Oh Gawd, you cracked it!

18 Apr

It must be that time of year.

The snow is melting, the site is unfrozen and it’s time to start setting boxes.

I’m getting the same question in my email over and over again right now. I’m also seeing the same questions asked in other places on the web, from “DIY to Specialty” blogs.

So, let’s address a question about HOW you attach the ISBU to the ground;

Dear Ronin,

Are you a Ninja? I’m just wondering because… oh, never mind.

(Editor’s note: No, I’m not a ninja, wise guy. I’m “The Man Of Steel.” Corten Steel. )

You’re constantly talking about putting ISBUs on pilings. You’ve even explained in great detail how to make them out of Sonotube casings.

But what if you want to just put the ISBUs on a poured concrete slab?

You know… not all of us want our ISBUs up in the air.

I want to just pour a slab and then possibly even embed some twistlock bases into them and then drop the boxes onto it. I  do know how thick the slab needs to be, as I hired an engineer to calculate it for me, so it’s all good.

If I set the boxes at a distance, say about 20 feet apart, I can get a pretty good size house for cheap.  I know that it’s true, you say it over and over again.

Man, you do tend to repeat yourself. 😉

In fact, after  I visited an ISBU Pool House you built in Fort Myers,  we  decided that we’re going to build a house like the “Shindig House” you showed on your blog. It’s exactly what I have in mind.

(Editors note: He’s talking about an ISBU Pool Cabana /Guest House we built  a few years back in somebody’s back yard, in South Florida. Essentially, it’s a pair of 20′ ISBUs with a common center section, covered  by SIPs and SSMR. I think it even made a few magazines.)

Here’s what I don’t get;

The guy who wants to pour my slab keeps telling me that we have to grade the area and then cover it with plastic before we add sand to pour the concrete on top of.

Won’t the plastic keep the slab from drying?

Then after he gets done, we can’t use the slab for at least a week. In fact, he says he needs to send somebody by to water it.

I’m trying to work as fast as I can. Is he right about this? Or is he just padding his bill?


Dear “Slabber”,

Your concrete guy is absolutely right.

Grading the site keeps stuff from puncturing the plastic he’s going to line your slab form with. You want to start with a smooth, clean surface  and you want your slab thickness to be consistent.

(You ARE dropping some rebar into it, right?)

You WANT to keep that slab “wet” for as long as you can (within reason). The plastic (usually 10mm sheets of visqueen or something similar) will help that happen.

This may confuse you even more, but then WET sand is usually added on top of the plastic, trapping in even MORE moisture.

THEN… you shoot your slab.

The idea is to keep the concrete wet to help it “cure” slowly and evenly. The end result is that this method produces a MUCH stronger slab.

When we shoot slabs for ISBU homes (or any other kind of homes or structures, for that matter) , I have people “water” them at least twice a day, (and sometimes even more depending on geographic location), just to keep things going the right direction. THAT is why he wants to revisit your slab. He doesn’t want your ISBUs to break it.

TIP: Depending on the end purpose of that slab, sometimes I’ll even shoot some sealant over it, to further slow down the drying process.

And you MUST wait at LEAST a week before you start playing on that slab. Two weeks is even better. If I can wait a month, I will.  The last thing you want to do is damage it by banging on it or loading it up, too soon.

A 40′ ISBU weighs about 8,000 pounds. A brittle (uncured) slab will cause you nothing but grief, once you set that big hunk of Corten Steel down onto it. Trust me on this.

So stop your crying and write that check.

BTW: You owe your concrete guy a frosty cold one. He’s looking out for you.


7 Responses to “Oh Gawd, you cracked it!”

  1. Jeremiah April 18, 2011 at 9:01 am #

    To add my own buck fifty to the conversation:
    concrete cures by a process called “hydration”, basically it hardens when wet. This is why concrete is so useful in building bridges and what not – it hardens under water.
    The plastic, or visqueen, under your slab also acts as a vapor barrier to prevent water wicking through the slab and into your home. Small punctures in that plastic layer can cause all sorts of problems down the road like mold, cracking, settling (from water getting in and pooling, then freezing, expanding…you get the drift). Concrete is still porous you know. A good idea is to also have at least 2 inches of rigid foam insulation under your slab between the slab and the visqueen. This prevents the transfer of heat and cold in the earth from seeping into your home (fyi metal conducts heat and cold VERY well – insulate, insulate insulate!).
    Lastly, I agree with Alex – buy your concrete guy a cold one cause he’s doing his job proper.

  2. Jeremiah April 18, 2011 at 9:08 am #

    Oh and I forgot to add, typical curing times for lightweight concrete are:

    walk-able: 2-3 days
    load-able: 7 days minimum
    full cure: 28 days

    It’s always preferable to wait the minimum 7 days before loading the slab. It’s IDEAL to wait 28 days for full cure. Depending on the size of the slab you may also want to take core samples for compressive strength testing. Not sure that would be necessary though.

  3. Faola April 18, 2011 at 2:22 pm #

    Hi, we too are planning on having the containers on a slab (after raising the land a metre to avoid those nasty 50 year floods that seem to come every 5 years) Do I actually need to engage an engineer to detirmine the concrete loading specifications, or is there some handy dandy formulae I can apply instead? thanks.

    • Jeremiah April 19, 2011 at 6:33 am #

      Faola, if you’re planning on raising the grade that much, you may want to consider using pilings. It would be much cheaper with less engineering and less site work (which also means less time). You may also have issues with water run-off raising the grade that much. Be sure to check with your local planning officials. Raising the containers on pilings would also allow you to get creative with exterior decking to create some unique outdoor spaces for your home.
      If you still decide to go with slab on a raised grade, pay the fee for an engineer to properly size your slab and rebar. The few hundred dollars you spend now will save you thousands later if your “formula” turns out to be wrong. Most structural engineers would stamp this kind of thing for maybe $600. Find a sole practitioner and haggle a little.

      • Renaissance Ronin April 19, 2011 at 1:45 pm #

        Good stuff!

        Faola, I’m with Jeremiah on this one. That’s enough distance to merit an intense “to pile or not to pile” conversation.

        Is there some other reason why you want to build up the site using fill? Is there some local ordinance that mandates it? Is it cosmetics? Is it WINDS?

        One of the things that people aren’t addressing about ISBUs is what I call the “uplift factor”.

        Uplift is when high winds hit the side of your ISBUs. Remember that an ISBU is basically just a huge billboard on it’s long axis. We’re talking “Sail City.” And to make matters worse, the uplift is magnified exponentially when wind gets under a container, between the pilings and grade (the ground). ISBU Housing is traditionally “low weight” in that it’s weight per finished square foot is less than other types of traditional housing. This means that with enough wind, you can tip, roll, and generally turn that container into a blender.

        Any good engineer will tell you that the mass of a foundation should both hold up your home and HOLD IT DOWN. If piles are used, then either serious attachment to piles via cast-in J-bolts, or welds to cast-in-place plates at the pile tops are in order.

        In my case, I recommend both systems working in conjunction with each other. In fact, Steel beams are often attached (using J-bolts), which act as “Sills” for that ISBU. Then you weld ALL THE WAY AROUND. Done properly, that ISBU isn’t going anywhere.

        With pilings… placing decks or even skirting around the base will decrease the amount of uplift on pile-supported ISBUs and it’s not “cost intensive.” In fact, you can use pretty “run of the mill” materials, like; sheeting (PT plywood/framing, earth berm, concrete block wall, or driven sheet piling.

        So… C’mon… more details, Faola! 🙂

        And it should also be noted that any engineer worth his salt will ask for things like “perk/perc tests” (slang terms depending on where you are geographically) and thus, they’ll KNOW what you’re slabbing on TOP of. This is crucial to insuring that your footings are properly formed and placed, and that the “palette you’re painting on” will bear the load and stay put.

        And for you “expert” naysayers: Look, I’m not trying to get you into every “construction affiliated office” there is as you build. You know I don’t roll that way. What I’m asking you to consider is that for a few bucks in the front end, you can build a TON of insurance into your building success, by asking the right people the right questions before you break ground… in fact, before you even draw any lines. It doesn’t have to cost an arm or a leg to get good answers. Shop around.

        GOOD Planning guarantees your success, as long as your idiot brother-in-law doesn’t show up drunk with the plasma cutter! 😉

        It should also be noted that a really well qualified engineer I know in Texas (he’s been doing it longer than most of us have been alive and he’s a friend of the blog), Allan Clark, wrote me a really cool explanation of all this and I’m going to build a post around it. So, I tip my hat to him!


        • Faola April 19, 2011 at 4:12 pm #

          well, yes, cosmetic is part of it, I guess I like the idea of being connected to the ground – I just never envisaged my home ‘floating in the air’, probably some relic of my childhood deeply embedded in my psyche. Also the current plan is for the containers to be in a rectangle surrounding an inner atrium, a bit easier with a raised earth platform than pilings. In addition, although I have not researched this, I was thinking of having the water tank underground in the centre under the atrium area – hence less fill, and no tank taking up space ‘outside’ as well as ease (I think) of draining the rainwater from the roof.
          Previously we were going to build an octagonal strawbale home on site so had the perc tests and all done. As I do not have the results here at work all I remember is that it was good drainage (30 minutes to drain the required amount?) A narrow river – the one that floods periodically – is about 20 metres from the edge of the platform area. It is down about 15 metres in a channel but the flash floods come roaring down from the hills…
          Ronin, in your comment you mention putting the ISBU in pilings and also using earth berms, if I understand that would essentially create the same ‘feel’ as having the house on a raised platform. Something to mull over.
          Wind is not really a problem, reasonable shelter from trees, but I will get the engineer to look at this too thanks.
          idiot brother in law hmm – he’s not bad – an architectural engineer in the family is a bonus, even if he does live on the far side of the globe (AND NEVER FINISHES YOUR HOUSE PLANS!)so is totally unreliable for actually helping.
          Currently doing a wee test project in the back yard (not on ‘the land’ we intend to build on) as we turn a container into a bedroom for the eldest offspring, sussing out necessary insulation etc. If she freezes this winter we are heading into we will know to put more in the house when we build!
          Thanks for the advice and elucidation, it is appreciated – just a bit jaded with engineers after the process of trying to get our straw bale dream through the pre-building process. 🙂
          (Ronin, forgive my long answer, but you did ask for more details)

          • Madrigorne April 20, 2011 at 9:04 am #

            plastic also acts as a vapor barrier against the evils of Radon!

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