Expert: Dan Griffin - 7/15/2008

Question
QUESTION: Currently, I have two 25m2 bungalows (both with concrete cisterns as their foundation) on two levels (one 0.75m lower than the other) with a 15cm reinforced slab terrace separating them. We would like to build a larger building on the higher level in place of the two bungalows fully utilizing the 5mx13m square area. This would mean raising the lower 8mx5m area to the higher level. The existing foundations are all constructed with reinforced iron and are 20cm thick. Can we build a new foundation on top of the older ones and the slab between the them? And if yes, what would be your recommendation for the type of foundation?

ANSWER: Theron,  I don't recognize your reference to cisterns.  I consider a cistern an underground tank, usually concrete or masonry, for holding water.  You've not indicated how tall the existing foundation walls are.  I am assuming that you are talking about two slab homes.  I assume that there is a spread footing in the ground with a 8" stem wall centered on the footing.  The slab may be poured on top of the stem or inside the stem.  If you are talking about a wood framed floor, then I have totally misunderstood you.  What is the exterior finish?  Are you building one story or more?

I guess you are trying to not remove the existing foundation.  An 8" stem wall should be adequate to carry the new construction, but I don't like the cold joint that will be created if you add to the existing stem wall, and it becomes riskier if there is substantial heigth involved.  What will you be doing for exterior grade when you raise the lower unit 2 feet?

Code will require a footing around the full perimeter of what you build.  The center terrace probably does not have a footing so will need to be cut back or removed.  The elevation almost can't be right for the proposed structure.

I think I need a bit more data to help you any more.



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

QUESTION: Yes the cisterns are concrete water holding tanks which also contain a central pillar holding up a slab which in turn supports the current bungalows, they are identical. So essentially I would like to, as you say, connect a stem wall to the lower slab and the terrace slab. The height of the stem wall would be equal to the height of slab of the upper bungalow. All three elements would be at the same level now. Then we would pour a new slab over the existing slab and the newly created stem walls. We would like to know if this is possible? For obvious reasons, we would like to preserve the cisterns. I don't understand your reference to the elevation for the proposed structure. Also, we will not using any wood, only concrete and indigenous stone.

Answer
Theron, it sounds doable.  Your code people will probably require a strucural engineer's stamp to ensure the original footing can sustain the load of a second floor  on the lower section.  He may well suggest a wood framed floor over that portion to reduce the load.  It will be an interesting project.