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-   -   Lower Slab Under Girders in Basement (http://www.diychatroom.com/f105/lower-slab-under-girders-basement-140864/)

benjamincall 04-20-2012 12:35 PM

Lower Slab Under Girders in Basement
 
I may have to add another girder in my basement to support an offset bearing wall above.

I also need to build a hallway along one wall of the basement. At best, the existing girder and the new girder, running perpendicular to this hallway, will leave me with a ceiling height just over 6' for a 3' stretch of hallway.

I would like to lower a ~3'x14' stetch of slab to be flush top of the footing, giving me an additional 13 inches of ceiling height. I need a minimum of about 8 inches to have a standard height doorway.

What is the most practical way to drop the slab in this area?

Jeeps 04-21-2012 11:53 AM

Wow, a tough problem you have. Sometimes getting the required head clearance in basements is not worth the efforts/cost vs. home value in the long run. jmo

Dropping the slab is simple= good back, breathing protection,fume ventilation and a concrete saw...

Problems with dropping the slab not so simple....

A 8 inch drop = weird numbers for steps, one step is too high to be legal, two 4 in risers is bad....

Many basements have water table infiltration issues. What happens if when you cut the slab, water starts coming up through it during heavy rains ? ...

A basement slab is usually poured as a monolithic pour with rebar or wire mesh in tact for a total mechanical bonding span. What happens when that bond is broken not only at the proposed 3' x 14' area, but also at the ends of the slab that is left originally ?

Lastly, will there be a issue with the local building codes having part of the slab below the footer ? I dont know these answers, just putting some things out here to consider. good luck

gregzoll 04-21-2012 12:20 PM

Jeeps, the big concern would be trip hazard. What happens if the lights go out down there and someone happens to be walking across that section, and whoops stumbles and slams their face into the corner of the other side of the slab, or stumbles and trips falling over one ledge, and stumbles while tripping over the other ledge.

Jeeps 04-21-2012 12:24 PM

Very true GregZ, lots of potential problems with the proposal.

gregzoll 04-21-2012 12:27 PM

But it would make a great place for kids to have boat races down there.

benjamincall 04-21-2012 01:35 PM

Quote:

Originally Posted by Jeeps (Post 903821)
Wow, a tough problem you have. Sometimes getting the required head clearance in basements is not worth the efforts/cost vs. home value in the long run. jmo

Dropping the slab is simple= good back, breathing protection,fume ventilation and a concrete saw...

Problems with dropping the slab not so simple....

A 8 inch drop = weird numbers for steps, one step is too high to be legal, two 4 in risers is bad....

If the BD wanted to be hard core about that one, we would either sacrifice 1/4" of height or build a ramp.

Many basements have water table infiltration issues. What happens if when you cut the slab, water starts coming up through it during heavy rains ? ...

Perhaps I would line the whole area with an impervious dimpled membrane, 2" XPS foam, and gravel.

A basement slab is usually poured as a monolithic pour with rebar or wire mesh in tact for a total mechanical bonding span. What happens when that bond is broken not only at the proposed 3' x 14' area, but also at the ends of the slab that is left originally ?

I don't know about that one. I noticed I have a small channel between the block wall and the slab along the 14' dimension.

Lastly, will there be a issue with the local building codes having part of the slab below the footer ? I dont know these answers, just putting some things out here to consider. good luck

Quote:

Originally Posted by gregzoll (Post 903837)
Jeeps, the big concern would be trip hazard. What happens if the lights go out down there and someone happens to be walking across that section, and whoops stumbles and slams their face into the corner of the other side of the slab, or stumbles and trips falling over one ledge, and stumbles while tripping over the other ledge.

That sounds like a three stooges skit. At least everything will be carpeted.

The 8-13" "ledge" ledge on either side Will have a framed wall sitting on it, so the only differene in elevation will be the single step or ramp at either end of the 14' hallway.

I'm more concerned about tall people hitting their heads when they misjudge the ceiling height. Until I tore it out of the walk-out basement, I hit my head all the time on the non-code (perhaps pre-code) stairway.

gregzoll 04-21-2012 04:14 PM

Quote:

Originally Posted by benjamincall (Post 903872)
I'm more concerned about tall people hitting their heads when they misjudge the ceiling height. Until I tore it out of the walk out basement, I hit my head all the time on the non-code (perhaps pre-code) stairway.

If your tall friends have not learned to duck by now when entering low areas, they will very quickly if they need to go down there.

benjamincall 04-23-2012 09:03 AM

Quote:

Originally Posted by Jeeps (Post 903821)
A basement slab is usually poured as a monolithic pour with rebar or wire mesh in tact for a total mechanical bonding span. What happens when that bond is broken not only at the proposed 3' x 14' area, but also at the ends of the slab that is left originally ?

I already have 1-1/2" channel that is at least 3 inches deep, running the length of that wall, so I don't think I'll be creating a new structural issue.

Has anyone created a sunken area similar to what I'm contemplating?

benjamincall 04-25-2012 01:29 AM

I think I may try something like Sani-tred, which, according to their somewhat heavy-handed literature, penetrates the concrete. Otherwise, I might try an old poly water tank or septic tank to waterproof this area.


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