The foolish man built his shed upon the sand

March 6, 2010 11:45 PM

Planning a shed on a beach. How should I build the foundation?

We're making spring plans for the cottage, the biggest of which involves building a new shed / pumphouse on the beach. I need some help choosing a foundation that will provide a good long service life, but won't break the bank (or my back since everything has to be hauled in by boat.) The location is about 50' back from the water, but the ground there is still mostly loose sand, the water table is only 6' deep, and it's subject to yearly freezes. I think my options are:

1. A floating slab poured on a plastic sheet, 4-6" deep. Probably reinforced with steel or mesh or both
2. Raised wood floor sitting on pre-cast concrete deck blocks. I don't know whether these would work on sand, nor how many I would need
3. Concrete sonotube posts. I'm not sure I can make these too deep though without running into the water table though.
4. Some kind of steel pile system (I remember reading about pile kits you could buy, but I can't find any just now)

Lend me your thoughts and experience. Civil engineers - I promise I won't hold you liable for your advice :)

posted by Popular Ethics to Home & Garden (13 answers total)

Best answer: How large is your shed and how critical is it that it stay level? Do you only occupy it in the summer?

1 and 2 if properly constructed (easier with 1) will stay flat but they wont stay level because the ground they are resting on is going to heave.

3 and 4 are really the same just using different materials. Piles are a good choice, to bedrock is best but a minimum of below the frost line would be acceptable. Hydraulic cements will properly cure even when submerged in water. Have you dug a test hole? Can you dig a somewhat straight sided hole or is the soil so loose that it collapses on itself?

If the shed is of modest size (say 10X10 or smaller) here's what I would do:

1. Start with four concrete piers sunk below the frost line. Hopefully that is above the water table. I'd construct the piers out of concrete, a 10" sonnet tube sounds about right. The bottom of the pier will need to be large enough to support the weight of the building and that is going to depend on the load bearing rating of your soil. You can buy special forms for this purpose but it is pretty easy to form them by hand at the cost of some wasted/over poured concrete. Make sure the footing extends at least 8" out of the ground.
2. I'd set the shed on two beams , one of each runs across two piers. When setting your anchor bolts you want to avoid where the beams sit on the footing. Ideally the anchor bolts will come up next to the beam and inside the wall of the shed so you can clamp the bottom plate of the wall down to the footings.
3. Once the concrete has had a chance to cure a bit, say 12 hours, start building the shed. Beams on footings one way and then the floor joists onto of the beams and running the other direction. Hopefully your piers are level to each other at this point but if not shim as necessary.
4. Continue to build your shed. The only tricky part is to leave the nuts of your anchor bolts exposed or hide them behind an access panel.

The reason you want to leave your anchor nuts exposed is when/if your shed gets out of level you can loosen off the nuts, use a bottle jack to lift up the low corner(s) and reshim the footings, then tighten the nuts back down. The same can be done with precast deck blocks though a shed on blocks is going to move more and more often than one set on piers. Also bolting your shed to a pier foundation will help resist the wind from blowing the shed over or floods floating the shed away.
posted by Mitheral at 1:01 AM on March 7, 2010 [1 favorite]

Assuming your shed is to be built in Ontario, CN, you could ask this question at the Ontario Building Code site, where you might actually get a definitive answer. That said, I think that the major problem your foundation is going to have to deal with, given the sandy soil, annual freeze/thaw cycles, and the nearness of not only the water table, but of actual surface water, is heaving, but this is greatly dependent on soil study results. Some sandy soils, with excellent drainage, never hold enough water to experience substantial frost heaving, while in other less well drained locations, heaving is brutal enough to rule out any type of slab foundation for a permanent structure. Really, only on-site soil testing can answer you question definitively, and given the proximity to surface water, I suspect you'll be required to pull a building permit for this project, which may itself, be contingent on having such testing done, in the pre-construction phase.

If you're required to pull permits and do soil tests, better explore that ahead of doing any work on the site, that could disturb it prior to testing, as this can result in invalid results, and denial of approvals. If you're not required to pull a permit, then, on preview, what Mitheral proposes is a reasonable approach, although even independent piers will not stay put in ground that is frost heaving as much as sites near northern lakes often do, particularly when the freeze line comes very near the water table.
posted by paulsc at 1:28 AM on March 7, 2010 [1 favorite]

I don't know about cost, but would screw/blade piles at an angle be appropriate?
posted by carnival of animals at 3:07 AM on March 7, 2010

One thing you should do first is look around in the area to see what other solutions have been tried, and have stood the test of time.

I am a fan of jet driven pilings. The key feature is that the materials are fairly cheap and light, and the installation won't kill you. Using a gasoline driven pump, and lake water, you can probe the ground and set piles deep enough to offer strength and avoid frost heave. You can tow wooden piles behind your boat, perhaps. Are storm driven waves ever a problem in this area? You can raise the structure a bit off the ground to let the water flow underneath.

I admit that I don't know about the implications of a freezing water table and driven pilings.
posted by Midnight Skulker at 7:02 AM on March 7, 2010 [1 favorite]

It depends on how far north you are. Three foot depth is fine in Wisconsin for a footing below the frost line.

It also depends on the size of the structure and the weight. If you are talking about something the size of an ice shanty, then anything major like you suggested is overkill. Four cedar posts driven to the frost line could suffice and they would be much easier to haul in an install than concrete. Then simply build your structure 4-6 inches above ground level have it rest on the posts. Even bigger sheds can be built with a driven post footing.
posted by JJ86 at 7:59 AM on March 7, 2010

Why wouldn't you use a skid foundation? Simple to build, sturdy enough for a shed, immune to frost heave, and probably puts you in a totally different category for building codes: e.g. temporary structure vs. permanent -- although a well made skid building can last a very long time. If it starts to subside after a few years, you can jack it up to the surface again. And if the waterline moves, you can just tow it somewhere else.
posted by seanmpuckett at 8:47 AM on March 7, 2010

Response by poster: Sorry to reply to all the questions so late. I posted just before going to bed.

Re size: I expect the shed will be somewhere between 10' x 10' and 15' x 15'. It's not critical that it stay level within reason. I'd like the doors to not require superhuman effort to close. It's only occupied during the summer. It will be used to protect boat motors and children's toys and such over winter.

Re: the building code and soil testing: I don't have a copy of the code handy (I should, but I'm avoiding forking over the $150), and I certainly don't have the budget to hire a geotechnical engineer to write a soil testing report for a shed.

Most of our neighbours just lay the wood planking right on the sand or avoid building on the beach entirely. We have lots of exposed bedrock in Georgian Bay. . The wood on sand route tends to only last 15-20 years though before the foundations start to rot (even with pressure treated wood). I'm hoping for something a little more long lasting so I don't have to do this work again when I'm less able to do it.

Does that help narrow down the choices?
posted by Popular Ethics at 10:10 AM on March 7, 2010

The following answers are dependant on the following assumption:it is a part time structure to be used as either a temporary camp/shed or some other outbuilding and if it needs rebuilding in 20 years no big deal. If you want something better you will need to get an engineer famalair with local conditions and do what they say.

I would (and have) use the concrete deck blocks. But don't just set them on the sand/dirt. A quick aside if you want to do a quick and dirty soil test take a handful of wet soil and squeeze. If it crumbles when you squeeze or after you stop squeezing it is probably sandy enough that you won't have frost problems and are good to go. IF it oozes or is cohesive you need to take measures to eliminate heaving. To provide a firm, solid foundation dig out about 2-3 feet down and about twice as wide as the deck blocks. Install a geotech filter fabric in the hole and pour in gravel(ideal)or sand (more portable). this will limit the heaving problems without having to haul in concrete. Also leave a decent airspace under the shed (like 4-6") to prevent dampness in the space and use treated wood for the joists that will be exposed to the ground. This shed will not be good for staying warm in the winter but will be decent and last for 10-20 years without unmanageable problems.

BTW this is probably not to code for any type of inhabitable structure and is pretty much boiler plate tool shed construction for wet environments, and is pretty much the same final result as a skid foundation but does not expose any wood to direct ground contact.

and another BTW-if you are using this a pumphouse for well you will want to insulate the crap out of everything or drain it before freezing is possible. Also if you use a skid or block foundation make sure you have a flexible coupling for any utilities or pipes that extend from the shed into the ground.
posted by bartonlong at 10:33 AM on March 7, 2010 [1 favorite]

Response by poster: bartonlong: Thanks for that. The whole cottage is a summer-only thing, and the pump is drained over the winter, so no need to insulate. I'll try that quick and dirty soil test, thanks.

Has anyone had any experience with small steel piling systems like the unfortunately named "Mega Anchor"? Such a thing might be easier than augering a hole and pouring a sonotube. carnival of animals & midnight skulker - do you have any links?
posted by Popular Ethics at 11:05 AM on March 7, 2010

I have spent my entire life living on the coast and if you're building on the sand 50' from the shore you're going to be lucky to get 15-20 years out of the structure. If there's any way you can relocate it to higher ground I'd do it.

I also wouldn't spend a bunch of money on something that can be taken away by a winter storm any time. Steel pilings= a bunch of money.
posted by fshgrl at 11:23 AM on March 7, 2010 [1 favorite]

fshgrl makes a good point. The sandy beach of a North American Great Lake has got to be one of the most challenging environments for building on the continent, given the weather that is endemic to that area. Making anything there even as "permanent" as a 20 year life building, of from 100 to 225 sq. ft. is going to cost real money, to say nothing of putting up something that will still be going strong 25 years or more from now. Here is a little 4 page .pdf file with pictures, of a number of recent, fully engineered concrete deep pile foundation structures built on sea beaches in Connecticut, that haven't fared too well, just from 2004 to the present. And while you don't have the corrosive effects of salt water, or the tide/current combinations to deal with that these structures did, you have far more treacherous ice conditions working against you in the long term than these structures generally faced.

So, good luck with doing it inexpensively. Were I in your position, I might, instead, go very small, and light, perhaps buying a couple of 4' x 6' commercial shelters that could be hauled down to the beach for summer storage, and hauled back up to higher ground, in winter, to provide outdoor dry storage away from the harsh winter beach environment. Small commercial sheds of this type weigh only a couple hundred pounds, and can be readily moved about, on a light trailer, much like ice fishing shelters, with retractable wheels, and can even be moved about fairly easily, on sand, by hand, for areas where getting a car or truck down to the beach is too difficult.
posted by paulsc at 4:33 PM on March 7, 2010 [1 favorite]

Response by poster: fshgrl, paulsc: We're lucky in that our beach is quite deep. The back of it is quite sheltered by rock bluffs and trees. I'm not too worried about storm damage or erosion (small trees seem to survive OK in this location.) That's a good issue to be wary of though.

I think I'll try Mitheral's procedure, but substituting the Mega-Anchor footing for concrete piers. I contacted the Australian Mega-Anchor company, and they offer worldwide shipping for a pretty inexpensive price. I can source the galvanized pipe locally. The fallback position would be to lay crushed gravel for drainage and build a floating raft.

I'll report back in the spring to tell you what worked. Until then, keep sending your suggestions. I'm very grateful!
posted by Popular Ethics at 7:00 PM on March 7, 2010

I'd love to hear your experience with Mega Anchors. I'd be concerned that the units are only rated (sort of, there isn't anything like a comprehensive data sheet on their site) for one tonne a piece. The piles I outlined or even your deck blocks are rated for that per square foot of bearing surface. A typical 20" diameter 5" thick footing that one gets with a 10" sonotube has ~2.2 square feet of bearing surface.

The good news is if you construct your shed properly and the mega anchors don't work out without failing catastrophically you can just jack your shed up, roll/slide it out of the way and then put more conventional footings under the shed.
posted by Mitheral at 12:39 AM on March 8, 2010