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I need to build a concrete base for a mail parcel box on the ground. The parcel box is 36.6cm deep, 49.6cm wide and 112.3 cm high. It weighs about 15kg.

  • What size should the concrete base be?
  • How is it best to fasten the parcel box the the concrete (anchor bolts vs dowels)? image of parcel box with dimensions

EDIT, added this illustration: diagram showing parcel box resting on concrete block on gravel

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    If there are any security concerns, more concrete is a cheap deterrent. I'd also attempt to slope the exposed edges of the concrete slightly so that water runs away from the box.
    – Tim M.
    Mar 6, 2023 at 21:14
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    6" of concrete? you might as well just plop it on a rock. Unless you live in the desert, it's 3' deep, minimum. Size of the protrusion to be determined by you, depending if you want it over hung (to shed water) or under hung (because... [reasons?]). Or more likely, w/e size sonotube is in stock. - I'd pour a post. So when this turns into a pile of rust in 10y, you unscrew it and buy another (that's slightly different these days now, so your foundation is wrong); none of this 'concrete business' ever again.
    – Mazura
    Mar 8, 2023 at 0:12
  • How's the guy supposed to weed wack around that w/o screwing up the paint and smashing into it with the lawn mower? 3' deep... and 3' off the ground.
    – Mazura
    Mar 8, 2023 at 0:17
  • What does the instruction manual say to do? Mar 8, 2023 at 15:51
  • @JacobKrall That's an issue because it just says to fasten it to stable surface. I also contacted their support which just said they unfortunately don't have an answer for this.
    – Mikki
    Mar 9, 2023 at 7:33

6 Answers 6

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Size will be an opinion mainly, but at least 5 to 8 cm(WxL) bigger than the box and about 8cm deep/thick.

Anchor bolts should be easy to use to anchor the box and will make the box easy to remove for maintenance.

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    Thickness/depth is highly dependent on climate. An 8cm (3") thick pad on the ground would fall over, or at least tilt crazily the first winter in my area due to frost heaving with a big/tall thing like that mounted on it. Might survive if it was much wider and harder to tip, but would still end up off-kilter.
    – Ecnerwal
    Mar 6, 2023 at 13:26
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    Yeah, climate may determine how deep it needs to be and what kind of footing, if any, is necessary. Still, +1
    – FreeMan
    Mar 6, 2023 at 13:27
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    There is the concrete pad which should be on a deeper base of packed stone. I think that is where some are getting the 60cm from. I doubt if they meant to have 60cm deep of cement.
    – crip659
    Mar 6, 2023 at 13:58
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    Nah, I meant 60cm (2') of concrete depth. I really don't want this to fall over if someone pushes it from the top. (That said, my business runs on the mantra, "Do it nice or do it twice", so you can understand how, in the absence of engineering, my inclination is to have a margin of error. We'll do 2' for a mailbox post without blinking.) Mar 6, 2023 at 14:45
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    We can agree to disagree. I think that a push to the top of a mailbox is like a push to that cabinet. Mar 6, 2023 at 18:53
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The depth depends on your regional frost line. For europe we have 80-150cm (varies wildly per source, depends on your location) frost line. That means, to avoid frost-heave, the textbook says the foundation must be deeper than the frost line.

An foundation of say, 40x50x80cm, is 0.16m³. According to the TDS of some random brand of concrete, an 25kg bag of dry concrete yields about 12.5L cured, so you need 14 bags of dry concrete, or about 330kg.

My personal handwaving says that half of that depth for something like an letterbox is still in the within-safe-limits area, especially if you add about 10-20cm of gravel underneath to remove pooling water. You can also add fieldstones you found during the excavation.

To attach a letterbox to an concrete foundation I would use anchor bolts, sealed with caulk. Or, even better, chemical anchors to prevent water ingres.

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  • So 55x42x40 cm (so depth of 40cm) would be ok for the concrete base if there is a layer of gravel 10-20cm, if I understood you correctly?
    – Mikki
    Mar 6, 2023 at 16:27
  • That's what I would be willing to try if I'd need to place a letterbox. To be on the safe side you'd must find out your local frost line and place the foundation accordingly.
    – Martin
    Mar 6, 2023 at 16:51
  • The concrete doesn't necessarily need to extend below the frost line. I suspect the OP could easily get away with a 10cm slab as long as there was gravel down to the frost line.
    – Huesmann
    Mar 7, 2023 at 12:57
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    Given that some parts of Europe can go years without even surface frost, and others have permafrost, that 80-150cm number is much too small a range
    – Chris H
    Mar 7, 2023 at 14:49
  • @ChrisH yes, I just used a higher range of frost line values as OP's bio indicated that he is in Luxembourg. But yes, you have a point here.
    – Martin
    Mar 7, 2023 at 21:22
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To use significantly less concrete while still being deep enough to be stable: Rather than pouring a single massive block, dig four holes (one for each corner/mounting hole location) and pour 4 small, but deep, columns with an anchor bolt in each.

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  • That's an interesting answer but it would be quite difficult to dig holes that wide, The diameter would need to be quite small.
    – Mikki
    Mar 7, 2023 at 15:08
  • Either a posthole digger or a soil auger (which has some overlap with some types of post-hole digger, but also overlaps with high-volume bulb planting in the smaller sizes) Or you dig a large hole, use forms for the concrete columns/pillars, and fill in with dirt after the forms come off. Or use the paper tube type forms and fill in around them before pouring concrete, just leaving them in place.
    – Ecnerwal
    Mar 7, 2023 at 15:54
  • @Mikki Yes, 10 or 15cm concrete paper tubes in the corners would be an excellent option here. Depending on how hard the ground is you might get away by just drilling holes with a soil auger. Then stick down a reinforcing bar in the center of the tube and bend it on the top and let the bar flow into a top, flat layer of 10-20cm thickness.
    – hlovdal
    Mar 7, 2023 at 21:15
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Is the box at risk of being struck by vehicles?

You might choose to have slightly more frangible anchors so they break first, minimising damage to your vehicle. Some hardwood dowells vertically with cross-pins inside the base should break off if your car backs into the mailbox.

On the other hand you might elect to use the strongest masonry anchors available, reinforce the whole base with a half-inch slab of steel, and even build an internal frame of rebar and tie that into your concrete base. The intention here is to make the vehicle into the crumple zone, not your mailbox.

Some parts of the world have regulations about positioning relative to the road, so check them out for your location.

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  • I had a generic residential mailbox of folded galv sheet metal, that got flattened by some yobs with cricket bats. I hammered it flat and installed a frame of 1-inch angle iron and a 1/4" thick lid. Sadly the yerks never tried it again.
    – Pete
    Mar 7, 2023 at 1:45
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    Also, snowplowing can send drifts into a mailbox even if vehicles itself don't strike it (we've lost mailboxes to that before). Also, some regulations may forbid masonry or otherwise immobile/highly damaging to errant vehicles configurations Mar 7, 2023 at 4:45
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    A mailbox presumably has to be built pretty close to the road, so that it can be accessed without getting out of your vehicle. This looks like a community box like you'd find outside of a post office? (As opposed to outside your house.) Most of the ones I've seen have a scoop that sticks out at the top so it's easier to reach from inside the car. Since this one apparently doesn't, it would need to be even closer to the road to be easily reachable. Mar 7, 2023 at 14:54
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    @DarrelHoffman I've never heard of people accessing mailboxes from within their vehicles!
    – user253751
    Mar 7, 2023 at 15:04
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    @user253751 Maybe it's just a US thing? I'm talking about the ones at the post office, not in front of your house. Though even the ones at your house, the mail carrier can often access from inside the vehicle. (In fact, in a country where nearly all vehicles have the driver on the left side, mail trucks have them on the right specifically for that reason.) Mar 7, 2023 at 15:33
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Don't forget to extend the footing (either concrete or gravel or similar) in front so you have room to use the box without standing in mud. Because directly in front of the box will be an unavoidable high traffic area so you won't have ground cover (grass) there for long. If there's a door in back of the box that will also regularly be used then extend the footing there as well.

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    That should not be an issue as the front will is pointed to the sidewalk and the distance will be about 20cm from that sidewalk.
    – Mikki
    Mar 7, 2023 at 18:37
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An alternative suggestion. To get depth (so more stable ground) and to keep the base of the metal off the ground (else it will rust in minutes, especially from condensation in the inside) could you attach it to one or two fence posts. Drill a drain hole in the bottom.

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  • The material is rust proof, so I think that argument is not so valid.
    – Mikki
    Mar 7, 2023 at 14:55
  • Concrete base for [treated 4x4 post to hang] parcel box, +1. Then this is a dupe of how to pour concrete for post, given my climate?
    – Mazura
    Mar 8, 2023 at 0:03

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