basement under new garage

[Zone_V]

I was hoping someone would have knowledge in this design dilemma. We're planning an addition to the house which will include a single car garage at living area level, and I'd like to utilize the potential space under this garage by extending the basement into this area also. I wouldn't foresee that carrying the weight of the vehicle would be such a problem, but rather a floor material/design that could be sloped toward a drain and be impervious to dripping snow/salt mixture, rain, the inevitable occasional oil drips, etc. Thanks for any ideas anyone might have. John

 

[boustany]

chances are, unless you live in an area with virtually no building enforcement, you will have to get an engineered solution. If I were you I'd look into precast concrete, but like I said, it'll be the engineer's call.

 

[bluecon1]

I have a 3 car garage (35' x 35') with a full room below, not one support column. I bought an existing house, so it was not my design. The previous owner did leave Engineered Blue-Prints with me. The floor is all pre cast concrete panels, as is the whole house. Quite unique, and not a squeek in the floor anywhere.

 

[Junkman]

I wanted to do this 20 years ago, but it was nixed by the building inspector. I have since learned that the companies that sell the pre-cast concrete slabs will also supply the engineering details to meet all the building inspection requirements. They also install the panels. There will need to be a complete 4 wall foundation poured to support the pre-cast slabs. After the slabs are set in place, there is a top layer of concrete installed over it about 2" thick. The pre-cast slabs have tube type holes in them so you can "snake" electrical wires through. This is a great idea, but it is also an expensive idea. Have the pre-cast concrete company run the number for you and decide if the additional cost is worth the gain. The concrete foundation for a slab is usually 4' high to cover the frost wall requirement. A slab floor foundation is usually 8' - 10' and the walls are 12" thick as opposed to 8" for a normal wall. There are also some other requirements, but they are not expensive.... Let us know what you decide and post some pictures....

 

[bluecon1]

Junkmans got it right. The supplier will engineer the whole thing. My panels are various lengths, have three hollow cores through them as well as 3 braided cables (post tensioned). . The 2" of concrete is used to fill the gaps between the panels, as well as to keep moisture from draining through (in the garage application). Should last longer than me here.

 

[dummy]

Precast or cast in place. Cast in place requires some big steel, ultradecking and engineering time. After the steel is down, in this case 4" of concrete was poured over the structure. The biggest problem with either design is the access to the created space. In the attachment, the homeowner opted to have the space open into the family room. IMO, not a very good route to get your storage stuff in and out. If you have slope for a walkout, install an overhead door for access.

 

[DaveNay]

Take a look at the Lite-Deck product from PolySteel.

Lite-Deck Foam Decking

I was doing some serious consideration of using this product a couple years ago, but we bought and existing house instead.

Dave

 

[yooperdave]

I have done this several times.

I would recommend a precast slab with a double topping.
The first topping for leveling & evening things out, then a chemical resistant membrane, then a wearing course of concrete over that.

Not cheap, but works well every time.

Good Luck

Yooper Dave

 

[Ben_W]

I have a 24' by 24' garage over a garage. I didn't want any posts in the middle of the lower one to support the upper so I would have more flexibility during use. I opted to go with 6 each 4' by 24' prestressed concrete slabs. The same thing that some bridges use for the decking. The contractor used 8" poured walls and had the slabs set on the inside 4'' that left 4'' outside for the upper garage sill plate. A 2" slab was poured on the prestressed slabs. You want the slabs to run parallel to the wall the downstairs garage door will be on so the opening won't have to support their weight. (The slabs have a slight upward bow.) I also had a door put in to give access to the basement of the house. It had to fit between one slab width to ensure good support of the slab. For property tax concerns it's cheaper than another garage. It is also a great tornado shelter. If I did it again I would go with 9' walls. The people the prestressed concrete was purchased from did the engineering. With the slabs being hollow a drain could be put in. Might want to check with the manufacturer on where to put it so you would't cut any of the cable used in them. Ben

 

[Junkman]

Ben.... garage drains have been outlawed in most jurisdictions because the building officials are worried about gasoline vapor getting into the drain and then an explosion following. The gasoline vapors are heavier than air, so they will settle in the lowest part of the garage.... ie... the drain. I put a drain into my original garage, but when I added on and tried to put a drain into that one, it was nixed by the town building inspector. He is very liberal and will allow just about anything that isn't going to jeopardies safety. He told me that the drain was a state mandated no no. He doesn't know about the other one, and I am not telling. I have since realized that a drain isn't really needed, because you are not likely to be washing your car in the garage....