Storm Room/Concrete roof


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Old 11-21-07, 05:22 PM
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Storm Room/Concrete roof

Hello Folks,
I live in a 1980's manufactured home with a basement.
The home will most likely, not surive a heavy storm with tornado type conditions. I need a safe retreat storm room, so I am building a poured wall, storm room attached to my existing basement. Footings are in and the walls are to be poured next week.

The roof will be heavy guage commerical roof decking, with a poured concrete slab above(poured right on the decking).

The slab roof will also act as patio, as it will be at grade level after backfill. Drainage considerations and flashing issues have been worked out.

My plan is to cover the slab with commercial rubber roof membrane (remant from large warehouse roof project), then small, smooth round stone, same as used on commercial flat roof systems.

So, here is my question:

Should I use a layer of 6mil poly sheeting between slab and rubber membrane to protech rubber from abrasion/friction by the concrete slab?

I have been told condensation will occur between the concrete and the poly film, thus trapping water.

There is no insulation planned for this roof as it is not intended as an occupied space, other than storm celler use, and as an access (Bilco basement access doors)
Any thoughts on this?

I do not want to use tar or asphalt products as the storm room walls may someday be used as support for an addtion to the home. The slab roof are would then be cleared of stone and rubber, hence becoming a crawlspace area.
There is plenty of room to do this as is is an extra deep basement.

the project will fast track once the walls are poured.
i want to be ready to move forward as soon as the wall forms are down....
Please advise, thanks!
 
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Old 11-21-07, 08:55 PM
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why not just lay,,,

1" insulation board on top of the slab ?,,, then your membrane ?,,, realize your stay-in-place forms'll eventually have some rust form but, in this instance, i wouldn't think its a big deal.

i'd use the insulation to prevent condensation which would infiltrate the slab & rust the rebar causing more problems.

just my $.02
 
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Old 11-22-07, 04:52 AM
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Thanks,

I have been thinking about something like that.
Are you saying that the insulation board will help move the condensation point to near the insulation board and away from the interior ceiling surface?

Also, what would the recommended slope per foot be for the slab surface? I have been told .25" per foot.
I'm sure that will drain fine, but it seems excessive. That's 3" in drop in 12 feet! Comments?
 
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Old 11-22-07, 10:28 AM
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how do know ? ? ? :-)

i'm not an insulation/heating guy, just a gc & conc specialist,,, but if it were mine (& congrats on the idea of protecting your family), i''d do it.

far's slope of the deck, 1" in 10' seems reasonable to me,,, but consider flashing on the house, prevailing winds, & usual direction of storms,,, eg, we use 3" in 100' for parking lot drainage,,, but your site conditions may dictate otherwise.
 
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Old 11-22-07, 05:55 PM
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thanks for the reply So-Elitecrete

do you know much about the additives used in the mix of concrete for waterproofing?
i'm not refering to brush on exterior coat, but rather the additive to the mix.
i't is suppose to help fill all micro voids in the concrete to minimize water pentetration.
is it worth the extra cost (I don't have any idea of $) see as i was to apply rubber membrane?
 
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Old 11-23-07, 06:12 AM
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not aware of any,,,

waterproofing additives for liquid conc but aci says 5,000psi conc's waterproof,,, i always preferr'd positive side waterproofing, tho.

you already have mtl on hand & i'd do the same,,, then again, 5gal of roofing cement'd do the same properly protect'd.
 
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Old 11-23-07, 08:28 AM
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Storm Room/Concrete roof

The only real good aditives to decrease permeability require extensive testing using the correct raw materials AND use in a closely controlled plant operation (not ready-mix plant or job site).

Even with perfect concrete, unless you cast in flexible barriers at joints, you are still faced with the joint leakage at the footing to wall and at the wall to slab joints.

If you control the gound water/moisture, then you can use the coatings and other measures that have a high degree of variability and life.

Its a little late for you now, but FEM (fema.gov?) has the most extensive design applications for "safe cells" to resist tornado and hurricane exposures for both above and below grade. The walls are either reinforced block or concrete (never wood or kevlar/fiberglass reinforced) with concrete roofs. There is also good information on doors, hinges, hardware and on ventilation. Been available and used for years.

Dick
 
 

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