Hello everyone and thanks in advance.
A number of years ago I made the mistake of buying a Quonset hut for use as a garage and workshop. This thing is ghastly – ugly, damp, dark, and so on. Part of the construction ‘package’ was the design for a concrete slab. This was a generic design so it had thicknesses, rebar, etc., for the biggest Quonset hut they sold. According to the local plan inspector I had to build the slab to their specifications so I had a massive, expensive slab in place which would cost me a fortune to remove.
Now, I would like to have a proper workshop built, so I am drawing up my plans to start the quote process. The existing Quonset hut slab is about 25’x37′, which is ok for a garage but too small for a workshop. I intend to add about 2000 square feet to this footprint.
I live in Southern Ontario so I need a full 4′ footing for any construction. I live on a farm, so I have to remove about 2′ of topsoil and if I want the floor of the new structure to be above grade I would have to refill about 3′ with gravel as I did for a small barn I had made. This is also very expensive (about $8,000), so I envision a ‘split level’ workshop – basically, a basement (4′ below grade, 4′ above), the adjoining slab, and a main floor 4′ above grade for the shop. The entire structure would be a rectangular building covering the slab and the ‘new’ area.
Here is my question: is it ok to build a foundation around the existing slab? I figure I’d have to leave a foot space between the slab and the foundation so they could put the forms in, so I’d have to have the existing slab extended to the new foundation.
My questions are: is this doable? Can a new foundation be placed around an existing slab? Are there any tricks to consider? Any issues to extend the existing slab to the new walls? Anything else I should think about?
This is not a DIY project – I will draw up the plans and have a contractor build it for me. I’d prefer to have ‘doable’ plans to initiate the discussion.
Thanks again.
Replies
Given any thought to a pole barn built around the slab?
Something to think about.
The slab is much too small
If I were simply interested in replacing the quonset hut, I'd have a pretty clear idea of what I could do. However, the slab is too small for a proper workshop, but the site is ideal for the workshop. So, I have to figure a way of expanding the space.
As noted, when I remove the topsoil, I'd have to pay a fortune to put a new slab above grade.
I understand that it's too small.
But, you build part of this pole barn over and around that slab, then continue to your new shop area. The change in level can be done there.
No problem
It shouldn't be a problem. You will just need to keep the existing slab either completely isolated, or completely tied to the new foundation. I'd recommend isolating the floor.
There are a lot of slabs on grade that are placed after the foundation wall is in place and then backfilled to gravel grade for the concrete. I just did a 5200-sf building that had 6 to 8-ft foundations, and then the grade was brought back up, and insulated with 4-inches of foam under the slab.
The problem I would anticipate would be soil sloughing in when you excavate for the footing. You might want to move the foundation and walls out a few feet from the edge of your existing slab so you can backfill easily, and then pour a closure strip for the floor that is tied into the existing slab with dowels to prevent differential settling.
Also: Consider making the step down height between the two sections match the bed height of your pickup truck. This will make is lots easier to move materials in and out of your upper work area.
Mingybear, I searched building a foundation for an existing concrete slab. I have the same situation 40x40. What did you end up doing? I hope you see this. It's been a while since you posted.
Thanks!