So I built an Underground WOOD shelter.

Mods: Feel free to move this if there is a better spot.


Disclaimer: For academic purposes only. If you decide to build anything, it will be at your own risk. Follow all local laws an building codes.

Why I did it:

My wife and I had the privilege of buying our first home about 2 ½ years ago. It is an original log cabin, built about 1885. We love this house. It is rock solid, beautiful, and just what we wanted. It sits on just over 4 heavily wooded acres in south central Minnesota. Our property is also at the top of a hill (this will come in handy).

Our house has one slight downside: No basement.
There is the old root cellar under the house, accessible from the inside. It is about 3ft by 5 foot, and about 4 ½ feet deep at its deepest. While this hole has kept the previous occupants safe for over 100 years, I felt I could do a little better for my family.

The mission:

We need an underground shelter that will protect us from up to and including an EF-5 tornado, any straight line winds, and because of our heavily wooded lot, debris. We are looking at about an 8x10 box. Just large enough to be comfortable. Planning an underground shelter is not something you take lightly. You are dealing with a LOT of load, from all directions (except down), high lateral forces, and of course WATER. Putting my family in a box not designed properly would be more dangerous than our little hole under the house. I consider myself a very intelligent person, with a great ability to visualize just about any mechanical device in my head. This will take some planning...

The options:

Knowing that I will be doing all the work myself, but having access to heavy machinery from our family farm, I knew I had to think on the cheap. First thing that came to mind was re-enforced concrete, either poured or filled block with re-bar. Off to Menards. Working with one of their designers, an 8x10x6 foot tall cement box would be about $4500 for materials alone (or more!). Although I cannot put a price on my family's safety, I simply do not have that kind of money.

In comes the internet. I have pretty good Google Foo, and scoured around for about 1 month or so looking for an actual design that was DIY friendly, (although I can build just about anything), actually based on calculations, material specs, and numbers, and CHEAP to do.

From cement culverts, to old fuel barrels, to shipping containers, and home made metal boxes, nothing was in my price range of about $600, or seemed 'right', structurally sound, or do-able for my situation. I decided that it was going to be wood or nothing.

Setting the Stage:

Now that I knew what building material I could actually afford, I had to design a properly made shelter that would take the extreme forces and figure out how to completely water-proof it. Now going back to the error-net, this time with a specific idea in mind. I set out to find, individually if need be, how every aspect needed to be designed to handle the stresses that this shelter would face.

The Research:

First: Walls. There is a building practice (that is apparently popular in Canada) of instead of cement block foundation, building PWFs, or “Permanent Wood Foundations”. PERFECT!! This part is easy. Here is how exactly to build a dimensional lumber foundation to withstand both the lateral forces of the earth, handle the load from 'up' (foundations handle a house, mine will hold some dirt. Should be similar), and how to water-proof it all.

Next: The roof. Here it got a little tricky. 8X10 square box, so 8ft span of beams.
Two questions must be answered before design can continue:
How much does dirt weigh, worst case scenario, plus snow load, and what can a beam hold at this span??

This took a little more digging.

Well, after calling around to structural engineers (most of whom would not talk to me about this without me actually hiring them), I finally got some numbers.
Now first: This WILL VARY from region to region. In my area, we have 1 foot of black, followed by clay until the center of the earth.

The Numbers:

OK, here we go. Building is 8x10 (outside dimensions). Total square footage is 80 Sq Ft.
The 'dirt weight' the engineer gave me at 2 ft thick soil on top of the roof:
Worst case compact scenario, barring any large rocks is 240 lbs sq ft.
Worst case scenario snow load for my region: 60 lbs sq ft.
Total dead load: 300 pounds per square foot.
Total load on 8x10 roof: 24,000 pounds.
That is a lot of weight. Now aren't you glad we are doing this right???? That is a lot of weight to come down on your head!!!

Great! Now, how much can what size lumber carry at that span??? That took more digging yet. Finally I found some span ratings charts that use the “allowable unit stress....” (this is something you can look for the species and grade of the wood you plan to use. Every mill or good lumber yard should have this information), along with other criteria,  in your span length to calculate spreadsheet style how much weight that one beam can carry.

I calculated that Southern Yellow Pine #1 (or SYP1) at a span of 8' and having 11 beams (for 1 ft on center studs and roof beams) will yield a safe load of 37,697 pounds, evenly distributed. That leaves a margin of about 13,700 pounds of safety. That is about enough to park 2 good size skid-steers on the roof after the dirt and snow. (DO NOT DO THAT!!!!!! If I really needed to tell you that, DO NOT even attempt to build such a structure). I figurd 2x10s would have worked, but for the extra $25 bucks, I opted for 2x12s.

For your reference: Dimensional Lumber Span Rating Chart can be found at this url: http://www.awc.org/pdf/wsdd/c2b.pdf   You may want to save a copy on your computer for reference, in case the internet breaks, or they take the information off.

How I built the structure:

Well, it was easy. I will not go into how to put it together board by board, but here are a couple hints:

1 ft on center for EVERYTHING.
Put down your plastic before putting up your walls.
Use SCREWS instead of nails
Remember: fasteners should not be used to do anything but hold the board where you put it. Do Not depend on the shear strength of the fasteners to hold your wood. Use another beam to back it up.
Remember the lateral forces, and re-enforce your walls so they cannot cave in when you back-fill.
I used 5/8” OSB for the sheathing. No figuring here, from my experience, at 1 ft on center, I decided it should hold fine. (and it is so far).
You will probably need a hallway. This is to allow more dirt to be piled around ALL of the actual shelter area. Same 1ft on center, except at only 3 ft wide and 6 feet long, I decided 2x6 roof beams were sufficient.
I used regular construction lumber (not treated) for the 2x6 studs. They should not get wet, and it saved money.
Roof beams were treated SYP1 2x12s. They are VERY CRITICAL, and only a couple bucks more each to get treated, plus Menards had them on hand, non-treated was 2 week special order.
Run your roof beam spans at 8 feet to keep the weight rating as high as you can.
Take your time and think about how weight will push in from different angles. When in doubt, more wood. REMEMBER THE LATERAL FORCES AT WORK HERE!!!!
I made the floor on the inside 'removable' to allow me to monitor moisture entry.
I used treated 'footers' for the walls. They will sit on the plastic, and they should be treated.
My footers are 2 boards thick: 2x10 treated on the bottom, then the regular 2x6 construction for the walls. Basically the walls sit on the flat treated 2x10s on the plastic (ground). The treated footers are not for structural support, just to give a bigger foot-print on the virgin clay (you must build on virgin dirt, or install cement footings), and keep the non-treated construction lumber off the ground.
And I will say it again: SLOW DOWN and think how the pressure from all sides will push on the structure.
One more thing about when you back-fill: Do it as evenly as you can to avoid un-equal stress on the structure.

Water-Proofing:

This part was easy. Just remember: Gravity pulls water down.

First, put down plastic (I used 3 mil, but 6 mil would be better), and put your walls up.
After you get your OSB on, caulk any seams or gaps. Then use foundation coating.
I used a bucket of foundation coating to cover the all the OSB. It is in 4.75? gallon buckets, in the roofing section at Menards. It is asphalt based I believe, and has the consistency of pait (I think you can actually spray it on) It cost about $22 on sale. I bought 3 buckets, not knowing how much to use. I used half of one bucket! While you are there, buy a cheap $5 broom to spread the coating on, so you don't PO the wife (lesson learned, and I owed her a broom!) Using the broom method, it took about 45 minutes to apply all the coating we needed. We literally poured the coating on the roof and used the broom like a squeegee. Get it on as thick as you can.

Next, fold up the plastic that you put under the structure (with a couple feet extra, I hope! I used 3 mil, 12 foot wide by 100 feet long plastic) up onto the still tacky foundation coating. Then, start at one end of your hallway, and go all around the structure in one piece of plastic, putting the plastic right up to the top of the walls. The bottom few feet of plastic: fan it away from the structure. Put a run of tile (I used 4 inch-make sure you use perforated) all the way around the structure, on the top of the plastic to a daylight drain (or sump pump, or whatever you choose to get rid of the ground/rain water).

Then, I decided to do some back-filling before I put the top plastic on. This was to make walking the plastic onto the roof easier. I back-filled to about 3 feet to the top of the roof. Then I put the roof plastic on. Again, the tacky-ness of the coating held the plastic on.
HINT: Be careful not to rip your plastic back-filling. The dirt will pull down on the plastic, so having someone in the hole to help hold it up is smart (and a little dangerous).

After I put the roof plastic on, I continued to pile dirt on until I had 2 feet on top, and made a nice little mound.

Finishing Details:

Make your doors, install electricity and phone if you want (again, follow all local codes and laws), and there ya go!

Make sure that you don't leave the plastic in the sun. Use roof flashing, or other suitable material to cover exposed plastic on the hallway.

My doors are simply 2xWhatever, 2 layers thick, running in perpendicular directions screwed together with big a$$ hinges and bolt latch on the inside. I have 2 doors, one on the inside to the actual shelter, one to the outside.

WARNING: Make sure you build a ventilation system if you decide to put up doors!! From what I have been able to find, the gov't says that 10 ft3/min is minimum per person. That can go way up in hot weather. I am building my ventilation system to provide up to 100 ft3/min. I have a family of myself, my wife and 1 child. So that will be sufficient for us.


Make sure you have in your shelter:
Food and water for 3 days minimum. (food that doesn't need water or to be cooked)
Sanitation supplies for 3 days minimum. (Baby wipes work in place of a shower)
Basic hygiene supplies for 3 days minimum
Fresh air system (or no doors)
Communication with outside world (in case debris traps you in). CB radios, or FRS/GMRS work well, and are cheap. Consider a whistle or marine canned air horn.
Flashlights, LEDs are best. Rayovac has LED flashlights at Wal-Mart for about $6 that run on 2 D Cell batteries for 60 hours.
Extra batteries for whatever electronics you have
Weather radio
Any medications you you or anyone in our family may need.
A medical kit is always a good idea
Spare clothes, and shoes/work boots and leather work gloves for every member of your family.
Avoid having any open flames (oil lamps, candles, gel flame heaters, etc), or devices that consume oxygen, or emit gasses. Instead, use modern LED devices for light, and deal with cold canned food. Its survival, not the Hilton. If you get cold easy: pack blankets in your shelter.
There are TONS of resources for emergency items lists: consult some.  


Other Things to have (or at least I have) in your shelter:

I will also hook up a 12 volt battery back-up system. It will maintain charge by a solar charger and/or 110VAC battery maintainer. It will power the ventilation fans, emergency lighting, and a CB (trucker's) radio with weather channel.
I will also have a 12VDC to 120VAC converter
Cell phone signal booster (I got free)
A couple cots, a few chairs
Games to pass the time.
Smoke detector
Fire extinguisher
Battery or wind up clock
Extra cell phone charger (one that runs off USB, with car adapter)
I have my own internet business, so things I need to run it from my laptop.
From what I am told, ANY old cell phone, whether it has a plan on it or not, will dial 911. I haven't personally tried to dial 911 from an old cell phone, but I will put it in the shelter with a charger left on. I have entered “911” on the key-pad and it will read “emergency”. I think it should work.

Oh, and what I actually spent on the thing: Buying basically everything new, but most things on sale, I spent about $700 on the structure.

I used the farm's skid-steer to dig the hole, and back-fill.

That is how I did it, and I feel safe.

If anyone can tell me how to post pics, I can.

Edit for pics

Edit:
Upload Pics to Photobucket
"Copy & Paste" IMG URL

Or email them and I will post them for you.

Thanks, I put some pics in for ya. Sorry, they are not in order.

I have built a similar structure as an underground shooting range. 25 yards of lane plus the shooting area. If you don't figure out some ventilation, the difference in temp between the ground and air will eat the thing....;)

Pictures?

Cool!  

Thanks.

I wanted to put this online because in my search for how to build a strong, cheap shelter, I was not the only one looking. You should not have to have a bunch of money sitting around for your family to be safe. If you buy a lot of the studs and OSB at auctions, etc., you can save even more money. I think $700 is cheap compared to the alternatives. And the span ratings for the roof beams are "safe" load limits. If a tree were to fall directly on the shelter, it should be able to take it.

Notice from the pics, it is built into the side of a hill, so it CANNOT flood during a heavy rain storm. In fast, in  the last 24 hours, we received just under 2.5 inches of rain, and it was perfectly dry in the shelter.

I am reminded of the "Utah Family" shelter from the nuclear survival books at the library. They built stuff using field expedited materials, but, it was only for limited time while earth/material could be cleared away that was affected by fall out.

I am a little concerned that you are using OSB for the walls underground, but, maybe thats not really a problem. Maybe a couple of thin coats of fence stain would help control/minimize moisture/rot ??? Just a thought.

Ventilation is going to be a big issue at some point, as others have pointed out... but, it all depends on how long  folks are in there as well as what you keep in it.

Use sealed 12vdc batteries, not traditional lead-acid as they tend to give off fumes.

You should cross post this in SF

What is "SF"??

Sorry for the stupid question.

Survival Forums

tag for later reading when not at work.

Good post OP!

Good work. Scrap cardboard will protect your poly film when backfilling and wont rot anymore than the dirt that is on top of it. Foam insulation board is also rediculously cheap, would help protect the poly and slow thermal exchange reducing condensation. Maybe a layer of foam board layed over the roofing sealer(bonded by virtue of the asphaltic sealer) and then the poly. A solar powered roof vent would be great although would add considerably to your expense, but adding a simply roof vent and an inlet vent in your door would provide some convection ventilation .

I have been mentally designing a similar structure, and will likely build one in the near future. Thanks very much for your time and detailed instructions, great job very inspirational.

If what I did can help any 1 family have a safe place to go, then the time it took to put it into writing is WELL worth it.

P.S. The reason why I didn't put a roof vent in is that I did not want something there to leak. I knew that with 2 sealed doors, I would need power ventilation anyway. I am going to use pvc pipe that I will route in the hallway (2 pipes: intake and exhaust) and use a 12 volt air mattress pump that I ordered on Amazon for 8 bucks. I don't know the CFM rating, but those things blow! The PVC pipes will exit at the end of the hall way (the part NOT under-ground), out the steel 'siding' with regular house type water-proofing procedures. I don't yet know how long my 12 volt system will run that pump, but I am probably going to wire in a dimmer switch to control the flow. I think wide open is a little extreme. I may do some calculations blowing up an air mattress with the pump to roughly figure its output capability. I am shooting for 75-100 CFM for my family of 3.

Great idea with the cardboard.

I actually want to keep the inside of the shelter as close to ground temp as possible for it to double as a root cellar. I may need a de-humidifier to control humidity levels.

Very nice. The only thing I may have done differently is used 2 x 8's instead of 2 x 6's for the walls. Also might add solid bridging between the studs at the halfway point to help distribute the load in the center of the wall span.

The part about the OSB was in relation to both the inside as well as the outside. I apologize for not being clear when I posted previously. (my bad)

Here is why I ask, if "water" (through any type of run off or moisture) gets into the wood from the inside, how do you plan to control it? This may actually NOT be a problem, but, I am very curious as to what you have heard.

I am building an ice fishing house this summer and I am torn between OSB and plywood.

I was wondering about this as well. But, 1-foot spacing on the 2x6 is a better for the plywood/OSB he has on top and sides for helping carry the load. Reduces any additional flexing and stressing across the unsupported spans.

I do like the idea of the cross braces, if for no other reason then to prevent any type of "buckleing" that might occur. Wouldn't take many, maybe 2 or three every other pair.

When going back and thinking if a "tree" landed on top, I guess a better question is how big are the trees adjacent to the structure that would be capable of falling?

My comments are "tweeks" and should not be regarded as a critique of the fine work. From my "back of the napkin" calculations, you could drive a full size car on top of it and not worry.

Run-off water will not be able to enter the shelter unless all the water-proofing fails. The shelter is built in to the side of a hill (at the top), and with how I did the landscaping after back-fill, it cannot run in from the surface.

For your ice-house: go treated plywood for the floor. With the snow, road spray, and constant heat/cold cycles, you will be happy with treated. All the ice-houses I have ever seen that don't use treated only last a few years.

cool! post some pics of the finished product once you're done.

Sorry I did not spell everything out with full construction details. I more-so wanted to detail the numbers of the forces at work, and the ratings of lumber for how to build such a structure.

Things such as water and drainage will very greatly depending on location. For example: I would not even consider building such a structure in a high water table situation where you are not on a hill. (I would be thinking concrete, steel or plastic shelter: Something that is inherently resistant to water from the material it is constructed of.)

Also, something I don't think I ever mentioned: The walls were built to 6ft high total interior height. 2x12 roof beams sit on top of that. I stand 5'8" in my boots, and my wife is a little shorter. I figured if we have guests that are tall and they need to go in there, they can stand with their head between the roof beams

Building taller, and I would recommend using 2x8 or better for the wall studs.

How is your shelter holding up so far?

Any changes you would make looking back?

Bump because I like this thread.

Built a drain into the shelter for the de-humidifier, so I didn't have to drill a hole through the door for a garden hose

Going good actually. Hindsight changes: make the hallway from 6 feet to 10 feet. The dirt is too steep without retaining wall. Also, making the hallway about 12 inches wider would have been nice.

Other than that, I would not make any other changes as of yet. Of course, bigger, or concrete would be better. But, for what I spent on it and what it does, I am very happy with how it turned out!.

I think about filling the bottom with concrete, but I don't like the idea of locking any moisture that could get down there from the de-humidifier.

Biggest thing I hate now is that the de-humidifier keeps it at just under 90 degrees in there!!! At least its a 'dry' heat at 25% humidity. The de-humidifier is required to run at all times (at least during the summer months. See what happens this fall). Now that the wood is getting dried out, I may put a timer on it to have it on a 4 hour on, 4 hours off to save electricity. Will have to experiment.

Planning on keeping it just above freezing this winter to store home canned garden veggies. Should need minimal electric heat to do that.

Have it now stocked with the basics, a roll-away bed, cots, water, canned food, extra cell phone (911 phone only), phone chargers, etc, etc, etc. Easy to make shelving with exposed studs!

I am hoping to get 5 good years out of it. If I get that, I would be happy. We are hoping to put a basement under the house in that time, and once we do, I will knock it in.

Will keep everyone posted if something changes that need to be made to the design. Like I said, I didn't get into specifics on how to put it together, but I will say: INCREDIBLE LATERAL FORCES AT WORK HERE! Just remember to brace everything!