I am pondering a home addition in the future and I am working on the (free ) plans right now, but I am stumped on how to make a slab to slab connection. Do I just butt the new against the old? I was thnking about saw cutting the old and making interlocking fingers with rebar rienforcment. I am worried about a hinge affect betweeen the new and old foundations, since it will be a drywalled wall. what is the standard practice? What works the best regardless of cost (and not just start all over new )

 

I would drill and dowel the connection with rebar (greased for horizontal movement). That is how the Engineers I work with usually handle the situation. Some have just specified a 1/2 " expansion joint. I would not build on or directly across the joint.....keep them independent.

 

Well, what type of footer is required out there for a foundation.... I'll give you a for instance... In Michigan, for any structure there needs to be a 42" footing unless your up north.. then they do a spread footing witch is like 15" wide by 18-24" deep because it is all sand... But If you were gonna do a addition and it required a slab... You would do a footing and after your footing is dug... You would jump down in the hole and drill at least 3 holes spaced equally apart in the existing footing and dowl it with at least 1/2" rebar.. Thats what we do up here...

 

Thats free plans for ya , you issue should be addressed in the plans , your in earth quake country [ I did construction there for 15 yrs. ] .
Do you have a permit , depending you planing department may have some answers ?
A lot more info before any answers match you question , how far to bedrock , what will the planing department OK , ect .

 

You will have a cold joint. If you have a footing adjacent to your new slab you may not be required to pour a new footing. I would drill the dowels (use smooth dowels, not ribbed rebar.....I misspoke earlier) ever 12"(remember to grease them). This will allow lateral movement but not allow vertical deflection. Remove any clay or bad sub grade.....compact good subgrade to 95 %. Carry your subgrade out about 5" past your building line. Don't worry about bedrock unless you have to cut through it.....follow local codes.

 

The way we do it, the way the inspectors want it, and the way I've seen it done many times, is to drill 1/2 or 5/8 holes into the edge of the old slab parallel with the ground and about 1 foot apart. Go in at least 6 inches or up to a foot. Blow the holes out with air and inject some construction epoxy. Then drive in a rebar that will bottom in the hole and stick out about 1 foot. This bar forces the epoxy to the bottom of the hole and squeezes some out at the slab edge. It absolutely ties the bar to the slab as though it was originally poured in place. This bar will then tie into the rest of your new slab rebar.

This really ties the two slabs together as one except for hydraulic water pressure from below. It will not allow any verticle movement of one against the other. Any "hinging" will be handled by the rest of the slab and it's stability with footings.

If you do this correctly you should be able to even tile directly over the joint.

If the new area is living space, you can really do yourself a favor and add radiant heat in the new pour.

 

That's the way we do it too. ^^^^^^ With the exception of washing the holes out with a bit of water to ensure there is no concrete dust left in the hole. We also started increasing our slab footings by about 25%. It does cost a bit extra, but we follow a program called code +. Essentially we increase code requirements by a minimum of 10% to allow for future improvements, additions or changes to the building code. i.e. minimum 16" stud spacing on ALL walls, commercial electrical wiring practices, double tied and min 10% extra rebar in foundations, etc. The joining process above works very well and if you take your time, and make sure that everything is spaced properly and compacted to spec, you should not have any problems. Make sure your exterior grading is sloped away from the new addition for a minimum of three feet. We do our best to ensure six feet.

DO NOT cut your existing slab. You will create stress points, and it will crack and fail. If your ground is properly compacted, your footers are formed properly, the slab depth is correct and consistent, you tie in properly, and your grading on the exterior is done properly, any movement of the slab(s) be uniform and consistent. Slabs are designed to float, and that is exactly what they do.

BTW, Raspy is 100% correct. If done properly, you will be able to tile over the joint with no problems.