We needed a storm shelter....
A few months ago we moved to a new house with no basement (a selection criterion being zero stairs). This conflicted with a high level of concern in the household about tornados and the general problem of large tree trunks flying at or landing on the house at high speed. So I promised that I would build a storm shelter as a high priority.
You can buy pre-fab storm shelters, but the below ground ones are very awkward to install, and the above ground ones do not inspire confidence, despite videos that demonstrate dropping a palette of bricks on them. I wanted to build something that would actually survive a tree trunk at 100mph.
In my court was the fact that I’ve had about sixty foam form concrete blocks in my shed for the past thirty years. I bid on them at an auction because I had recently built a whole house with them on my farm property, so I appreciated how much they cost. I kept them all this time thinking that maybe some day I’ll want to build a really small, really strong room. And guess what? That day actually came after only three decades.
Foam form blocks are a really wonderful building system. Mine are 48 inches wide, 16 inches tall, and 11 inches thick. The hollow space inside, which will be filled with concrete, forms a waffle pattern with posts and beams about 8 inches in diameter and about 2.5 inches thick in the web between. With rebar in every post and every beam, this gives a strength comparable to a solid 8” thick poured wall. The foam forms stay in place, and become insulation (a lot of insulation).
(Less wonderful is that my 30-year-old foam form blocks were in terrible condition, with lots of dings and holes. This required a lot of remedial form-strengthening, but saved a lot of money compared to buying new ones, not to mention the satisfaction of actually using something I’d been saving for three decades.)
Buildings always start in the mud, with step one being digging the foundation trench. This requires someone who is actually good at running an excavator (i.e. not me). Around here we do foundations a minimum of 38 inches deep. Normally the load-bearing capacity of a 16” wide footing is plenty, despite the soil being terrible (for construction purposes—it’s great for growing corn). But because this structure has so much concrete for its size, I had to actually calculate to be sure it would be enough. The structure weighs about 50,000 pounds (about 99% of that is concrete), and the load bearing capacity using the city’s number is about 64,000 pounds, meaning over the tiny 6x6 foot interior I could have over 400lbs per square foot of extra weight before it began to sink into the ground. That’s fine.
Footings are normally about 6 inches thick, but with this one being so small that would be a very wasteful thing to do. Why? Because you can’t order less than two cubic yards of Redimix concrete at a time (otherwise it’s not worth them sending the truck). So my footing is more like 16 inches thick, just to use up the whole two yards. Might as well, and that way I can use less concrete on the next pour, because I’m starting from a higher level. The rebar structure starts here, well underground.
The foundation wall is two courses of foam form, which brings the level to about a foot above grade. I filled in the space inside with dirt and random old bricks we wanted to get rid of, and laid down some insulation board over it to insulate the floor from the ground.
Then we poured the foundation wall and the floor in one go, using about 3 yards of concrete. The floor has a 1-foot grid of rebar, while the rebar from the footing continues another 2 feet above the level of the floor.
When the floor and foundation wall were solidly set, the next step was to install the door frame, which is made of 6x6x3/8 angle iron, with anchor bolts on all sides. (The same anchor bolts were embedded in the floor, allowing the frame to be bolted down after being set in place.)
Then came drama! I cut a hole in the house, committing to actually finishing the project.
I added the next stage of rebar, tying it to the lower level. These rods reach up to within two inches of the top of the ceiling, tying the walls to the ceiling/roof. In retrospect, I should have bent over the tops of the rebar to form hooks, though that would have made it harder to thread the foam blocks down over them. In the end I remedied this mistake with more rebar, as you will see. (And the building inspector reluctantly admitted that hooks are not actually required, just highly encouraged.)
Each course of foam form had to be threaded down over the rebar from the top. I added a hoop of horizontal rebar at every level. (There is a total of about 1200 feet of rebar in this structure, and a lot of wire ties.)
Because of the terrible condition of the foam form blocks, once the wall was finished I completely encased them with 1/4” luan boards screwed to the metal mounting strips embedded in the foam forms. (I got about a four-foot high stack of luan cutoffs on Facebook, hoping to use its for laser cutting things, but it turned out to be in bad shape, water damaged and delaminating—in other words, perfect for one-time use as form boards.)
Here you can see the finished foam form wall, and the 11 inch extension above it that is the form for the ceiling slab. At this point there is still no more concrete above the floor level.
Next I added a sturdy wood platform at the level of the bottom of the ceiling. Now everything is ready to pour concrete.
The best way to get concrete up there would have been to use a pump truck, but their minimum charge is very high. Totally worth it for a larger project, but way too much for the tiny amount of concrete I needed. Instead we used a small excavator and filled the bucket many times to ferry the concrete up. (This is a thing people do.)
The specs for foam form concrete say you should not fill more than three courses (four feet) of height at a time, to avoid having the pressure of concrete at the bottom be too large. I decided that with all the extra reinforcing I had done, I was going to risk doing six feet of the wall on the first pour. This would allow me to pour only two feet of wall plus the ceiling at the same time, on the next pour. And I was proved right, because there were no major escapes of concrete beyond the luan shell.
I don’t have any pictures from above, but if I did you would see us dumping concrete into the middle of the ceiling, then me pushing it off into the wall, and Toby running the vibrator all the way down to the bottom of the wall.
A note about vibrating concrete: it’s very important! The device shown here has a motor in the yellow head, and an eccentric weight at the far end. It is amazing to see the way a stiff, heaped up pile of concrete will instantly liquify and flow when it has that vibrating tip lowered into it.
Without vibration, foam form concrete would not be possible, because huge gaps and unfilled areas would be left. But even normal poured forms need to be vibrated to get out the inevitable voids and bubbles.
I heard a story that they failed to vibrate the concrete in one of the large pylons on a highway interchange just outside of my town, and when the inspector saw the result, the entire column had to be demolished and redone at a cost of a million dollars.
With the walls about 3/4 full, it was time to prepare the fourth and final concrete pour. I put in two 1x1 foot grids of rebar, one just above the bottom and one just below the top of the ceiling. You can see that the top layer consists of staples that come up one wall, go across the top, and then down into the other wall, effectively anchoring the wall and ceiling. This mitigates the lack of hooks on the top of the floor-to-ceiling rebar.
This is enough reinforcement that, together with the 11 inches of 6-bag concrete, I can say with confidence that if the large nearby tree lands on the roof, the roof will win. I think.
The portion of the concrete ceiling supported by the wood platform will weigh about 5000 pounds, enough that I didn’t want to take any chances with the platform giving way with us standing on it.
(Concrete weighs about two tons, 4000 pounds, per cubic yard. This project used a total of 12 to 13 cubic yards, or around 50,000 pounds of concrete in four pours.)
I had a bad infection in my leg (it’s a whole other story) when we could arrange concrete, excavator, and weather all at the same time, so I had to hire a guy, who turned out to really know his concrete. Totally worth it for the quality of work.
We again used an excavator (this time with a bigger bucket, thankfully), and just kept adding concrete until it overflowed down into the walls, and eventually filled the whole remaining forms. The platform did not collapse.
I had ordered extra concrete to fill in a patch in front of the house, and make a pad for our garden shed. Once a truck is coming, a bit of extra concrete isn’t that expensive.
Before the concrete had set I pressed in treated lumber 2x4s with lag screws facing down. These will be used to hold down the roof. Which turned into an issue later, as you will see.
Concrete takes a while to harden. The standard specification is that it reaches full strength at 28 days, though in fact it will continue to get stronger even after that, as long as it doesn’t dry out. It reaches 50% of final strength in about five days.
So while I was waiting for it to be safe to remove the supports under the ceiling, my attention turned to the door.
I had found an auction with a nice quantity of 2x8 inch square (well, rectangle) tubing with 1/4 inch thick walls. (I.e. very heavy, very strong stuff). I combined that with various other bits of square tube and angle iron I had lying around to piece together this door, which Toby expertly welded together.
I made some escapable slide bolts to keep the door closed during a storm. The idea is that if the door gets bashed in and bent to the point where you can’t slide the bolts open, you can just take off the wing nuts and the whole thing will come apart.
After much consideration, I decided there was only one way to get the door mounted. Keep in mind this thing weighs 200-300 pounds….
After carefully measuring and drilling the holes, we convinced Maribel to go into the room with a handful of wing nuts. Then Toby and I stood up the door and braced it in place, then shimmed it into position.
Then we fed machine screws in through the holes and Maribel added wing nuts from the inside. Fortunately she is not claustrophobic! Once the nuts were all on, we could open the door, and thanks to careful measurement, it actually opens perfectly into the opening of the outer door.
Finally it was time to remove the supports and all the luan boards to reveal just how bad the foam form blocks were. But really, the amount of concrete leakage behind the boards was entirely manageable, and easily knocked off with a hammer and chisel. The actual desired concrete is at least two inches back from the surface, behind the foam wall.
There are four small, high windows. Enough for ventilation, but not enough to let any tree trunks in. The rebar going through the window openings gives a nice prison-cell look, eh?
You’ll see in later pictures that the windows are small, openable windows meant to be used in glass block walls. I used the same ones in the round house at my farm, where the first floor is also foam form concrete, also as a tornado shelter (but much bigger).
I like using prefinished flooring for walls, because it’s much more robust than typical wall materials, but mainly because I have a lot of it from various auctions that sell remainder quantities. (I.e. not enough to do a whole house, but enough to do one room, especially a small room.)
Wiring is run behind the flooring, which is separated from the foam by some furring strips (actually prefinished MDF baseboard, which I also have a lot of).
On the outside, you can see I found some shingles that almost perfectly match the existing roof. Unfortunately you can also see how shallow the slope is, about 4 inches over 8 feet.
This is due to a small miscalculation. I should have moved the structure a foot or two to the right, so the roof could come out of the wall without a complicated joint, and still have enough slope. (I did think about and measure this issue, but somehow didn’t do the measurement right, plus I added an inch of thickness to the ceiling concrete at the last minute).
As the building inspector was quick to point out, this is less than the minimum slope allowed for using asphalt shingles. He was going to make me redo it with rubber membrane roofing, but I called him back with three theories on why I shouldn’t have to do that, and number 2 was a winner: that this isn’t the roof. The actual roof is the solid 11 inch thick poured concrete slab, and everything above that is purely decorative. So I got to keep my inadequate roof, which will probably need to be replaced some time in the future with a rubber membrane.
Back inside, the ceiling is suspended (to allow wiring above it). Earlier in the year I had gotten a small trailer load of 2x2 ceiling tiles for a few dollars at an auction, intending to use them in our round house at my farm property. But when I actually opened a box to use a few here, I discovered that this plan would never have worked. They cannot be cut, only used whole, and the round house design requires every tile to be cut.
Why can’t you cut them? Because they are incredibly fancy, incredibly expensive passive thermal regulating tiles. Glued to the back of every tile is a flat, sealed chamber containing what is described as a salt solution which melts at 72 degrees.
As the room gets warmer, the salt melts, absorbing heat and keeping the temperature at 72 (just like ice keeps the temperature of a drink at exactly the freezing point until it’s all melted). Then when the room cools, the salt refreezes, releasing that same heat and keeping the room warmer.
Anyway, I had no idea I was buying such fancy tiles, and now I don’t know what to do with the rest of them.
Maribel keeps plants indoors over the winter, so I installed a pair of grow lights I got from the closing out auction of a hydroponic lettuce growing operation in Indiana. (I feel like they went out of business because they were trying to grow regular lettuce instead of the devil’s lettuce, which would have been far more profitable.)
Anyway, the lights are super bright. They emit only blue and red light. Why? Because plant are green, which means they reflect green light, which means it’s pointless to waste electricity shining green light on them.
There followed much annoying detail work putting in the floor, trim, framing in the passage between the two doors, etc.
The last step was painting both doors, then installing trim everywhere and filling a million nail holes in the walls and trim.
And here is the finished product!