Pouring a Slab Overlay Question

[julian abram]

We are going to convert our back porch into a sewing/hobby room for the wife. The porch is 10'x12' on a slab which is 4" lower than the main house slab. My question is about the necessity and technique of bonding the overlay to the old slab. I know in most construction the process is to drill and epoxy rebar into the adjoining slab but that kind seems like overkill for a small slab project like this. I thought maybe just drill some rebar pins down into the old slab floor then pour the overlay on top.
Any thoughts about bonding these slabs together?

[Ralph Boumenot]

Why not put down pressure treated floor joints on top of the old slab? It would be quicker and cheaper than pouring a new slab. And it would be for easier to level a wood floor.

[Wayne Lomman]

Julian, a 4" overlay slab is fine - just drill in some pins as you said and put in a layer of reinforcing mesh. Cheers

[Chris Padilla]

Head over to "johnbridge dot com" and either ask them or do some searching there for bonding old to new concrete--they are the masters over there. It isn't quite as clear-cut as you might think but it does depend on, perhaps, the type of flooring you might want to put down and how that flooring will bridge from your home's concrete to your new sewing/hobby room concrete. For example, if you want tile bridging, then you must honor the transition joint otherwise the tile will most likely crack because the new concrete will move more than the old concrete. There are also products you can put down to help old bond to new concrete. In some cases, it can be just good old thinset but I suspect not in your large area case. There are paint-on type products for that.

[Jerome Stanek]

Why do you need to bond it 4" is the same as poring over stone. I pored a 3.5" slab over a slab 45 years ago and it still looks like new.

[Fred Perreault]

I would second putting down some sleepers over the old slab and having a wood floor. Softer, warmer and easier to install, and you could pump some foam insulation into the cavities. If not, a new 3.5" to 4" slab overlay would work fine, and even better if you could use fibermesh concrete which is very crack resistant.

[julian abram]

Fellows, thanks for sharing your thoughts on this, I'm kind of kicking around the sleeper ideal. I guess I would use PT 2x4's turned on edge for the sleepers with a vapor barrier under them. How would you attach the sleepers to the concrete?

[Wayne Lomman]

Julian, space your floor joists at 18" - 24" apart and fix each one down with steel brackets screwed to the side of the joists and masonry anchored to the existing slab. The function of the brackets is to stop the joists falling over and also to stop uplift of the new room due to wind forces. The frequency of the brackets depends on your local wind conditions but I would use 4 per joist if it were at my place - windy Tasmania. Cheers

[Mike Henderson]

Why not put down pressure treated floor joints on top of the old slab? It would be quicker and cheaper than pouring a new slab. And it would be for easier to level a wood floor.
I did this years ago and it worked well. I eventually sold the house so I can't give an current update.

The only issue was making sure that everything was level in all directions. The old porch was slanted for water runoff and was generally not a good reference - the people who poured it were not that accurate, I suppose because it was just a porch. It took some time to get things setup and I had to keep checking as I went.

It probably would have been faster to pour concrete - you make sure the forms are accurate and then level to the forms.

Mike

[julian abram]

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I did this years ago and it worked well. I eventually sold the house so I can't give an current update.

The only issue was making sure that everything was level in all directions. The old porch was slanted for water runoff and was generally not a good reference - the people who poured it were not that accurate, I suppose because it was just a porch. It took some time to get things setup and I had to keep checking as I went.

It probably would have been faster to pour concrete - you make sure the forms are accurate and then level to the forms.

Mike

Yes, I'll face the same issue in getting the new floor level. Since this was a porch the slab slopes about 1-1/4" over the ten feet width. If I go the sleeper route, guess I would need to use 2x6's instead of 2x4's and rip a taper on them. Not sure the sleeper method would save any time over pouring concrete. Probably six one way or half a dozen the other. More thinking required.

[Todd Mason-Darnell]

We did almost the same project as the OP 10 years ago--except our front porch was 10 x 17. We poured a slab over the top with a bonding agent and pins every 18-24 inches (1/4 rebar drilled 2" into the existing slab and coming up 3" into the new slab).

Worked great. In talking to several concrete guys, the pinning may have been overkill.

Depending on the area of the country you are in, I would be careful about the wooden sleepers. Even with insulation and a vapor barrier, a 4" cavity is a great area for moisture to get trapped and/or a place for creepy/crawlies to move in.

[Chris Padilla]

I think concrete will be more expensive overall but I think the time involved with either one will be about the same simply due to the leveling you'll need to do with tapering the 2x6s.

Sleepers is likely something you can do 100% on your own.

Concrete will likely require extra help beyond yourself.

[Wayne Lomman]

Julian, speaking with my carpenter's hat on, tapering the floor joists is unnecessary work. Just use appropriate thickness, decent sized packers under the joists at 3' - 4' intervals to bring it all up to level and then fix everything down. This is quick and easy, structurally sound and allows better underfloor ventilation. Make sure everything is vermin proof before you lay the floorboards. You will know what the local pest problems are likely to be.
I could not do it this way if I were building a house in New South Wales as the termites there are so voracious that you have to make a timber floor frame at least 18" off the ground so that you can inspect regularly. Even pressure treated timber is not a guarantee - they won't eat it but they will cross it to get to the rest of the house. However, here in Tasmania we have no invasive termites so it is an acceptable construction method. Cheers.

[julian abram]

Hey Wayne, thanks for your good input. I was looking at your location and just realized you live in the country where all those weird, crazy animals live. I'm sure I'll never get there but would love to see some of that "down under country" someday. Again Thanks!

[Chris Padilla]

Are you getting permits for this work, Julian? If so, you will find out what your city/county may require for such an endeavor. I like Wayne's idea for handling the slope...makes a lot more sense and sure is a ton easier and faster as well. However, me being me, I think along with shimming to level, I'd sister on ~1' lengths of 2x material at the shim point just for the extra support and then fasten that down to the concrete.

Do you know if you have any water percolating up through this slab? A quick test: tape down a 1 sq ft piece of clear plastic on the slab somewhere (or even at a couple spots) and see if moisture is an issue. If moisture is an issue, you may wish to install a vapor barrier first before putting any wood (even PT wood) down.

[Jerome Stanek]

Like has been stated before with sleepers there is a good chance that you will have a nice habitat for small animals and snakes that eat them also ants and termites. I would hate to step out side and confront a rattle snake