I am trying to extend my back patio like this enter image description here

Eventually, I want to extend the full 18 feet, but for now I am planning to just do the 5 feet by 5.5 feet section in blue. This section will be attached to the existing patio and the house. My plan is this:

  1. Dig out the existing ground by 8 inches deep. This will provide for a 4 inch gravel subbase and a 4 inch patio.

  2. Pour the gravel subbase.

  3. Use stakes and 2x4s to make the frame.

  4. Put in the rebar grid.

  5. Glue asphalt-impregnated fiber board to the existing patio and house to create an isolation joint.

  6. Mix and pour cement.

  7. Screed concrete and smooth out top with 2x4s.

  8. Seal concrete.

My first question is this: Will I need to insert the rebar grid into the existing concrete and the house foundation? In other words, do I need a better connection between existing concrete and the new concrete or will the isolation joints be fine?

Also, is there anything that I am missing in this plan? Or is there anything that I should be aware of when taking on this project?

Thank you.

  • 1
    Effort wise you should just consider doing the whole 18 feet at once. You will also eliminate the extra joint at the 5 foot mark.
    – Michael Karas
    Jun 16, 2019 at 15:40

2 Answers 2


I would add rebar on that end and not leave the new slab independent. I would get a mat and dig a little more dirt out build your 5 foot pour. Let the mat run by three feet and is ready for the next section of pour and lock them together. And do each pour this way til done. The slab will be somewhat as one pour. And finish each end as an expansion joint.

  • +1 for adding rebar along the edge for the balance of the pour.
    – Lee Sam
    Nov 13, 2019 at 21:21

I like your ideas of 1) pouring a square...it’s less likely to crack, 2) extending rebar into the existing slab to tie the two slabs together...to keep the two slabs from differential settlement, (I’d use 6 pieces of #4 rebar about 12” long and drill a hole into the existing slab about 6” with 6” extending into the new slab...this makes the spacing about 12” on center along the edge of the slab), 3) using asphalt impregnated fiber board to isolate the new concrete.

The items I’d change are: A) only dig down 4” so that the new slab rests on undisturbed soil (there’s no reason to have it on gravel), B) do not use a reinforcing mat...it is not large enough to be temperature steel and is not needed anyway when it’s only 5’ x 5’-6”. C) You indicate you’re pouring “cement”, when actually you want concrete (with aggregate).

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