Fixing a hole ...

[David Nebenzahl]

... where the rain gets in (with apologies to Lennon & McCartney).

I need to fill a hole in a garage floor. But it's an odd situation, and I'm not sure what the proper way to do it is. I ask here with much trepidation: tried to find answers locally, but I have no "go-to" guy (or gal) I can get good answers to these kinds of questions from. So I'll take my chances here. (If you *must* speculate without actually knowing what the hell you're talking about, go ahead: I'm hoping to get at least one or two answers from knowledgable people.)

The garage floor in question is actually above a room below. It's in a hilly part of Oakland; the garage is level with the road in front, but the ground slopes down sharply, and there's a room built below the garage. (The garage is actually only a roofed area with no walls.) The occupants are planning on finishing the room below, so this hole needs to be filled first.

There's a hole built into the floor, apparently made for the purpose of working on a vehicle from below. (Kinda weird, as it's almost 8' above the floor, requiring a ladder to reach.) It's covered with a wood (2-by) cover. The cement slab, which is about 4" thick, has a ledge molded around the edge of the hole to hold the cover in place.

The hole is about 3 by 4 feet.

The end result needs to be able to support the weight of a person or an automobile. It will probably *not* be driven over much (the existing cover is never driven over by the current occupants of the house), but it obviously needs to be able to withstand this weight.

Climate is mild, rarely freezes.

Sketches:

Here's what I *think* I should do:

o Drill holes into the edge of the slab, using rotary hammer & SDS bit. o Epoxy rebar dowels into the holes, long enough to overlap at least 18" or so. Bind the dowels together with wire. (How many dowels? spacing?) o Add wire mesh over rebar? (What type?) o Add new joists across the short (3') dimension of the hole, using joist hangers, plus cleats at the edges. (The existing hole is completely framed by joists.) Two new ones should do it, giving a spacing of about 16". o Place plywood (1/2"? 3/4") on top of the joists and cleats. o Pour concrete into the hole. Finish top surface smooth to match existing floor.

So, whaddya think? Will this work? I'm not sure I'm up to finishing the concrete myself: learned that skill years ago on a construction crew, but I'm not at all in practice. But the rest I can do.

[David Nebenzahl]

On 7/9/2010 7:16 PM David Nebenzahl spake thus:

Before anyone pounces on me, the order above is obviously wrong. Correct sequence would be 1-4-5-2-3-6.

[RicodJour]

I don't quite understand what's going on with the wood joists below the slab. Are they full length and adequately supported at both ends? Or is it just a sort of boxed frame at the hole?

If the existing 4" slab can take a vehicle (how?), the trick would seem to be patching the hole and keeping vehicles off of the patch. If you can accomplish that, then I'd probably just pick up some metal decking (you only need a smallish piece, so you could buy a damaged sheet that couldn't otherwise be sold), cut it to fit tightly in the hole, seal the edges/gaps with spray foam, and place the concrete.

There are a number of advantages to this approach, not the least of which is you'd use less concrete and the patch would be a lot lighter.

I'd bolt down some barriers/guides to make sure that a vehicle would not drive on the patch.

R

[David Nebenzahl]

On 7/9/2010 7:54 PM RicodJour spake thus:

No, they're fully supported. About a third of the slab above is over the room below, supported by the joists (about 2x10 as I recall).

So you're saying the patch could be less than 4" thick with the metal decking?

Could do, although they (occupants) might not like that. Are you saying I'm not going to be able to make a patch strong enough to drive over?

[Evan]

Well, since this is clearly an advanced structural issue, why not consult a licensed structural engineer in your jurisdiction...

The concrete being used for the slab floor of the garage or carport or whatever is more than likely better than the ready mix you will find at a big box home repair store...

You need to find out what kind of concrete (what load rating) is in use in the structure and match that in any patch you attempt to make...

Your proposed ideas are sound, yet you should have your plan endorsed by someone with credentials before you create something which could potentially collapse and hurt people in the future due to a variety of situations you have not considered...

~~ Evan

[Colbyt]

California, earthquakes, old non-compliant construction, those are the top 3 reasons that I suggest you run fast away from this.

I can not imagine residential construction, (my assumption), where that room would ever have be intending for finishing and I very much doubt that it can pass the muster now.

I am not convinced that your plan, while extremely well thought out, will meet the codes. Personally I would not get involved in this without a written certification from a structural engineer.

If you feel the need to go ahead, I second the suggestion of another poster to use plate steel. The road crews temporarily cover some fairly big trenches around here with 1/2 or 3/4" steel plate on the roads where the big rigs drive. You would need to confirm the thickness as I have never stopped to measure it. -:)

[HeyBub]

Leave the hole.

Put in a circular staircase (or trap door).

"It's a FEATURE!"

[Pete C.]

While I don't agree with using a steel plate for this (I think properly applied concrete and reinforcing is best), I do think you have a *lot* of other code compliance issues to address on this one. Items that come to mind include sealing the entire floor area so that no spilled gasoline or gasoline vapors can make their way to the room below, and ensuring that the supporting joists are actually adequate for the weight of the slab + vehicle. 2x10s don't sound correct to support 50# per square foot of concrete plus two? 1,000# per square foot concentrated loads from the vehicle tires.

[hrhofmann]

How do you manage to get involved in impossible situations like this?

[LdB]

In earthquake country? Don' even think about it without approval from a structural engeneer and the inspectors. You want to take the responsibility for a car falling on someones head?

LDB

[David Nebenzahl]

So I'm thinking along a couple of lines here:

1. Metal plate might not be a bad way to go. But instead of plopping the plate over the hole, with brackets to hold it in place, I'm thinking of a plate flush with the floor, built up to the height of the wood cover (~1-1/2")) along the edge. I'm planning on getting a price on this Monday. Once fabricated, I could just caulk the hell out of the ledge, place the plate, then caulk the gap to seal it.

1. I'd like to get a couple bids from concrete contractors to fix the hole. I'll both find out how they'd do it, and also get prices: it might turn out to be cheaper overall to just have them do it, rather than do it myself.

At this point, the only thing that gives me pause is the possibility of weakening the existing slab by drilling into it for rebar dowels. Everything else seems like overkill, and the people I've talked to locally agree with me on that.

[cshenk]

"David Nebenzahl" wrote

This wouldnt add substantial weight and would fill the hole but probably would not support a car.

Code spec issues come up as well as liability. It doesn't sound to me like the setup is code-spec now but it may be ok as the room under isn't living space. I'd have that checked out. This could be a very costly mistake if done wrong and someone later is hurt.

[Wayne Whitney]

I'm pretty sure that if you drill an oversize hole (e.g. 9/16" for

1/2" rebar) and fill it with a structural epoxy adhesive, you won't be weaking the existing slab and it may end up stronger than previously. However, this is a definitely a job that calls for an engineer, so stop beating around the bush and hire one.

Cheers, Wayne

[RicodJour]

I don't know how the existing condition meets any code in the first place. A suspended 4" slab to support a car? Wood joists to support concrete? The whole thing is a kludge.

The liability issue should not be ignored. I'd rather walk away from a little bit of money, than open myself up to a boatload of liability. Can you imagine the lovely glossy picture enlargements, showing the car covered in debris under the ex-patched hole? A jury would love them.

R

[DerbyDad03]

re: "... where the rain gets in (with apologies to Lennon & McCartney)."

Ya know...those same 2 gentleman refer to "car crashes" in a couple of other songs they've written.

Perhaps you will be quoting from those songs a short time from now.

re: "The end result needs to be able to support the weight of a person or an automobile."

A person *or* an automobile? Quite a range there, don't you think?

Seems like this needs some serious code-related advice from the local authorities. Fumes, liquids, structure, insurance, weather, what else?

This is more than just a "fixing a hole", this is a full fledged engineering project.

[Tekkie®]

DerbyDad03 posted for all of us...

Fumes are a valid concern! I think this is beyond the scope of the OP and requires an on site engineering design/review.

This sounds like what happens in the Philly area when the "low bidder/buddy" gets the job to demolish or repair a structure by untrained or unknowledgeable people. He may or may not be judgment proof because of lack of funds but does this excuse one morally?