Garage slab elevation standard

[CJSpud]

Question:

 

Is there a "standard" attached garage slab elevation that is "typically" used in the home construction industry for a home that is using 11-7/8" I-joists and 23/32" sub-flooring (3/4" nominal)? 

 

Should the slab elevation be set based on the recommended 7" stair riser height (±) or does it more have to do with how much stemwall curb is desired to keep the wood framing above the slab surface in case the owner wants to hose down the slab for cleaning?  What are Chief designers/architects using for the garage slab elevation?

 

I do mostly ICF designs with those the garage slab is usually the same height as the subfloor.

 

Thanks.

[chief58]

The garage slabs around here is determined by the final grade and to have a few cmu or conc fnd walls exposed so if someone wants to wash the car or hose down the floor its not against the wood part or the garage, when I built my house it was a site decision and I have 2 setps and a landing to my man door to the housee

[parkwest]

If you go 6" below the top of foundation wall at the back wall with your slab, and then slope the garage slab 2" down toward the garage door, you should end up with the required clearance between your final grade and the wood framed garage wall.

[CJSpud]

Thanks Ray.  I was sort of coming from a fairly level site scenario with my question. I realize that site conditions would certainly affect where an attached garage's slab, stem walls and footings would need to be placed based on the site grade elevations.  So I imagine the goal is to try to set the slab elevation such that you wouldn't have to haul in a bunch of engineered fill and then compact it to 95% compaction? 

[chief58]

5 minutes ago, CJSpud said:

Thanks Ray.  I was sort of coming from a fairly level site scenario with my question. I realize that site conditions would certainly affect where an attached garage's slab, stem walls and footings would need to be placed based on the site grade elevations.  So I imagine the goal is to try to set the slab elevation such that you wouldn't have to haul in a bunch of engineered fill and then compact it to 95% compaction? 

Curt this is true most of our garages are set on the high side of the property to as we can have some room for adjustment of the garage floor and the footers are just trenched footings so we don't have to bring in a lot of fill for there

[CJSpud]

David:

 

Sounds like your talking 20-1/8" from top of subfloor to top of slab next to the house.  If the landing in the garage is at subfloor height, that works out pretty nice for stair riser height (3 @ 6.71").  I think that might work out nicely for the stick-built house I am working on.  The site isn't surveyed and there is some slight slope from the left side of the garage to the right side.  At worst I think only one more step would be needed .... unless some fill is placed inside the stem walls on the downhill side and compacted.

[CJSpud]

Ray:

 

I think this project's garage floor will need a little extra base brought in due to the slope of the lot.  Is there a minimum curb height on the high end of a garage slab that is commonly used in your area or is it all over the map as far as what builders are doing?

[Doug_N]

Here in Ontario, there must be a minimum of 6" between the grade and any siding or wood.  Although this doesn't address the original question it does set a starting point for planning floor level elevations when planning a floor layout.  I would suggest two risers between the ground level floor and the garage floor.  This way there is not a big encroachment of stairs into the garage space, and in most cases no handrail requirements either.  

[javatom]

I use -21".  It satisfies the 6" foundation exposure requirement and makes for easy math on the risers (3 at 7").

[chief58]

1 hour ago, CJSpud said:

Ray:

 

I think this project's garage floor will need a little extra base brought in due to the slope of the lot.  Is there a minimum curb height on the high end of a garage slab that is commonly used in your area or is it all over the map as far as what builders are doing?

the height is all over the map here, we start as if we were going to put the garage on the high side of the lot and stake out from there and then determain the garage floor height, the foundation walls most always are the same at main foundation and garage height and floor doped down from there

[CJSpud]

Thanks for the comments on the 6" exterior finish exposure. 

 

So my garage rough ceiling elevation is 109-1/8" by default so my garage roof eaves etc. match the house.  When I change the garage slab floor to -21" below the subfloor, the room specification graphic shows the stemwall at the same elevation of the slab (-21").  The "SWT To Ceiling" setting changes to 130-1/8" (i.e., 109-1/8" + 21" = 130-1/8"), which matches the new "Rough Ceiling" elevation in the Room Specification dbx.

 

 

I want to change the "SWT To Ceiling" setting from 130-1/8" to 123-1/4" which will give me a curb height for the foundation stem walls in the garage of 6-7/8" which will match the top of the basement foundation walls.  For the Floor 1 Room Specification dbx for the garage, it looks like this should be a simple undertaking by changing the value of the "SWT To Ceiling" setting to the desired 123-1/4" setting.  However, when I do that, the ceiling moves down to 102-1/4" while the SWT stays exactly where it was ... even with the top of the slab at -21".

 

 

If I go down to Floor 0 and open up the garage Room Specification dbx, the settings are different and it appears that I have two different options to move the top of the stem wall up to 123-1/4" as desired:

     1.  Change the Floor To SWT setting from 0" as shown to 6-7/8" (21" - 12-5/8" - 1-1/2" = 6-7/8"), or

     2.  Change the Stem Wall setting from 26-7/8" to 33-3/4" (26-7/8" + 6-7/8" = 33-3/4").

However, trying  option 1 results in the slab dropping the added 6-7/8" while the stem wall top stays where it was rather than going up.  This makes the ceiling height in the garage go to 137", not the 130-1/8" it was suppose to be, and the overall height of the stem wall remains at 26-7/8" as shown below:

 

 

Trying option 2 just makes the stem wall taller but the top stays fixed at -21" while the bottom and footing are lowered by the 6-7/8".

 

 

I did go into my foundation defaults and change the setting for "Garage Floor to Stem Wall Top" to 6-7/8" but that didn't seem to work any magic for me in the Room Specification dbx changes I have been attempting to make.

 

 

So I am apparently having another senior moment trying to get the garage floor and stem walls setup the way I want them [top of garage slab @ -21"; top of garage stem walls @ - 14-1/8" or 6-7/8" above the garage slab].  I would prefer to use the settings in the dbx's if possible rather than trying to move wall tops/bottoms around in elevation cross section views.

 

Can anyone spot something really obvious about where I am screwing up trying to make these changes? 

 

PS:  I may be wrong about my desired stem wall top elevation of -14-1/8".  If the sill plate is part of the stem wall calculation, then I probably should be using -12-5/8".

 

 

 

 

 

 

 

 

 

[CJSpud]

Eric:

 

Let me see if I can do that.  My project file is 36 Mbs before zipping or stripping.

[CJSpud]

Eric:

Here's a simple plan.  Everything seems to be working when it is setup correctly from the beginning.

 

GarageSlab&StemWalls.plan

 

One thing I didn't do with my project plan is put a check in the "Automatically Rebuild Foundation" box in the Foundation Defaults > Foundation dbx.  I am a little hesitant to do that because of some editing I've already done to the plan regarding some interior basement bearing walls.  Maybe I just need to bite the bullet and do that and see what happens.

[CJSpud]

Eric:

I went ahead and did the auto rebuild and that fixed the garage floor and stem walls to what I wanted.  Now I need to go back and do some editing again of some steps I had in the garage stem walls; redo the basement walkout wall to include extra depth for frost protection; and fix the location of the terrain ... it seems to have gained a foot or so in relation to the building.

 

[javatom]

If I understand your foundation question correctly, you control the stem wall height with item C in the DBX

[Doug_N]

In X11 the dialog box and stem wall height over the garage floor looks like this.

 

 

[CJSpud]

javatom:

 

As Doug pointed out in his last post above, and as shown in my images, Chief has reworked the room specification dbx'x (as Doug shows in his post too).  I actually like the graphic much better now.  For whatever reason, I could not make changes to the values I noted in my post (and images) without messing up some other setting.  By auto rebuilding the foundation, I was able to get the garage stem walls with the curb height I wanted.  That finicky behavior cost me a bunch of time and work today.  Moral of the story ... set up your defaults properly ahead of time to avoid situations like this.  Unfortunately for this project, it was initially set up for ICF construction and with that I didn't need any garage curb heights, etc.  Making the switch to stick-built was not such a good thing.  We really should have some flexibility with making changes in the room spec. dbx for garages without having to rebuild the entire foundation IMO.  Guess I should put that in the suggestion box.  Unless of course, I am totally missing something ... which is entirely possible.