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rat slab (post #77038)

What do I specify when I go out for bids?


2,200 sq. ft. crawl space with a grade to floor height ranging between 4' at one end and about 10' at the other.  A full basement is not in the budget, but it would make for some really useful storage space.


So, I would like to put in what some on this forum have called a 'rat slab', i.e. a thin layer of cheap cement.


The grade has about a 1' in 10' slope.


I'm not expecting anything more than a garden rake finish, some cracking is inevitable, and no structural strength required. 


In my mind, this means they strip off the topsoil when they do the grade work for the rest of the foundation, put down a vapor barrier, and pour a couple of inches of cheap cement.  Is that really what I ask for?


 

(post #77038, reply #1 of 59)

You still have topsoil in a crawl space?

Is this house already built?

 

 


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(post #77038, reply #2 of 59)

Ground graded smooth, otherwise you won't get any kind of a consistent slab thickness. 1" thick slab will be a waste of money.


Install some kind of drainage system at the low point so that if the crawl does leak, the water has a way to get out.  Floor drain, or better still both floor drain and several sections of perforated pipe below the slab.   A drain tail that exits to daylight is required.


If required, don't forget the termite treatment before placing the vapor barrier.


6 mil poly vapor barrier with no voids.  You may even want to run it up the walls some and glue it to the walls - if so, use black poly.


2 1/2" slab thickness of 2500 PSI mix, bull floated but not finished.  Use concrete rather than cement.


 

(post #77038, reply #3 of 59)

Exactly the information I was looking for


THANKS!


 


 

(post #77038, reply #4 of 59)

My opinion... a rough, raked out rat slab will be worse than dirt. Have them float it out at least a little bit.

(post #77038, reply #5 of 59)

I might disagree with half of what he said depending on conditions - got any answers for me?

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
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(post #77038, reply #6 of 59)

Think about that slope for a minute.

Now try placing crete on a plastic VB.

Even with low slump, most of it will be at the bottom while you end up with only a stain on the plastic at the top.

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #77038, reply #7 of 59)

That slope is stopping me from commenting.


SamT


Anyone who doesn't take truth seriously in small matters cannot be trusted in large ones either. [Einstein] Tks, BossHogg.


SamT
A Pragmatic Classical Liberal, aka Libertarian.

I'm always right!
Except when I'm not.

(post #77038, reply #8 of 59)

I'm still a waiting to see if he is going to tell us if that is the natural slope on an unbuilt site full of topsoil or not.

I'm thinking that somebody who needs to specify for bid how to do a rat slab is trying to GC his own build but lacks a LOT of experience still

Then there is the "Can't afford a full basement" - but The only way I can imagine ending up with a slope like that inside a foundation is if it is ledge, but he has topsoil there...

Something is screwy here.

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
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We did the best we could...

(post #77038, reply #9 of 59)

In my mind, this means they strip off the topsoil when they do the grade work for the rest of the foundation, put down a vapor barrier, and pour a couple of inches of cheap cement.  Is that really what I ask for?


 


 


"This is a process, not an event."--Sphere


And I'm a legitimate certifiable Tool [JOBSITE WORD].--Dieselpig

 

 

(post #77038, reply #10 of 59)

I saw that.
So you agree?

It is hard to have a discourse with the guy. He says he has exactly what he wants now, right or wrong. He read my request for info and ignores it...
All we can do is wish him luck.

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
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Oh Well,

We did the best we could...

(post #77038, reply #11 of 59)

Gotta love the cut-and-run posters.  I think he got the answer he was looking for.


My guess is he's GCing his own place and doesn't know what to tell the excavator or foundation guy.


Mark's advice sounded ok to me, except for the slippery vapor barrier, and if it's on ledge I'd throw in some radon venting.  I don't know how big an issue that is outside of Maine.


I'm curious--what else would you disagree with on the given advice?


 


 


"This is a process, not an event."--Sphere


And I'm a legitimate certifiable Tool [JOBSITE WORD].--Dieselpig

 

 

(post #77038, reply #12 of 59)

Like I said, it depends on the answers this OP never provided.

But assuming we are right and this excavation has not even started yet, I would explain that it will only cost another two nickles or dimes to excavate enough that he can have a full basement someday if he wants it, unless he is on ledge. Then he can still do a rat slab but it owuld cost but another quarter to have a real floor. He seems to have assumed that he can't afford a full basement, but the few facts he gives are contrary to that.

The size of house in SF and the slope he indicates shows this house to be about 40x60 or more likely, 36x66.

Pretty good size, wouldn't you say?

So he can afford some space down there. The slope would afford a fine walkout basement. The extra appraised value would more than make up for the cost of creating the rough shaped space.

OTOH, if this is on ledge, then he needs a drainage plane under this rat slab, or maybe no rat slab at all, since there is already rock there. He would only need to use a pressure washer to clean it, but he might need to plan for drainage and radon removal. That would place the plastic VB in a different laoction in tha plan.

i'm not picking on Mark I think it was. I'm just suggesting that since the OP only gave half the necessary info to get a decent answer, the answer he took away with him was not worth much. Not because Mark was wrong, but becuase it was based in part on assumptions and was generic information. Good genericly, but maybe not specificly

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
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Oh Well,

We did the best we could...

(post #77038, reply #13 of 59)

Sometimes work gets in the way of life.

We have a construction loan and are working on the foundation drawings. The next step after that is permits.

The house is 33x70 - to fit within a boundary of some very large live oak trees.

We looked very long and hard at a partial basement - The budget was just not there for it. The concrete retaining walls are ~$10 per sq. ft., and a full basement would have required a lot more concrete.

The house is going on the part of the yard with the least amount of top soil.

I lack a WHOLE lot of experience. But I'm trying to make up for it with stubborness.

The foundation is a stepped pier and beam foundation and I just got the foundation engineer to replace some of the concrete walls with cripple walls.

No, I do not know what to tell the excavator or the foundation guy, that's why I've been asking lots of questions.

No Radon in this area - but lots and lots of termites. I will have a professional come in with Termidor and I will come in afterwards and treat with Boracare.

If you can't put the concrete on the plastic, what is the answer?

(post #77038, reply #14 of 59)

Paul,
You can put concrete on plastic.( I think there are better underslab moisture barriers than plastic personally)
I think what the issue here with the concrete poured directly on plastic was the slope of the ground under the plastic, not with the plastic itself..
Most of the time any area that may conceivably be used , or would have a slab of any kind put on it is cut to a flat grade by the excavator. Then a gravel bed is spread out and levelled. Then the concrete. Exception noted for ramps, driveways etc. in terms of flat or level grade.

"Rat Slab" is a generic term that has come to mean a relativly rough finshed slab that is either temporary in nature, or as in your case simply to seal an earthen floor without regards to the finish being smooth .

If I recall correctly from earlier threads then you are looking at someday maybe finishing a portion of the basement are, but not all of it. Your excavator (after he sees the site and prints ) should be able to guide you in terms of how much of the basement he can safely cut to level without impacting bearing of the building. Certainly your engineer can do that for you anyway.

What I understand is that you want a partial area of the crawl/basement area to be covered with a slab . Is that correct?


Life is Good

(post #77038, reply #15 of 59)

The hope was to cover the whole 2,230 sq. ft. with the rat slab.

The HVAC equipment and water heater will be in the crawl space. I had not planned on anything level except under the water heater.

There will be a full size door as an entry way, but no stairs or other openings. The crawl space will be insulated and conditioned.

(post #77038, reply #16 of 59)

OK, I misunderstood.

So I will ask why even bother with the rat slab?

Level the area for the utilities , level a walk way that area and just pour those.


Life is Good

(post #77038, reply #17 of 59)

I am not at all familiar with a pier stepped foundation with crawl space so I'm a bit blind on this.

But I can say fairly certain that it would cost no more to excavate the rest of that space to make potential someday for a full basement than it will cost to pour a 3" slab in there.
So My choice would be to dig all that soil out when the digging is happening..

Then if money is way too tight, pour the slab later.

The reason you can't pour on THAT plastic as described earlier is that plastic is slippery when wet. A pitch like that will let it slide right off. There are other moistop products that are tougher and that have a bit of a weave, and are not so slippery. but you would still want no more than half that slope. Since you need the slope smooth more oir less to pour, it is relly easier for most equipment guys to smooth it flat than to smooth it sloped, depending...
There are other ways to do it too, such as gunite, but you are talking budget...

So to get a lower slope that you can pour onto and expect it to stay you have to dig more soil out anyways.

Unless when you comment on topsoil means you have ledge (bedrock?) close to surface, that changes things too.

BTW, a stepped footing foundation is harder to do and thus more expensive than a straight one, except for cost of the crete itself.

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #77038, reply #21 of 59)

The beams are only going about a foot below the surface in many places. More than a partial basement would require digging into rock, since it isn't very far down to limestone on the up hill side.

I realize that the stepped foundation is more expensive than a straight one - however, a straight one would add a lot of rock digging, and make the concrete wall around the crawl space about 10' high and we are talking about 200 linear feet of wall. Right now, the tallest wall is 4', and most of it is 3-1/2' tall. Six foot high of concrete x 200 linear feet x $10 a sq. ft. is an extra $12,000, and an additional $5 to $20K of excavation. A stepped foundation is a bargain by comparison.

(post #77038, reply #24 of 59)

OK, see this is the info I was after with my line of questioning. First time there has been more than a bare hint that there was rock ledge in the way. There may still be details I am not seeing from here, but with ledge there, I don't see any need for a rat slab or we also call it a mud slab. Just cleaning the dirt off the ledge exposes rock that pretty much does the same thing.

Maybe I have a mental blank here and you can educate me.

The soil you say is maybe 3-4 feet deep at the uphil side. So you will dig holesfor piers down to the ledge and a trench along tops of them.

Is this all trench form and no wood or metal forms?

then with this grade beam in place you build a concrete wall atop that
Is that to be CMUs or re-inforced, formed concrete?

So the inside of this area is the dirt you speak of that has to be dragged down closer to level to get crete to stay on it. Once that is out of the way, the piers are exposed....This is where I really get lost.

It is so much easier and better to my mind and way of thinking to just dig it out to ledge in the first place and place the footers right directly on ledge

Now too, there is something strange about the figures you are working with.
What I ddescribe does not necessarily mean adding that 6' of wall height all the way around. You already said you have that att he deep end, so this shouls only be about 60% of the way around - again, be aptient with me since I'm in the blind both about site and your local methods in Texas hill country - so the amt of crete would be much less.

Then, the price for it - I am on an island and crete is expensive here, but we figure slightly over $250/yd placed in forms for footers or walls formed. It costs no more to form an 8' wall than a 5' wall, labour-wise. So the only cost is the extra crete.

But let's say I was pouring enough extra wall at my local rate to do the amt you say you would need, it would cost me about an extra $7500.

In my concept though, you would only need half as much crete extra and that would cost no more for labour so it might be an extra $2500 rather than the $12000 you thought.

So I ask myself what is missing in this?

I put myself in th e place of a crete guy in you l;ocation talking to you and start scratching my head.
First thing I say is "This guy is not sure what he is doing here ($$) and this stem wall is stepped ($$) and it is short so my labour per foot has to be higher ($$) and maybe this is far out of town($$) so I better quote high enough to cover my butt for all this unigue work"

If that is what we are looking at, it might explain the high price. Maybe the guys there rarely do poured concrete walls for residences so the only ones with wall forms are the commercial guys. That would make him more expensive. Or the guy is quoting high to help defray the cost of buying forms...
I don't know but the dollars you are dealing with is definitely up there.

Then the excavator. Half a day for a trench pour dig vs two days for a full dig?

Let's be generous and think 16hrs at $150. Double that in caase it is particularly slow. Five grand sounds like tops.
I have never paid twenty grand to excavate a cellar let alone an EXTRA 20K !

So there is my thinking, maybe an extra $7500 for excavate and extra wall to end up with deeper space and no rat slab - ledge rock instead - versus $3500 for crete and labour to place a rat slab inside the space you first described.

But you end up with more space and a more solid wall foundation. Another option for consideration would be using CMUs for the walls if that is the locally dominant type used. Then you can drop the price possibly.

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #77038, reply #25 of 59)

You might want to read through this thread: http://forums.taunton.com/tp-breaktime/messages?msg=87719.1


Other aspects of the foundation are discussed.

Matt

(post #77038, reply #28 of 59)

The (4) 2x14's have been replaced with a 5x14 gluelam.  One of those is 10' long, the others are all 8', so should be reasonable for the framers to work with.  The floor joists are 2x12's.  According to the span tables, the engineer appears to be aiming for L480 worst case, so I'm pretty happy with what I have of his current design. 


I'm still waiting to see the detail sheet though, and the devil is in the details.


 


 

(post #77038, reply #26 of 59)

piers down to rock (or refusal as the foundation engineer calls it)

On top of the piers, formed grade beam walls (at least 12" thick), reinforced concrete, in wood forms, 3' to 4' high, stepped, with 2x6 cripple wall from the top of the grade beam wall to the bottom of the floor. The cripple wall varies in height between about 1' and 6'.

The rock is about 4' down at the top end and about 7' down at the bottom end of the slope.

at least 99% of the houses done in this area are slab on grade. Everybody (but me) brings in enough fill to level the lot - the good contractors compact it.

In this area, the expansive clay soil starts to become a real issue when you talk about retaining walls. Big footers, lots of steel required, etc... With balanced fill on both sides of the walls, except for 33' at the garage end, I get away from most of those problems. Digging out the crawl space would lead to a LOT of those problems.

Your $250 a yard for formed walls is right at the $10 a square foot that I am told is standard in this area. I don't know what is standard in your area, but in mine, any concrete wall with unbalanced fill is going to be 12" to 18" thick due to the expansive clay soils.

(post #77038, reply #27 of 59)

Just to reiterate what we know. All cut and paste follows.


The house is 33x70


2,200 sq. ft. crawl space with a grade to floor height ranging between 4' at one end and about 10' at the other. 


The grade has about a 1' in 10' slope.


[The two above para's do not compute. ST]


The [bed]rock is about 4' down at the top end and about 7' down at the bottom end of the slope. [2:10 slope]


The foundation is a stepped pier and beam foundation


The beams are only going about a foot below the surface in many places.


On top of the piers, formed grade beam walls (at least 12" thick), reinforced concrete, in wood forms, 3' to 4' high, stepped


The HVAC equipment and water heater will be in the crawl space. I had not planned on anything level except under the water heater.


There will be a full size door as an entry way, but no stairs or other openings. The crawl space will be insulated and conditioned



SamT


Anyone who doesn't take truth seriously in small matters cannot be trusted in large ones either. [Einstein] Tks, BossHogg.


SamT
A Pragmatic Classical Liberal, aka Libertarian.

I'm always right!
Except when I'm not.

(post #77038, reply #29 of 59)

Since budget is of high concern, why not go to a pier and bond beam foundation vice pier and grade beam?


This would save (1'x2'x200') 26yds concrete @ $4000. It would also eliminate all the pony wall$ up to the floor, all the extra foundation $teps and all ek$cavating.


No VB or rat $lab. Insulate under subfloor instead of foundation walls.


 Floating slab and conventional framed mechanical room.


You could color coat the piers or put latice around the area.


Later, it would be real easy to shovel out a flat area, put in a locked block retaining wall and pour a small slab for storage space.


Oh, a bond beam is a floating beam the ties the tops of the piers together. It does not need to rest on soil. Typically around 18" deep and as wide as the pier diameter.



SamT


Anyone who doesn't take truth seriously in small matters cannot be trusted in large ones either. [Einstein] Tks, BossHogg.


SamT
A Pragmatic Classical Liberal, aka Libertarian.

I'm always right!
Except when I'm not.

(post #77038, reply #31 of 59)

Budget is always a concern.  I really appreciate the suggestion, but I am having trouble following it.


If the grade is 10' below the level of the floor - I only see three possibilities.


1. a LOT of fill dirt - not the right answer for this lot


2. 10' concrete wall - expensive


3. concrete wall and cripple wall.


So, what am I do dense to see?

(post #77038, reply #32 of 59)

The problem with insulating under the subfloor instead of at the crawl space walls is now all my duct work is in unconditioned space.


The house has a brick exterior from the gradebeam all the way to the rafters, so just piers supporting the house is not going to work.


That did give me another idea though.  Would it make sense to turn the stepped pier and beam foundation into a stepped floating slab - like the garage but with steps a thinner slab and a lot less steel in the floor?

(post #77038, reply #33 of 59)

Not to try to re-enginmeer this foundation but just for information purposes.

I am curious why the design isn't for a simple inverted "T" foundation.

We also have expansive clay soils here and yet use it all the time on engineered foundations. (Soils bearing assumed at 2500 PSF.)

Why the grade beams, and piers?

And here even with unbalanced fill the engineers will design a re-bar placement and tie the floor system to the tops of the walls so as to allow us to pour an 8" wall most of the time.

I have done dozens of 50ft. or so long walls 8-10' tall at 8" thick and sitting on a relatively small footing.
Actually sometimes it is a cost saver to pour the slab as it acts as a compression member to hold the bsae of the poured walls in place.

Just asking ?'s here.


Edited 3/30/2007 11:37 am by dovetail97128


Life is Good

(post #77038, reply #34 of 59)

Paul - there is another data point that you may not have considered.


You mentioned expansive clay soils.  When you backfill with enough gravel, this problem goes away.


If this were my house I would do what I could to have the excavator dig something square, flat and easy to form - spend the money on rock removal rather than the labor on complicated formwork - and get a basement for a similar price.


Of course if this were my house, I'd use ICFs too, and everything would be in conditioned space.


Best wishes on your project.


 


Treat every person you meet like you will know them the rest of your life - you just might!
Treat every person you meet like you will know them the rest of your life - you just might!

(post #77038, reply #35 of 59)

I strongly suspect that digging down 6 feet into a limestone ledge is not something I want to pay for.  I'm sure it would be a lot easier to form, but I doubt that it would be cost effective.  Before you hit 6' into the limestone, the soil test report claims it has gone from medium hard to very hard.


The crawl space will be insulated and in conditioned space.


I've read and studied the ICF's a lot.  My personal opinion is that they are an answer looking for the right question.  I think there are cheaper ways to solve the issues that ICF's excel at.  But, that is another thread alltogether.


Everything I do on this house that is non-conventional in this area is another headache and battle.  Some of them, such as the pier and beam foundation, I am willing to do battle.  Where I can, I'm keeping the construction as conventional as possible and still end up with the house I want.


 


 

(post #77038, reply #37 of 59)

I gotta agree with Piffin on this one.  Dig out the dirt, leave the ledge, make as much usable flat space as can cost effectively be done at this time.  Do it that way because, expensive as it is now, it'll be vastly more expensive to do after the house is built.


Look around the rest of the plans and budget for stuff you can put off and finish after you've moved in.  Maybe live with one bathroom to start with, and do the rest later? 


 


 


-- J.S.


 

 

 

-- J.S.