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Discussion Forum

slab versus raised foundation

KWOLSEN | Posted in Construction Techniques on October 10, 2009 06:35am

Currently working with architect to add about 800 sq. ft. to a ranch style house. We have the design pretty much nailed down in terms of new roofline and layout. I’ve received preliminary drawings.

The existing house is on a poured concrete raised foundation. So, I was planning on making the new addition the same poured concrete raised foundation as well rather than a poured slab. Can you guys give me some feedback on doing a raised foundation versus a poured slab.

The addition sizes are pretty simple, basically two rectangular areas. One is an extension to a kitchen that is about 17 ft x 8 ft. The other is 30 ft x 17 ft. The two areas are sperated on opposite ends of the property ( a patio is in between).

Cost difference ?
Is one easier for architect meaning he will select since less of his time to make drawing?
Easier to poor one versus the other ?
In earthquake area, what happens if quake wastes the slab ?

I’m doing alot of the work (framing, elect, plumbing) myself but will job out the foundation part. I really want a raised foundation since it gives great access under the house for plumbing maintance, mods, wiring etc.
thanks

Edited 10/10/2009 11:38 am ET by KWOLSEN <!– KWOLSEN –>


Edited 10/10/2009 11:43 am ET by KWOLSEN

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  1. Hudson Valley Carpenter | Oct 10, 2009 07:13pm | #1

    Raised foundation is better in earthquake territory.   I'm currently living in such a house, built in the 50's, in the San Fernando Valley. 

    During the North Ridge quake, many of the slab houses in the Valley suffered serious damage and were red tagged.  None of the houses in this nearby tract, all on raised foundations, were damaged at all.  

    1. KWOLSEN | Oct 10, 2009 08:15pm | #2

      good point, thanks for the info, I'm in nearby Arcadia just ready for the big one.

      1. Hudson Valley Carpenter | Oct 10, 2009 11:03pm | #3

        I've been in this house for numerous good sized "rollers", the type of quake which comes through like a series of small waves.  That's where you really understand how a raised foundation works better than a slab, because the house just follows the energy of the waves. 

        A monolithic slab couldn't do that.  Each wave would toss the house up and down without any give from the structure.

         

  2. oops | Oct 11, 2009 12:43am | #4

    What is a poured concrete raised foundation?

    1. Hudson Valley Carpenter | Oct 11, 2009 01:29am | #5

      What is a poured concrete raised foundation?

      Is the word "raised" throwing you?  That description is added for the typical home buyer, to help him/her understand the difference from a slab on grade foundation.  

      1. oops | Oct 11, 2009 02:02am | #6

        If it is raised, what is it sitting on? In what manner is it raised? Is it sitting on some type of stabilized fill, on piers or what. Is it a slab sitting on some type grade beam that is sitting on footings or piers? Or is it just floating up in the air. I can see that if it is floating on air, that it would work well to withstand an earthquake.

        1. Hudson Valley Carpenter | Oct 11, 2009 02:10am | #7

          You're over thinking it.  It's just a typical concrete foundation. 

          1. oops | Oct 11, 2009 02:49am | #8

            In my part of the country a typical foundation (over simply stated) is a reinforced, monolithic concrete slab on grade. Is that the same as a raised poured concrete foundation.
            I'm really not trying to be stupid. It's just that I am not familiar with that terminology and was curious.

          2. Hudson Valley Carpenter | Oct 11, 2009 03:21am | #11

            OK, a poured concrete foundation follows the perimeter of the building.  It begins with a concrete footing, two to three times the width of the foundation wall. 

            The footing is poured below the frost line, on compacted soil or gravel, with integral rebar.  The rebar is bent 90 degrees and it protrudes vertically out of the center of the footing so as to tie into the foundation wall. 

            After the footing has set up, the foundation wall forms are set in place.  Then the concrete is poured to the proper height. 

            After the forms are removed, the wood floor is the first thing to be built on the foundation.  The floor can be a couple of feet above the ground (crawl space) or there can be a full basement under it. 

          3. KWOLSEN | Oct 11, 2009 03:34am | #12

            thanks for the info, was wondering if they ever pour the footing and foundation wall at the same pour ? I have carpenter/concrete worker brother who will be helping but more info the better. He builds large concrete projects,not home additions, and assume my architect will show the foundation details on his drawing. Also, how do they tie new foundation into existing foundation, drilled holes with that 2 part expoxy stuff with rebar or what?
            thanks.

            Edited 10/10/2009 8:35 pm ET by KWOLSEN

          4. Hudson Valley Carpenter | Oct 11, 2009 05:31am | #14

            thanks for the info, was wondering if they ever pour the footing and foundation wall at the same pour ?

            Yes, that's one way to do it but only when the foundation wall is relatively short.  SoCal is one place where that is the case. 

            The footing is formed first, then the rebar is set in place, then the foundation wall form is suspended over the open footing form on speaders.  

            When the concrete is poured, care must be taken to keep the wall form in place and plumb. 

            I've never done a foundation that way but it appeals to me because the foundation is stronger as a single pour and becase it requires less work.

            Also, how do they tie new foundation into existing foundation, drilled holes with that 2 part expoxy stuff with rebar or what?thanks.

            One good method is to drill muliple holes in the old concrete, then put smooth steel pins in them, sticking out as far as necessary, six inches or more.  The steel pins are like rebar, just smooth so as to be strong and tight in a drilled hole.  The steel pins are cut from longer stock.

          5. 6bag | Oct 11, 2009 06:22am | #18

            smooth pins are called slip dowels.  they should be greased or a cap should be added.  they are used to allow movement when warranted.  if you don't want any movement you should use deformed rebar as pins.  the deformation allows the concrete to bite on the pin, this will minimize movement.

          6. designbing | Oct 11, 2009 11:08pm | #23

            I also live in Arcadia.  To answer your question I have done single pours and separate pours.  My perference is single.

            Bing

          7. brownbagg | Oct 11, 2009 11:49pm | #24

            since this is mainly a carpenter site, there is no way anybody can convince someone that a slab on grade is better. the idea was with a crawl space maintance on ulity is possible. Once the slab is place maintance on ulity are not needed. Only thimg under there is sewer pipe.

          8. oops | Oct 11, 2009 04:58am | #13

            It's not like I am not versed in construction and terms. I've been in the business over 45 years. Long enough to know that types of foundations are regional. I simple never heard that term and thought maybe that it was unique to his local.
            Around here that type of foundation would be called a "pier and beam". A perimeter beam on a footing or piers with interior piers supporting a wood floor system.(rarely used in these parts in that our frost line is zero inches and monolithic slab on grade is less expensive. Also common here is post tension slab foundation. Where steel cables are used as reinforcing in lieu rebar. The cables are tensioned a few days after the pour by tightening the cables hydraulically.
            Anyway, thanks for your time and interest.

          9. Hudson Valley Carpenter | Oct 11, 2009 05:38am | #15

            In earthquake regions, like SoCal (Southern California), multiple piers are an advantage.  This1250 sqft 1952 house has about twenty piers under it.  They allow the house to rise and fall gently during quakes. 

            Edit: I see you in Ft Worth.   I lived and worked there for a couple months, back in 1981.  Here's a link to a story I told on BT about how that came about, and some funny results.  http://forums.taunton.com/tp-breaktime/messages?msg=125363.16

            Edited 10/10/2009 10:42 pm by Hudson Valley Carpenter

          10. oops | Oct 11, 2009 06:01am | #16

            Good story. Nothing like having your victim schedule your heist for you.

          11. Piffin | Oct 11, 2009 04:06pm | #20

            and a well told story it was! 

             

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

          12. Hudson Valley Carpenter | Oct 11, 2009 10:13pm | #21

            Thanks Paul.  I imagine that when you and I eventually get together, we'll have a fine time, swappin' stories.  And making some new ones come to life too.

          13. Piffin | Oct 11, 2009 11:02pm | #22

            Look forward to it. 

             

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

          14. florida | Oct 11, 2009 06:08am | #17

            Around here a raised concrete foundation is a typical foundation with courses of block as high as you need to get. That's back filled, compacted and then a 4 inch slab on top.

    2. KWOLSEN | Oct 11, 2009 02:54am | #9

      just wanted to be clear that it wasn't some cinder block type foundation that I've seen. my definition of a poured raised foundation is one that is made with wood form work, then filled with concrete.

      1. oops | Oct 11, 2009 03:03am | #10

        Thanks for the reply. I just had not heard that terminology before and was curious.

        1. Piffin | Oct 11, 2009 03:55pm | #19

          glad you worked them over on this to save me the trouble. To me, a raised foundation is a block wall, crawlspace, and wood framed floor.When he said raised poured concrete, I had to assume that the 'crawl' space had to be filled with compacted soils, or that the walls were poured concrete, but the floor was still wood frame with a crawl under it. One of those things where terminology used assumes everyone knows, but is different for various regions.To the OP first question, I almost always prefer to emulate same as existing foundation, because adding on with a different can cause settlement / movement issues between the two portions. 

           

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

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