Tornado Shelter Roof
I expected pouring the 3” thick concrete roof for the tornado shelter to be a real challenge, but it turned out not to be nearly as bad as I had expected.
Pic 1 shows the shelter before the roof. Notice the top row of concrete blocks have been dished out to act as forms for holding the concrete.
Pic 2 shows the form boards in place. I used 4x8 OSB (3/4” thick) for the base. Pic 3 shows what held up the OSB base; concrete blocks stood on end. I built six pillars of block so that the floor would be supported roughly every two and a half feet in all directions. I did take care to make sure the pillars were vertical (used a level). On top of the pillars sets a 2x4 frame, with the OSB sitting on top of this frame. There were some small gaps around the OSB that concrete could go through so I used 3” wide strips of aluminum (.020” - twenty thousandths thickness) to close those gaps. They overlapped the concrete block by 1/8” to ¼” in most places and so they wouldn’t move were held in place by one dry wall screw on each piece.
I was afraid the weight of the concrete might push down too much in the middle, so I wedged 2x4’s there to help support the middle.
Rebar was then installed in both directions. Each rebar piece had 90 degree bends on the ends so the ends could stick down into the top of the concrete blocks to give the concrete more to hold on too and to better secure the roof to the walls. Unfortunately I don’t have a pic of the rebar installed.
Then came the concrete; I mixed it; girlfriend placed and smoothed it. One piece of rebar stuck up more than desired, so we just made the slab thicker there than other places. My plan was to have 1-1/2” between top of concrete roof and bottom of floor joists (the thickness of the sill), so the slab could be made up to 1-1/2” thicker locally and still not hit the joists.
Pic 4 is the completed roof slab. I left the forms up for three days and kept the roof as wet as much as possible while curing, but I didn’t cover it with plastic (should have). A week later now, no cracks.
When I took out the block pillars the OSB base didn’t want to come out easily, so I have decided to leave it in permanently. It will give us something to screw the lights too and after it’s painted we think it will look nicer than a concrete roof.
Pic 1 shows the shelter before the roof. Notice the top row of concrete blocks have been dished out to act as forms for holding the concrete.
Pic 2 shows the form boards in place. I used 4x8 OSB (3/4” thick) for the base. Pic 3 shows what held up the OSB base; concrete blocks stood on end. I built six pillars of block so that the floor would be supported roughly every two and a half feet in all directions. I did take care to make sure the pillars were vertical (used a level). On top of the pillars sets a 2x4 frame, with the OSB sitting on top of this frame. There were some small gaps around the OSB that concrete could go through so I used 3” wide strips of aluminum (.020” - twenty thousandths thickness) to close those gaps. They overlapped the concrete block by 1/8” to ¼” in most places and so they wouldn’t move were held in place by one dry wall screw on each piece.
I was afraid the weight of the concrete might push down too much in the middle, so I wedged 2x4’s there to help support the middle.
Rebar was then installed in both directions. Each rebar piece had 90 degree bends on the ends so the ends could stick down into the top of the concrete blocks to give the concrete more to hold on too and to better secure the roof to the walls. Unfortunately I don’t have a pic of the rebar installed.
Then came the concrete; I mixed it; girlfriend placed and smoothed it. One piece of rebar stuck up more than desired, so we just made the slab thicker there than other places. My plan was to have 1-1/2” between top of concrete roof and bottom of floor joists (the thickness of the sill), so the slab could be made up to 1-1/2” thicker locally and still not hit the joists.
Pic 4 is the completed roof slab. I left the forms up for three days and kept the roof as wet as much as possible while curing, but I didn’t cover it with plastic (should have). A week later now, no cracks.
When I took out the block pillars the OSB base didn’t want to come out easily, so I have decided to leave it in permanently. It will give us something to screw the lights too and after it’s painted we think it will look nicer than a concrete roof.
P.S. The holes in the walls in pics 1 and 4 are for vent fans and for emergency communication, should a tornado bring the house down on top of us (blocking our escape).
posted by Tony @ 8:31 AM
4 Comments:
Thank you for the post, being a non-engineer I'm trying to learn here but am unsure about calculating the load the roof can handle as well as figuring out how to support the roof once it is in? Did you have some background that you used in the calculation or just a lot of building experience? I'm new to your blog so you may have discussed this elsewhere.
Thanks.
Joseph:
The tornado shelter is free standing in that it doesn't support any of the weight of the house. Free standing here does not imply it isn't well connected to footers; believe me, it is. There is vertical rebar in every 2nd concrete block cell of the shelter, whereas in the house itself, it's rebar every 3rd cell.
Love the post - Thanks! Quick Question: What size is your shelter and how far apart were the rebar in your rebar grid. thanks a million - John
John:
The shelter is about 5x8. In the floor and ceiling the rebar is on a 1 ft square grid. In the walls every other cell has rebar and grout in it.
Post a Comment
<< Home