Expert: Dan Griffin - 5/13/2009

Question
QUESTION: Hello, I currently have a screened-in porch which I would like to extend so it will be big enough to fit my hot tub.  It is built on top of a concrete slab and is 8' wide by 17' long.  I am planning on tearing down the old porch and building the new one from scratch since the existing one is in pretty bad shape.  (I have plenty of experience in this area - it's the concrete I'm inexperienced with)  I would like to extend the slab to 10' wide, 18' long.  It appears that the old concrete slab is 8" thick and is resting on what appears to be brick and other stone.  Not sure how deep it goes, I've only dug down about 18" so far.  Frost line here is 24".  How should I go about doing this?  Any help would be greatly appreciated.

ANSWER: Mike, it sounds like you are adding 2' of concrete one way and 1' the other.  This is not the best way to deal with the concrete as long, skinny concrete is prone to cracking and any add-ons can settle differently.  All things are possible, but I would be amazed if your existing porch were over 4" thick.  Often the edges are dug down deeeper to help resist cracking or to give a finished surface to the sides.  I assume your porch is one or more risers above the exterior grade.  The brick and rock you are seeing was a convenient way to get rid of trash and served to occupy the space under the slab.

 If you think you or anyone would ever consider "closing in" the patio for additional living space, go ahead and create a perimeter footing now as you are almost that large already.  YOu would need to deal with drainage issues if you use a raised stem wall, but it would simplify the wall work and would be the method I would use.  Any additonal slab work could just be filled in around the perimeter after the footing.  This method would allow the porch walls and roof to bear on the new stem wall.

If you plan on the roof and walls resting on the porch, just add the additional concrete.  Any  new concrete will always show in color  and the existing rounded edge as a joint.  I would dowel into the existing slab with rebar and add a continuous bar.

Hope this gives you some good ideas.



---------- FOLLOW-UP ----------

QUESTION: Hi, thanks for the response.  I have a couple more questions as I'm still unclear on a few things.  I'm not 100% sure I understand your terminology, but I'm pretty sure I get what you're saying and it sounds like it's kind of what I had in mind in doing.  Basically, 2 of the 3 walls of the new porch will rest on the new section I'm adding, the 3rd wall will rest on the existing slab (the porch is against the house, so the house is the 4th wall).  So let me see if I got this... I need a footer.  But how big and how thick is what I'm unsure of.  Was thinking 12" wide by 8" thick running the 18' long and then the 10' wide.  Was planning on putting it 32-36" deep.  or is that too big/thick?  This would mean that I'd only have a 12" x 17' x 4" thick slab I'd have to add afterwards (4" thick over 4" of stone?) - rebarred into the existing slab of course.  When you say a "stem wall", do you mean cinder blocks?  So I'd place them on top of the footer and build them up flush to the existing slab?  Ideally what I'd like to do is have the stem wall 4" below the top surface of the existing slab so when I pour the new slab it runs over the new stem wall and would be 4" thick all over so what I would be staring at in the end is just a slab and no cinder blocks showing (I know there will be color variation and seam line but I'll be tiling over)  Also, how do I address the drainage issues?

The other alternative to everything I just said that I thought of would be to just put rebar into the existing slab, dig down 32" (one foot in from the outside perimiter and have the second foot 8" deep (4" for stone, 4" for slab) and just pour the whole thing in one shot.  Please see this image will show what I'm talking about:

http://www.buildeazy.com/newplans/eazylist/concrete_floor_example.gif

The reality is the room I will be adding is not going to be that much weight.  It looks like they got away with only a 4" thick slab layed on top of scrap brick and stone with no actual footer and it has stood fine for 10-15 years.  Hopefully I'm going the right route, but I would prefer to do it correct if you know what I mean.

Thanks

Answer
Mike, I'm sorry it has taken me so long to get back to  you.  AS you have already noted, the amount of add-on concrete is minimal in relation to concepts of footings, finish, and missing slab.  It is very diffilcult to create a void filler and yet, filling up the entire space would use up a lot of concrete.

I have put up an outside form and a sacrificial inside form that would hold sand on one side and be buried and abandoned after the pour.  This would allow pouring the slab on top and the side(s) at the same time.  If you are good enough with form work, you can create a raised stem on the outside edge higher than the slab that your new wall can rest on above the wet floor.  

I think I would install a floor sink and piping to provide a way to deal with the inevitable water involved with a hot tub.