Replies

1. 
   freeman2 | May 19, 2004 02:01am | #1

   'round these here parts it is mucho xpensive to do perimeter type foundation, i have found that slab foundations (monolithic pour) are far superior in every way especially with potential earthquake issues, in fact here in central calif. it is hard to get any insurance (i'm not talking earthquake ins.)on anything other than slab also the building dept. here frowns on anything but slab on grade. use 12"x12" spacing on your 1/2" rebar and pour. keep in mind i'm not a pro, but i have built several homes here (owner/builder) make sure you get at least 2500 psi concrete or higher i used 5000 psi with fiberglass mixed in (i forgot what they call it) wow!! you will never see a crack even without control joints! cost approx. 96.00$ per yard here with fiberglass in the mix. hope that helps, have fun!

   fm2

   "the large print givith, and the small print taketh away"          Tom Waits

   "those with accurate observation are often called a cynic by those who have not got it" george bernard shaw

1. 
   brownbagg | May 19, 2004 02:12am | #2

   90 % of all house here are slab on grade. we do it this way for the termite problem. but then it doesnt freeze here either

2. 
   Piffin | May 19, 2004 02:16am | #3

   Many of the differences derive from factors you have not yet adressed; Soil type, ground water levels, climate, and predominate local practice, as well as intended purpose of the structure.

    

    

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1. 
   newbuilder | May 19, 2004 06:19am | #5

   Hey!  Wow! -- everyone seems to be 'voting' for slab on grade!  I really would rather do it that way but it seems to me that all I see and hear about are perimeter.  I'm glad to be wrong regarding what I 'assumed' would be the general concensus in this case! 

   As to conditions, use, etc. .. this is going to be a sort of 'gathering hall' (hopefully).  It's in Seattle .. and it's at the very top of one of the highest hills here so 'water table issues' is definitely NOT a problem .. (in fact .. EVERYTHING flows AWAY from this point).  I guess the only other question would be 'floating (separated) floor or all-of-a-piece. 

   Very happy to get this considered feedback .. thanks so much!

   Any further thoughts greatly appreciated!

   Thanks!

   Terry

1. 
   FrankB89 | May 19, 2004 08:23am | #7

   I'm also in the PNW and have done both mono slabs and perimeter foundations.  Both have their merits.

   In the meeting room scenario you describe, the slab is probably the best option. Slabs are most times less expensive than a perimeter foundation and subfloor all things being equal.

   Being in Seattle, I'm sure you'll have an inspector making sure your subgrade is compacted and you have a compacted gravel or sand base, and that your reinforcement is per spec.

   You don't mention it, but if your project has restrooms, or a kitchen or any other need for utilities, especially a waste system, the rough-in must be trenched in, tested and locations of toilets, showers, tubs and the like and their respective drains and vents must be precisely located prior to pouring concrete. 

1. 
   newbuilder | May 19, 2004 12:15pm | #8

   Thanks much for all the good info guys.  Sounds like a slab-on-grade is more of a likelihood than I had thought.  Seems like all the books and soforth (including 'For Pros by Pros .. Finehomebuilding's Foundation and Concrete Work) tend to give most of their space to perimeter forms ... ICF's .. etc.  But a slab has seemed simpler and seemed more to the point to me all along.  thanks again.

   Terry

1. 
   brownbagg | May 19, 2004 02:42pm | #9

   thats one of the problem with a forum. The county is so big, there are so many different method of construction. what might be ok on the gulf coast would never apply up north. so there are no wrong way, just what better suited for your area. alot that other people do is not wrong just that group of community has never done it that way.

   We use slab on grade because we can not have basements around here. water table too high. Bugs are big problem so with rodent, possumn etc. so they like to have concrete for the first six inches touching ground. Termite do not have a flavor for concrete. We have some crawl spaces but not many. They either cut footer in ground then block up with fill inside block, or slab on grade

3. 
   Catskinner | May 19, 2004 06:15am | #4

   Following on Piffin's observations;

   what is your frost depth?

   is the soil on your site cohesive?

   how is the surface water situation year-round?

   most of all, is the site level or fairly close to it?

   DRC

1. 
   newbuilder | May 19, 2004 06:26am | #6

   --Frost depth here -- Pacific NW -- is not a biggy .. donno what it is exactly .. but it's pretty mild .. RARELY hits 22 degrees F. here.

   --I guess I dont know enough to answer the 'soil cohesiveness' question ... but I CAN say that I see people 'in the neighborhood' excavating, putting up forms, and pouring with seemingly no special preparations.

   --surface water situation ... there is none (see previous post)

   --yes ... site is level as can be

   thanks

   T

4. 
   Dez | May 19, 2004 10:21pm | #10

   Hi Terry,

   I grew up in So. Cal where slabs are generally the rule, not the exception. I too am here in WA. (Snohomish, Co.) where, conventional foundations (footings and walls) are the general rule. The idea of getting the foundation and floor all in one, is pretty attractive, considering materials/labor savings.

   All the suggestions/advice others have posted is spot on. Good stuff to get you thinking.

   Personally, my first home in WA. state I built on a slab. Never again in this state for me! I was pretty green being a new transplant, and didn't give the moisture up here the proper consideration. Not good! Once I got everything all insulated, added heat, my problems started. Moisture infiltration via capillary action, condensation, mildew. Radon gas can also be a problem to consider.  It can get ugly.

   I'm sure if you have a good site, and do what the others have suggested, you can have a successful slab. Check out the link below. I hope it works. Good luck to you with your project!

   Dez

   http://wwwems..epa.gov/iaq/homes/hip-basement.html

5. 
   DanH | May 19, 2004 11:06pm | #11

   Your foundation (unless it's engineered to be "floating") must go below frost depth. So you must excavate that deep around the perimeter regardless. If that's, say, one foot, then you've just excavated a more or less standard footing for a slab, but if it's 3-4 feet then you're going to end up doing something resembling a conventional footing even if you pour a slab on top.

   Another consideration is the plumbing. With a slab you need to imbed most of the drain plumbing in the slab, and changes after pouring the slab are very expensive. With a conventional platform you have much more flexibility.

6. 
   user-28141 | Nov 23, 2004 09:01pm | #12

   I would like to piggyback on the topic, raising the same question for my situation. 

   Western Maryland, 31" frost line, this will be a 20x20 shop attached to the back of my attached unheated garage.  The floor will be about 1 foot above grade at the highest point, sloping to channel water away from the house.  

   I've been told of two main options, perimeter foundation (either poured concrete or 8" concrete blocks) with a pier in the middle supporting engineered floor joists, or a poured foundation, lined with 1" rigid insulation boards laid vertical around the perimeter, filled with rubble, 1" rigid insulation boards laid horizontal next to the vertical insulation, with a concrete pad poured over that. 

   This latter appeals to me because it is supposed to keep the concrete floor from being so cold in the winter (cold would have to travel past the vertical insulation and past the horizontal insulation to cool the pad)(I'm heating the shop with a safe propane heater) and there is no crawl space, vented or otherwise, to mess around with.  The crawlspace would be inaccessible (not 18" clearance between dirt and joists).  Don't want termites, although there's 8-12 inches from any dirt to wood, so that may not be a problem.

   Shop will be for woodworking (tablesaw, bandsaw, etc.), no plumbing, one story. I'm perfectly willing to either use rubber mats where I'm standing or later come in and run 1x over 6mil sheeting, covered by tongue and groove flooring, but later, much later,hopefully.

   Am I missing anything? Is one just dramatically cheaper than the other?

   Thanks for any advice.

1. 
   wivell | Nov 23, 2004 10:35pm | #13

   Shysterfish,

   I'm located in southwestern PA.  My garage is 24 x 32 with radiant floor heating in a concrete slab.  It was constructed in the manner you described except I used 2" rigid insulation.  I keep the temp. set at 60 and it seems to barely run.

   Scott