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  1. #1
    Join Date
    Mar 2005
    Location
    Illinois
    Posts
    138

    Default Building three season room on post/pier w/slab on grade

    Howdy all....my first post. For starters I'm just a carpenter, not an engineer or architect type. I've been lurking for a while and really enjoy reading here....I learn far more here than I do on the job site on any given day.

    I'm gathering information to build a three season room on my house this summer. The plan at this point for reasons of simplicity and cost is to build on post and pier. The addition will sit roughly 48 inches above grade (attached to house) and the piers will extend below frost to roughly 42 below grade (I live in the 'burbs around Chicago).

    I'm interested in pouring a slab under the addition and enclosing the 16'x20' area with 2x6 insulated walls (secure, dry storage). My concerns and reason for this post are with the slab.

    In what way should I handle the slab so I don't have any issues with heave from frost?

    I'm assuming a turndown slab with rebar in the edges and running in the pier holes too? If this would work, any suggestions as to how thick the slab should be (thinner is better for cost purpose (just storing boxes and such underneath - nothing heavy -), however I obviously don't want too thin where I'll have problems)? What about how thick in the turned down edge?

    I know these questions will be answered at some point by either my village when I drop off the plans or by my architect, but I'm trying to get a jump on things...besides, I'd like to learn about it first as I'm not at all familiar with the foundations/footings aspect of building.

    Thanks a bunch.

    Tom

  2. #2
    Join Date
    Jun 2004
    Location
    Central SD
    Posts
    606

    Default Re: Building three season room on post/pier w/slab on grade

    A lot will depend on your building officials. This may be termed as non standard construction for your area, and require an engineer. A slab supporting walls,even if the walls do not support the sun room, may be required to be on the foundation. A technique I have used several times is to drill your piling( 12" /18" dia. apx 6' apart) Pour a grade beam over the top of the piling. This can be integral with the slab.This way the piling are supporting the floor as well as the sun room away.

  3. #3
    Join Date
    Mar 2005
    Location
    Illinois
    Posts
    138

    Default Re: Building three season room on post/pier w/slab on grade

    Thanks dakota...this sounds a little like my plan of a turned down slab on grade. The slab would be maybe 12" or 18" in thickness at the edges and would basically be supported by the piers (12" diameter, 6 total piers, 3 under the rim joist and 3 at mid-span for the support beams). My piers would be poured up to grade. I was planning on a single pour (monolithic?), piers and slab all at once.

    Like you mentioned, the walls are not supporting the room and in fact will not have any bearing on them at all. My concern is if the slab heaves at any point between the piers...that would be a major. I thought a 12" thick edge with rebar all tied into the piers would be sufficient to resist heave from frost...I'm not so sure now.

    I don't quite follow the idea of the grade beam, I'm not real sharp when it comes to foundations and such, definitely still feeling my way along. I've only this past week learned about turndown slabs on grade;)

  4. #4
    Join Date
    Sep 2004
    Location
    Rimbey, AB, Canada
    Posts
    333

    Default Re: Building three season room on post/pier w/slab on grade

    To prevent the frost from heaving the grade beam or thickened edge slab between the pilings you would use void form. It is a waffled styrofoam panel that you would pour over top of. When the frost lifts the soil below the beam, it will crush the void form instead of lifting the slab. This means that all load must be carried by the pilings. Typically a 2' grade beam between the pilings will support the intermediate loads. But as always, check with a local expert who is familiar with your specific situation.
    ____________
    Darren Dolman

  5. #5
    Join Date
    Nov 2004
    Location
    central NH
    Posts
    67

    Default Re: Building three season room on post/pier w/slab on grade

    I'm in New England, with different conditions than Chicago, but NOTHING can prevent frost from lifting something if it gets underneath. I would strongly recommend a continuous frost wall. (4'-0") A little more $$ but a much better bet. Then build wood walls on top of that up to the room floor.

  6. #6
    Join Date
    Feb 2005
    Posts
    237

    Default Re: Building three season room on post/pier w/slab on grade

    water expands by approximately 10% when it freezes. So for anyone to make a generalization on the heaving condition (good or bad) is foolish.

    Depending on your soils you will or won't have a problem. Certainly the safe bet would be the better but w/ concrete at $80 yd^3......... who knows.

  7. #7
    Join Date
    Mar 2005
    Location
    Illinois
    Posts
    138

    Default Re: Building three season room on post/pier w/slab on grade

    Thanks all for the input.

    I'm getting real apprehensive about going with the turn down slab on grade now. I'll spend this next week talking with my architect, village engineer and my concrete guy and see what they think.

    The slab isn't critical for me, it just meant a solid footing for me to build some walls on and secure dry storage too...if the slab doesn't seem like it'll work I'll have to figure some other kind of ground cover/floor.

    I'll drop back in after I hear what they have to say.

  8. #8
    Join Date
    Mar 2005
    Location
    Illinois
    Posts
    138

    Default Re: Building three season room on post/pier w/slab on grade

    To keep my cost down I've decided against the slab on grade under the sun room. The area under the room was only for storage and I can achieve a clean dry storage area less expensively by simply framing in a wood floor. Concrete would undoubtedly be nicer but would also be much more expensive and require much more work.

    I'd like to ask a second question unrelated to the slab if I could.

    This sun room is being built on post and pier with the floor roughly 50 inches above grade. It is 16' wide and extends 20' out from the existing house.

    The floor will be framed with 2x10's and the piers will be 6x6's. Both the top and bottom sides of the floor will be sheathed because it will be insulated. The walls will be sheathed with ½ OSB and the interior walls with ½ drywall.

    The room is mostly windows and the 6/12 roof has 6 30½ x 55" skylights in it. It has a gable roof (scissor trusses) that will tie back into the main roof with an overlay. The ridge heights of the sun room and main roof are within a few inches of being the same height.

    My plan is to support the room with 6 6x6's. Three at mid-span (8ft off the house) and the other three on the outside rim (16ft off the house). I'll notch the 6x6's so a double or triple member (girder) 2x8 or 2x10 can sit on the 6x6's and support the deck.

    It was mentioned to me that I might have a problem with sheer because A) the room is build on "stilts" and I don't have a slab or foundation to tie the walls into B) the roof is scissor trusses C) being mostly windows I don't have the strength from being mostly sheathing.

    I would have assumed with the walls and roof tied into the walls and roof of the existing home (and sheathing both sides of the floor) that I would not have any issues with wind causing the building to shift or move or twist.

    Will sheer be a problem with the way I'm building this room? I'll try to attach a sketch I made to better illustrate my concern.

    Thanks.

    I had problems uploading the .dwg file I had so hopefully my description will suffice for the moment.

  9. #9
    Join Date
    Jul 2004
    Location
    Western suburbs of Chicago
    Posts
    5,554

    Default Re: Building three season room on post/pier w/slab on grade

    Are you going to be enclosing the underside of the porch? Seems the biggest issue might be the potential for sway if the posts aren't braced. Is this the shear issue you're worried about?

    If so, you can put diagonal bracing in from post to post, or you could maybe enclose the area beneath the floor with kind of a sheathed curtain wall that is tied into the rim joist & posts.

    Either way should limit the potential for sway. Just depends on the look you want.

  10. #10
    Join Date
    Mar 2005
    Location
    Illinois
    Posts
    138

    Default Re: Building three season room on post/pier w/slab on grade

    Quote Originally Posted by gburnet
    Are you going to be enclosing the underside of the porch? Seems the biggest issue might be the potential for sway if the posts aren't braced. Is this the shear issue you're worried about?

    If so, you can put diagonal bracing in from post to post, or you could maybe enclose the area beneath the floor with kind of a sheathed curtain wall that is tied into the rim joist & posts.

    Either way should limit the potential for sway. Just depends on the look you want.
    Thanks gburnet...and yes, I am concerned about sway because I really didn't want to diagonal brace the posts below the room because it would take up much of the storage space below.

    I am enclosing the area below with insulated 2x6 walls and some sort of plywood or gravel floor, haven't decided which yet. Those walls would definitely be tied to the 6x6 posts and the rim joists.

    I hadn't really given much thought to the room swaying because I figured with it being tied into the main structure (roof and walls) it would be fine...a friend of mine is who mentioned sway and said I might have some problems.

    Unless anyone else here really thinks this might be an issue, I'll assume I'll be alright.

  11. #11
    Join Date
    Mar 2005
    Location
    Illinois
    Posts
    138

    Default Re: Building three season room on post/pier w/slab on grade

    Well, I guess one of the advantages of constructing something yourself is you can change your mind countless times before you begin and you are only wasting time, not money.

    I'm heavily leaning again toward a foundation with a slab at grade. It's been pointed out to me that you can dig a trench down to frost and pour directly in the trench for the foundation walls...and then continue to pour the slab in a single pour.

    For myriad reasons I would prefer this option, cost was the single biggest factor against it. I have a old buddy in concrete who recently started his own operation coming out this week to give me a estimate.

    One of the concrete guys at work figured for a side job, on the high side, maybe close to 2 grand. The job calls for roughly 10 -12 yards of crete. His guesstimate included a 4" 16'x20' slab and three foundation walls (two at 20' and one at 16') dug 42" down, maybe 10" wide. He said that would include whatever pea gravel fill was needed as well as framing the slab up.

    Having an old cronie do the work and working along side him...I'm hoping the cost may be even less than that. Anyone out there familiar with this type of construction work think this guesstimate is in the ballpark?

    Tom

  12. #12
    Join Date
    Jan 2005
    Location
    Victoria, BC, Canada
    Posts
    28

    Default Re: Building three season room on post/pier w/slab on grade

    I am completing a very similar project. I went with a center engineered floor beam and eng Ridge beam. You can view it at http://www.aye1.com/569leaside/569.html to see where I am at (planning some short diagonal bracing at the tops of the support posts)
    Steve Mahovlic, Carpenter

    to view my latest project - www.aye1.com

    "He who is wise will not criticize
    when other men fail at the game" - Gordon Lightfoot

  13. #13
    Join Date
    Dec 2004
    Location
    NW Iowa
    Posts
    83

    Default Re: Building three season room on post/pier w/slab on grade

    Getting in a little later here, but that wont stop me form adding my $.02, hehehe

    I have done this type of addition, only with 6 feet underneath. I did it with 3 midwall and 2corner columns. Sway was defiinitly a potential problem. I poured the footings below frost line, 3'X3' x 12 deep at the mid point in the wall, and the corners, Then I poured 5, 12"X12" by 10' tall support column on top of the footings to outline the building. This eliminated the sway factor much better than wood could ever do. Then I used double microlams for rim joists, and tied the floor joists (tjis) onto the microlams, with hangars.

    This is completely open underneath. I installed 1 inch of foil faced foam, sealed to the bottom of the top chord of the tji, which gave me a 1 1/2 inch air space, above it, then added 9inch insulation into the cavity, and finished it off with soffit plywood on the bottom. The air space allowed me to run water lines out above the air barrier(foil faces) in conditioned space. The finish floor was topped off with hydronic infloor heating and 1- 1/2 inch of concrete for cover, then floor tile. The drain was built into a small conditioned chase below the foil foam, but completely sealed from the insulation.

    In your case, since you want to enclose it, and have a slab underneath, why not look into a shallow frost protected foundation. It seems to me to be a perfect situation to utilize this techniqe, without being completely disruptive of the surroundings.
    Last edited by Rollie Peschon; 05-08-2005 at 07:23 AM.

  14. #14
    Join Date
    Mar 2005
    Location
    Illinois
    Posts
    138

    Default Re: Building three season room on post/pier w/slab on grade

    Quote Originally Posted by Rollie Peschon
    In your case, since you want to enclose it, and have a slab underneath, why not look into a shallow frost protected foundation. It seems to me to be a perfect situation to utilize this techniqe, without being completely disruptive of the surroundings.
    Heya Rollie,

    Honestly I'm not real familiar with different types of foundations and I'm definitely learning this on the fly. I'm trying to familiarize myself as much as possible before my concrete guy comes out to give me ideas and estimates.

    Would you explain in just a little more detail what a shallow frost protected foundation amounts to? The more I learn before my guy comes out the more options I'll have and with more options comes a greater likelihood of getting exactly what I need.

    Thanks,

    Tom

  15. #15
    Join Date
    Sep 2004
    Location
    Easton,MA.
    Posts
    437

    Default Re: Building three season room on post/pier w/slab on grade

    Check your building code book, here in Mass. the info is in the code book.

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