £52. balast bulk bag (at least 855kg) is £62.50 mixer £11.50 for the week (HSS special offer)

This gives £126 but I am aware that I am slightly under (sand + gravel above = 1135kg). In fact, if the 'one ton' bag is only 855kg then I could be looking at a bag and a half. [Hang on - aren't the bags supposed to be one cubic metre? In which case one bag will be fine (unless ballast shrinks by more than 1/3 when you add cement and water). However in that case there is something screwy about the weights from the on line calculator!]

I do note that postfix is £7.30 a bag at TP so I may achieve better prices elsewhere, although TP were the cheapest for the concrete posts.

All this is looking like scarily large numbers! Do I really need a full metric ton of ballast and 8 bags of cement to secure

12 concrete posts? I nearly finished this post once then had to go through again doubling all the quantities when I spotted my initial sizing mistake.

Has anyone else done a similar exercise of building a fence with concrete posts who can reassure me that I am not wildly out with my calculation?

Help!

Dave R

[The Medway Handyman]

Far too complicated. 12 posts, do yourself a big favour & invest £20 in one of these

sell it on FleaBay afterwards or hire it out to neighbours.

Nice neat round hole about 8" diameter x however deep - I'd agree on 2'.

I bag of Postcrete/Postfix per post generally, if the hole goes a bit bigger due to lumps of rock, roots then 1.5 bags per post.

Couple of tips. Never assume fence panels are actually 1.8m wide - they vary +/- as much as 3/4". Concrete fence posts are 'kin heavy, get them in the right place first time - you need a helper.

[David WE Roberts]

O.K. - round holes are probably easier than square ones and I am already eyeing up a couple of similar hole diggers.

I am a bit concerned about the amount of concrete around the post - the soil here is quite sandy and I want this fence to be rock solid and to last for

50 years+ (working from the fence at our previous house which was still going strong in the concrete post department after over 20 years, with just the occasional fence panel replacement).

One of the concrete posts for our existing fence (bolt through, not slotted) is leaning over but it seems to have 4" of concrete on one side only. This suggests that 2" of concrete all round might do better. Question is, would

3" or 4" be even better or just overkill? Perhaps 9" or 10" diameter holes?

I think I will have to go for regular 1.8m spacing as I need to be able to replace fence panels as required - I think I will have to select the panels to fit, or if not possible then pad narrow ones out with an extra strip of wood.

Postfix is £3.99 a bag this Friday and Saturday at HomeBase if you 'bulk buy' 4 or more bags (just read the 15% off flyer) so I may go for that.

Reduced my plan to 11 posts and a 'U' bracket for the panel which meets the house rear wall (no space between the soil pipes for a concrete post). 20 bags of Postfix might be about right. I will also check with Travis Perkins as they seem to do 2 sizes of bag.

I was planning to pick the posts up with one hand and toss them into the hole, but you are probably right ;-) Will have a test lift with my good lady and see if we can shift one together or if I need to start waving beer tokens about.

Thanks very much for the advice.

Dave R

[The Medway Handyman]

Well worth the money.

Are you using concrete gravel boards?

Needs to go al round. Nothing wrong with bigger holes, you can do that with the post hole digger.

No, what I'm saying is don't space the post at exactly 1.8m. If you space at exactly 1.8 & have a 1.82 panel its a problem. Measure all the panels & space for the widest one, you can then pad any narrow ones. Oh - and never assume a panel is square.

Good price that.

[David WE Roberts]

Yes - and I was wondering last night if we would be able to get these in after the posts had been set, or if they needed to be fitted before the next post was concreted in. If I go for 9' posts (to allow the 6" gravel board and 6' fence panel to sit below the top of the post) then unless you can wiggle them in we would have to lift this gravel board 7' in the air. [O.K. for me at 6' tall, but perhaps not for my beautiful assistant.] On the other hand if we lay down the gravel board before finally concreting in the post we have the issue of getting the concrete in past the gravel board on one side. We could raise the gravel board a bit on bricks or wood, then lower it when the concrete is in. Won't really know what works until we fit the second post! At least the gravel board will help with the spacing.

I may just space for slightly over 1.8m and measure any panels we buy to make sure we don't get any that are vastly outside spec. If I space out too much I may end up having to pad out most panels. In my previous place I suspect they spaced the posts to fit the original panels as they were not consistent and the undersize panels were a b*gger to replace. I will need a little 'wiggle room' to allow for expansion of the wood when wet.

Thanks again for your advice.

Dave R

[dennis]

If you use a dry mix you can just shovel it in and poke it down with a stick without having to worry about the gravel board being there.

You don't need concrete anyway, telegraph poles are shoved in a hole dug with a post hole digger and then back filled without concrete (in most cases). They don't fall over very often.

The best way under normal circumstances is to remove the minimum of material needed to get the post in the ground as disturbed ground is what gives under load.

(If digging holes was the best way to support something they wouldn't use pile drivers

[The Medway Handyman]

Yes.First post, gravel board, second post etc.

You don't want to do that, trust me :-)

Why not go for 1' gravel boards & 5' panels? That way you can stay with 8' posts.

Not a problem with Postfix/crete. You pour it in dry & then add water.

[David WE Roberts]

Looking to go to the full 2 metres (6' 6") fence height - so 6' panel and 6" gravel board.