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Old 01-01-2012, 04:58 PM   #1
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Building a garage with underground mancave/bomb shelter


Hey guys!

Here is my situation...
The lot I am on is 120 x 50 (6000 sq ft).
The house I have is 1600 sq ft and is pretty much centered on the lot. My house is the only house that is like this. All of the other houses in the neighborhood are built closer to the front. So I have an even back and front yard and everyone else has a smaller front yard but a bigger back yard. I have a one car garage now but I want to build a bigger garage back there. With the zoning rules set by my county I cant build a massive garage.

Here are the rules I have to follow:
  • Open yard space requirement 10% of lot (600 sq ft)
  • Accessory building cannot be more than 15% of lot (900 sq ft) and no more than 85% of the main buildings footprint
  • No taller than 15 feet or 18 feet with a 3/12 pitch
  • Has to be 6 feet from the house
So after factoring in all of those rules I figured I could build a 28x32 garage and just barely meet the 600 sq ft open yard space requirement.

I want to have a decent size backyard and I want to have the biggest garage I can build that meets my needs. But somehow those dont go hand in hand because of the size of my lot and the way the house is set on my lot.

I want to have at least a 2 car garage with a separate area for my mancave. I could do that with 32 feet wide, BUT that would make my yard only 600 sq ft.

So thats where the idea of having a smaller 2 car garage to allow for bigger yard space and have my man cave underground.

Does anyone know if this is possible and if the county will buy off on it? I live in Pierce county in WA state. And what would have to be done structurally to make this possible so that the cars dont fall through the ceiling while im playing call of duty. Any input is greatly appreciated. Thanks!

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Old 01-01-2012, 05:14 PM   #2
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Building a garage with underground mancave/bomb shelter


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Originally Posted by m.glisson003 View Post
The lot I am on is 120 x 50 (6000 sq ft).
Any input is greatly appreciated. Thanks!
Sell this place and start over somewhere that has the room.

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Old 01-01-2012, 05:31 PM   #3
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Building a garage with underground mancave/bomb shelter


Anything is possible with enough money. That said, any utilities in the area would have to be moved. Is the house on septic? If so, the field may be too close. Since you're building a basement, you also have to think about the water table and proper drainage. Also, the building department is certainly going to have requirements (not just structural) for building a living space UNDER a garage. I would suggest you contact your local building department and an architect to find out just what this will entail.
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Old 01-01-2012, 05:39 PM   #4
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Building a garage with underground mancave/bomb shelter


Lack of proper egress, moisture, total lack of light, very high cost to build the foundation.
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Old 01-01-2012, 05:59 PM   #5
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Building a garage with underground mancave/bomb shelter


I can see this costing a small fortune.

Is an 18' garage too low for a loft area above?
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Old 01-01-2012, 08:06 PM   #6
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Building a garage with underground mancave/bomb shelter


28 x 32 garage is a decent size.

Would a 24 x 38 be acceptable to you? This way, you could stick with the 24' module.

How is the grade on the lot? Any slope?

The construction is fairly straightforward. You'll want to establish the elevation of the garage floor and decide on a basement ceiling height. As mentioned, you'll want to take the water table into account, which may influence the garage floor elevation and basement ceiling height. Nothing wrong with having the garage floor 3' above the present grade if you have the room to get up to it. You are going to have plenty of dirt on hand to fill in around the perimeter of the building.

I would have a foundation/basement wall company come and form up and pour the basement walls, including a bearing wall down the middle. They will take care of your perimeter drainage tile on the interior and exterior and you will probably want to install a sump pump pit. They can also waterproof and insulate the exterior. I would insulate the exterior with 2" of polystyrene.

You will want to incorporate a staircase down into the basement, egress windows, and an exterior staircase would be a good idea as well. Have these all formed up and poured as a part of the foundation/walls. You can cover your exterior access with a set of doors or leave it open. If you leave it open and have some slope away from the building, you can have the exterior access walls slope away from the building similar to the slope of the grade. The length of the exterior access will depend on how deep it is from the finished grade, get it? You can just build a set of stairs on the inside out of pressure treated wood later. Be sure to put a floor drain at the bottom of the stairs which will tie into your sump pump pit. Make the staircase 4' wide.

Now, for the floor. I see three options here.

1. A self-supporting/leave in place form system. This will be offered by some firms which supply ICF's. I believe you will find it is styrofoam with steel and it is ribbed or of a corrugated form. It will be placed and formed up around the perimeter to accept the proper thickness of concrete. The concrete will poured, and the support will remain in place.

2. You can have pre-formed panels of concrete placed on the top of the basement. A crane will place 12' long panels, say 4" thick, on top of your basement walls. They will be of some uniform width and add up together to be the length of your building. Now you can top this off with another "wear layer" of concrete and you have your finished floor.

3. You will have a form system built in the basement to support the concrete as it is poured, then it will be stripped once the concrete has cured.

You will have to work with an engineer or a rep from the company selling whatever system you want to go with in order to determine the thickness and type of the concrete, the amount and type of reinforcement to be used, etc. You are looking to answer the question of what it takes to span 12' with the use of the floor as a garage.

Good luck and let me know how you come along.
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Old 01-01-2012, 08:40 PM   #7
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Building a garage with underground mancave/bomb shelter


For garage questions like that, Garage Journal is a good place to find the answers. Just about any type of garage you want can be seen over there.
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Old 01-01-2012, 09:17 PM   #8
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Building a garage with underground mancave/bomb shelter


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Sell this place and start over somewhere that has the room.
Thanks for your helpful advice. But I have to do the best I can with what I have.
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Old 01-01-2012, 09:23 PM   #9
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Building a garage with underground mancave/bomb shelter


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Originally Posted by iminaquagmire View Post
Anything is possible with enough money. That said, any utilities in the area would have to be moved. Is the house on septic? If so, the field may be too close. Since you're building a basement, you also have to think about the water table and proper drainage. Also, the building department is certainly going to have requirements (not just structural) for building a living space UNDER a garage. I would suggest you contact your local building department and an architect to find out just what this will entail.
House is not on a septic system. And I will give them a call. Thanks
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Old 01-01-2012, 09:24 PM   #10
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Building a garage with underground mancave/bomb shelter


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Originally Posted by Jay 78 View Post
I can see this costing a small fortune.

Is an 18' garage too low for a loft area above?
I'm not really sure. I figured after the first story and roof I would be close to 18 feet.
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Old 01-02-2012, 08:23 PM   #11
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Building a garage with underground mancave/bomb shelter


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Originally Posted by cleveman View Post
28 x 32 garage is a decent size.

Would a 24 x 38 be acceptable to you? This way, you could stick with the 24' module.

How is the grade on the lot? Any slope?

The construction is fairly straightforward. You'll want to establish the elevation of the garage floor and decide on a basement ceiling height. As mentioned, you'll want to take the water table into account, which may influence the garage floor elevation and basement ceiling height. Nothing wrong with having the garage floor 3' above the present grade if you have the room to get up to it. You are going to have plenty of dirt on hand to fill in around the perimeter of the building.

I would have a foundation/basement wall company come and form up and pour the basement walls, including a bearing wall down the middle. They will take care of your perimeter drainage tile on the interior and exterior and you will probably want to install a sump pump pit. They can also waterproof and insulate the exterior. I would insulate the exterior with 2" of polystyrene.

You will want to incorporate a staircase down into the basement, egress windows, and an exterior staircase would be a good idea as well. Have these all formed up and poured as a part of the foundation/walls. You can cover your exterior access with a set of doors or leave it open. If you leave it open and have some slope away from the building, you can have the exterior access walls slope away from the building similar to the slope of the grade. The length of the exterior access will depend on how deep it is from the finished grade, get it? You can just build a set of stairs on the inside out of pressure treated wood later. Be sure to put a floor drain at the bottom of the stairs which will tie into your sump pump pit. Make the staircase 4' wide.

Now, for the floor. I see three options here.

1. A self-supporting/leave in place form system. This will be offered by some firms which supply ICF's. I believe you will find it is styrofoam with steel and it is ribbed or of a corrugated form. It will be placed and formed up around the perimeter to accept the proper thickness of concrete. The concrete will poured, and the support will remain in place.

2. You can have pre-formed panels of concrete placed on the top of the basement. A crane will place 12' long panels, say 4" thick, on top of your basement walls. They will be of some uniform width and add up together to be the length of your building. Now you can top this off with another "wear layer" of concrete and you have your finished floor.

3. You will have a form system built in the basement to support the concrete as it is poured, then it will be stripped once the concrete has cured.

You will have to work with an engineer or a rep from the company selling whatever system you want to go with in order to determine the thickness and type of the concrete, the amount and type of reinforcement to be used, etc. You are looking to answer the question of what it takes to span 12' with the use of the floor as a garage.

Good luck and let me know how you come along.
24x38 wouldnt be bad. I like the idea of going unerground though. Even if it costs a little more. Since space is limited and to have the biggest backyard space it would be better for me to have a detatched garage with a smaller footprint but enough space inside to meet my needs.
The lot is completely flat.
So I guess it sounds like the next thing I need to do is talk to my county and make sure it will be okay and then talk to an engineer. If I could get away with it I would prefer to use wood. Much cheaper alternative. I was doing some research and came across several people that used lumber and it was structurally sound. Thanks for your helpful advice!
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Old 01-02-2012, 10:09 PM   #12
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Building a garage with underground mancave/bomb shelter


Having usable space, or a basement, under a garage isn't all that uncommon, but it's typically done with pre-stressed plank with a rubber membrane, followed by about a 4" topping to protect the rubber, much like cleveman explained. I would imagine you'll still need to install something similar to the rubber membrane with the wood floor as well as a "sacrificial" wear layer over the top of it. Can't say I've ever seen it done with wood though........

I would honestly look harder at pre-stressed plank, as it can easily be designed to clear span those dimensions, so you won't need to cut up the lower level with additional bearing walls or post & beams. I'd be aprehensive to park cars on top of a wood floor with these long spans and expect it to perform well long term.
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Old 01-02-2012, 10:24 PM   #13
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Building a garage with underground mancave/bomb shelter


I worked on a customers house that had a basement under an attached garage. It was (if memory is correct) about 15' wide and 30' long. I remember having to cut the new floor joists we put in to bring the 'garage' floor up to the level of the house.

It was 2 by 10 fir on 12inch centers. spanning the 15'. They had sagged up to 1/2 inch in the center. It was 3/4" T&G plywood on the floor.

Under where the vehicle parked there was a large rectangular pan made up of a sheet of 1/8" metal and angle iron around the edges all welded together. It had a drain in the center of it.

Not sure if it would meet code, I think probably not, it was on a farm and no codes. The house was built in the late 60's or maybe early 70's, so about 40 years old. It had not fallen in or cracked any floor joists.
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Old 01-02-2012, 11:23 PM   #14
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Building a garage with underground mancave/bomb shelter


You could sink the garage floor a couple feet below grade then build on top.

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Old 01-03-2012, 12:03 PM   #15
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Having usable space, or a basement, under a garage isn't all that uncommon, but it's typically done with pre-stressed plank with a rubber membrane, followed by about a 4" topping to protect the rubber, much like cleveman explained. I would imagine you'll still need to install something similar to the rubber membrane with the wood floor as well as a "sacrificial" wear layer over the top of it. Can't say I've ever seen it done with wood though........

I would honestly look harder at pre-stressed plank, as it can easily be designed to clear span those dimensions, so you won't need to cut up the lower level with additional bearing walls or post & beams. I'd be aprehensive to park cars on top of a wood floor with these long spans and expect it to perform well long term.

Yes I've read that its actually common! Even if I did have the space to build another out building I wouldnt want to have to leave one building and go to another or pay taxes on another building. So I think its a good investment.

And most of the sources I have read said they they used pre stressed concrete or span crete. You make a great point that does sound like a much better idea than having beams or bearing walls below. And it last much longer is a great incentive too. Thanks for the input.

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