I've worked construction for many years but never did concrete work. One of my best friends from my school days now runs his fathers old concrete business and he refuses to pour any pad thinner than 4". They do good work and rely on repeat business and word of mouth so I trust his judgement.

 

Most house slabs in my area are 4". For inside shops and garages where heavy equipment tractors 2 post lifts are going to be is usually 6".. Also look at pricing the concrete with a fiber mesh in it and check the psi on it.

 

I will look into fibre mesh as well as the rebar to increase it strength

 

OK guys I'll simplify what the OP is looking for:

A poured concrete ceiling... 4" would be a minimum. No need for 2 layers of rebar, use #4 instead of #3, space the mesh every 12" place the rebar in the center, use roofing panels under the concrete and build wooden support to hold it all in place for 30 days.

Rancher

 

Correct, but use thicker R panel (corrugated roof panels) as the base for the pour. Leave in place once the concrete has cured.

 

You need to be asking an engineer not internet people on survival forum. That's a serious project right there....

 

The required load bothers me: it is too low. What rancher said would work, but I would have used a thicker slab, with two layers of #4 rebar. Also it would be stronger with a slight concavity on the bottom (with rebar matching the shape).

 

How many buildings in Mexico collapse during earthquakes? Lots.

 

^^^ Who cares ?

 

www.concretefence.com Have you thought about buying prefab slabs? They make the so they lock into place. An example, I knowbit isnt roofing but they also staryed building houses like this...

 

prefab slabs are out of the question due to carrying them to where they are required.

it has to be done by hand not machine

 

I think 200# per square yard is awfully light. 2 average Americans standing next to each other would be double your loading . Make sure no one can drive over it. Concrete is really good at compression, tension not so much. Half your roof will be under tension if it's flat, even a slight arch would be better IMHO. A 1 PSI over pressure is like 1300 # on your square yard.
http://en.wikipedia.org/wiki/Effects_of_nuclear_explosions

 

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https://www.google.com/search?q=cor...2F%2Fwww.finehomebuilding.com%2Fhow-to%2Fqa%2Fconcrete-porch-floor.aspx;400;333

This is the system you should use corrugated steel decking. I highly recommend you run this buy an architect/engineer,There is different things that come into play .Such as the 10 inch wall bellow and the footing it is built on.Any opening such as doors and garage bays are going to require steel beams .Also if you need a water proof situation bellow you will have to design it to have some pitch. I am using this system on a deck for a new house I am pouring a 4 inch slab . putting down torch down rubber than another 4 inches of concrete above.Make sure the rubber runs all the way the ends

 

A tow motor o forklift takes a 8 inch slab. That said if it was me would do no less than 6 inch, but more tan likely a 8 inch slab.

 

I would go 6'' and do it like this guy did

www.robertsprojects.blogspot.com/2010/02/rootcellar.html

down on the right side of this page click on root cellar

 

I have built 2 structures similar to this (8'X8"). I used a double layer of Strong Panel siding as a pan, a double rebar mat, 6 bag fiber mess concrete and I poured it 7" thick. One of the buildings is 19 years old and the other is 10 years old.

 

cheers for the info lads.

I have recently spoken to a structural engineer whilst holding a bottle of jack in front of him.

he is going to look into it for me, but cheap means slow responses

 

I have recently spoken to a structural engineer whilst holding a bottle of jack in front of him.

Ahhh I see where the problems lie, Jack is a cheap American whiskey that people pay way too much for, it was made popular by Clint Eastwood movies. You should have dangled a nice bottle of 18 year old Glenlivet in front of him, he probably could have worked out the solution on the back of the napkin for ya.

Rancher

 

Good plan getting pro help. Much better to only build it once correctly than to do it over.

 

I just did this - 10 feet wide roof, 8.5 foot span 8 inch thick poured walls.

There are a number of one way slab calculators on the net for this :

http://www.oytec.net/edu_ref/slab_designer.htm

Basically, all of the strength of a concrete span is in the steel, not in the concrete itself,
so adding thickness of concrete does not add much if any strength to the structure. That being said, most roof pours are 6 inches + which is what I used, and the reason for this is that the rebar is supposed to have 3 inches of coverage top and bottom to 'protect' it and to make sure the coverage is enough to transfer stresses between steel and concrete.

It probably also depends bit on the strength of the concrete mixture - how many sacks of concrete used.

I used a 6 inch slab, roughly 12 inch centers of 1/2 inch rebar and I expect to pile several feet of dirt on top. The slab calc says this is fine.

The main thing you are doing right, IMO, is making sure your span is small - you only need an engineer when you start going longer spans.

 

loving the link, tried quite a few different designs through it

any way yes jack is cheap , so am I !! I keep the decent stuff for myself

as for the engineer spoke to him today whilst he was at work and he ran the calculations through the system and came out at 4" is ok for 500lb loading which is massively over my requirements, so 4" it is, we are also using thinner rebar as well 1/4" mesh rather than bar pieces which is also a lot cheaper.

over designing is good and in most times I normally would but here there is cost and time and material handling issues so no I don't want to.

to do this slab at 4" will take 8 weekends of prep work, to go to 6" adds another 5 weekends to the project time line and more people to do the slab pour in one hit over one day.

logistics are a killer to many projects, lol

 

We front porches on house with room under them 6 in. To 8 in thick with wire mesh rebar.never had any issues. That was 6 bag mix. But if mixing your self. Better mix fast to keep from forming a cold joint that willl cause weak spot and could crack

 

i have been working with a architect and engeneer on a simular deck let me get your facts straight

10 span wall to wall
9" block concrete filled
200lbs per yard load on top of it ( giveing me the feeling you are putting 3' of dirt on top

in my project which does not include being buried i have steel i beams every 6' supporting steel corugated decking then 4" of concrete over it so if you are thinking of somehow free spanning 10' i am guessing you will have a failure and 4" thick is way to little even the plans a kits i see for 10' wide shelters and rootcellars have steel supports so http://www.americanbombshelter.com/bomb-fallout-shelter-kit.htm

this is the kit i am talking about

 

6 inches would be about right. Tough enough for even pretty heavy vehicles driving over it (just in case). And yeah, 2 layers of rebar will be all you'll need. And what'll help, I think, is getting that at a slight degree, like a roof, so that water will go off the sides.

 

Are you joking or serious? This is just about the worse engineering of a load bearing concrete slab I have ever seen. Just tell me you're kidding. 2 layers of rebar? Really?

 

Look at Ferro Cement boat construction.

If you built a curve into the roof it can be thinner and it will be stronger.

 

now theres a topic I have not heard of for years now, seen a couple being built and tried to sail one once,

as for the replys many thanks people and the ideas are great but because of the site specific details, we have gone for and are planning to organise the 4" roof slab, its the most economical cost option for what we require

 

Soil weighs about 75 lbs per square foot. Hope this helps with sizing. Also, you are only spanning 10'. 4" with rebar would probably be ok. The rebar and wire mesh would help if it cracked to keep it from falling. If you put 3' of dirt on top, you would have complete insulation from the weather. You would have to go about 8" thick to hold the weight though.

 

the concrete roof is more for security reasons than a structural walking on driving on structure.

it is going to be covered but by a pond liner and moss and sage grasses with little or no soil. we are planning to place on top of the pond liner to add old carpet in a single layer to allow the plants some thing to hold on to.

we have started to load the materials up to the site.

there will be 3/8 rebar in the layer of concrete as well as mesh. but there will not be any structural steel supports in it due to costs and loading out issues.

I must ask though , how do people consider the risks with getting companies in to do "private works" is there not a risk of too many people finding out or at least being local enough to cause security issues, when this sort of works are being done

 

I must ask though , how do people consider the risks with getting companies in to do "private works" is there not a risk of too many people finding out or at least being local enough to cause security issues, when this sort of works are being done

It all depends on the amount of time between the end of the project until you really need it. I visited a bunker near High Wycombe that I bet many of the locals don't know is there.

Rancher

 

this forum is a great place to debate the finer points of wheat storage or how to pack a bug-out-bag. And we probably have members who know exactly the answer to your questions but, for obvious reasons, can't put their engineer's stamp on their post in reply to you. Therefore, I really recommend you ask this in a more appropriate forum. If you have reasons for not asking an engineer, there are great resources online, if you can do the math, that will help you figure this out for yourself.

Engineering of standard building features is pretty standardized into tables, charts, graphs, or at worst, formulas. The thing the engineer brings is knowing which to use when but you could even figure that out yourself. What you don't know are the variables: what is the load-bearing capacity of your soil. It can change easily within the length of a wall. What about the block wall you're topping? Was it laid correctly? Were the joints done right? Was the rebar and grouting correct? Is it exactly level and plumb?

What you are asking can absolutely be done but it takes a lot of work and forethought to make it reliable and safe.

If you have questions about how to store rice for long term storage, though, ask away; you're in the right place.

 

This is one reason why it really should be 6" thick. I would not trust even a professionally prepared batch to make 4" thick roof, because nothing is ever perfect. The cement can be old and compromised, the mix can be made imperfectly, the gravel not uniformly distributed. On and on. I always use 3x safety factor making concrete for important jobs. But even for a professional job it should be 2.5x, in my opinion. As far as making the roof in small batches, it is not too bad: extra rebar goes a long way (with a power vibrator). Also, columns may be added for extra insurance. No biggie, but still a lot of work. And nobody would mix it by hand, I think. A couple of small power mixers are easy to get to the site.

 

huh? I'm dumbfounded. I look at my typing and I see logic. You must see wah-wah-wah-wah-wah.

What makes you think that in any world other than Superman's Bizarro World, anyone other than a master craftsman concrete person could do better mixing their own concrete than your local concrete yard? If those people did it wrong, no contractor would call them. They'd be out of business very quickly.

And why a 3x or 2.5x safety factor? I've read that lumber yards love do-it-yourself home builders. Build-your-own types over engineer everything. When a 2x8 floor meets every spec required, they jump to 2x12's because they don't trust (rightfully) their building skills. Where a 6x12 beam would work, they'll put in an 8x14 beam. Where 4 inches of concrete would work, they put in 6 or even 8. Of course I still wouldn't buy the house they built. My in-laws bought an owner-built home. The edges of the slab are 6 inches thick (inside of the footing). In the middle of the rooms, the slab is falling apart and is as thin as 1.5 inches in many places.

Where do you get get 6 inches thick? A random number generator or do you have an extra finger on one hand and you just counted your fingers? The last time I did actual math to design a two way slab similar in size to the OP's slab, the number was actually between 4 inches and 5 inches where it rounded up to 5 inches - and that was with a significantly greater load capacity than the OP specified (though I would never plan an outdoor underground roof with 23lbs psf load rating - that's just plain silly on it's own. But, properly engineered with a reasonable load rating, I would expect the thickness to come out between 4 and 5 inches. I wouldn't be terribly surprised to see the numbers say 4 inches - but not with a silly two layers of rebar.

So, the OP, and you, come up with random numbers for the thickness of a slab with no supporting mathematics and then you talk about throwing in a column. What's missing there? Umm.. a beam? And what about a footing under the column?

This is not random stuff you just throw in. That's the whole point. This requires engineering. Someone with any ability to read, do math, read charts, etc. can take advantage of the decades to centuries of engineers who have captured their knowledge in formulas, charts, and graphs, so it is possible, for the right person, to do DIY engineering on this. Problem is, most people are not the right person. You have to be totally obsessive about numbers and research. You have to be willing to accept what the numbers tell you, and you have to be able to implement the design. If you can't actually do the work to the quality upon which the formulas are based, you will likely kill someone.

 

gentlemen, please, whilst your points are all very very valid, we do seam to be getting very over heated here.

I have run all the calculations and design through a bottle of jack and a structural engineer to confirm every thing. due to reasons you have stated very clearly here we are going for the thicker slab, but in doing so we have gone from $1,000 costs to $4,000 plus costs.

there are many logistical reasons why I am doing what I am doing they way we are doing it. I wont go into to many details due to site security reasons but I am willing to discuss the project via PM's if you wish to be fully informed.

the main reasons I thought this would be a good topic to discuss on this forum is the construction of buildings in bug out locations within the united states. I thought would have a lot of logistical issue getting men and machinery and supplies to their chosen locations

I see a lot of similarities of someone building a bug out shelter in the mountains and woods and so on to my own build, miles from anyone and off grid and off roads. I don't see someone building a bug out location and a two laned road to it, when they don't want people to know its there?

I started this thread with a general idea that a four inch slab would suffice because I have worked on many older buildings that have this set up and that's why I wanted to chose this style of build, security and strength were the two main points. rather than the ability to park a car on it, hah ha haha ha ha

After seeing the reply's here I did go and find a structural engineer to design and calculate the roof thickness, construction, rebar design, rebar coverage, camber to the roof, life span and construction of the slab within the area it is to be built and its construction methods. we have also looked at the below ground the floor slab is on, its foundations, the floor slab the walls it is to go on and the change of ground shift and slippage (subsidence) and have resolved all issues before going down the route I have decided on

I have also looked at other cost options.
pumped concrete and pre mixed
pre mixed and manual transport
mechanical transport and pre mixed
mechanical transport and manual mix
and several others, they all have positives and negatives and considerable costs

I have gone for the best cost option for the best solution for my project that's within my time scale, on perhaps another area or site other cost options can come into play, but for this site and this build we have decided on manual carting and manual mixing

we will be using gauging boxes and water containers and I will be quality checking every thing, the concrete will be vibro pokered and allowed to set under the best conditions possible for this area and location, getting the maximum strength and best practise possible

as for my own skills. they are more than enough after 35 years in the construction industry and 25 as a construction site manager, I am well able to supervise this pour correctly

many thanks for your in-depth and detailed responses, HP

 

gentlemen, please, whilst your points are all very very valid, we do seam to be getting very over heated here.

I have run all the calculations and design through a bottle of jack and a structural engineer to confirm every thing. due to reasons you have stated very clearly here we are going for the thicker slab, but in doing so we have gone from $1,000 costs to $4,000 plus costs.

I'm glad you made the right choices. But this is a public forum and the bad advice you get is bad advice for anyone who reads the forum. Bad advice that can leave people dead can't be let stand here or anywhere else. Unfortunately, you're not the only one who would go to an anonymous forum to learn how to do structural concrete.

The extra 3000 dollars in cost is a small investment in the safety of your family.