five.five-six
Senior Member
- Location
- california
I have a job to install a small generator on a roof. The roof it about 12" thick concrete deck with asphalt cover. How do I go about flashing the penetrations on such a roof?
Flashing a concrete deck roof penetration.
- Thread starter five.five-six
- Start date
- Status
mgookin
Senior Member
- Location
- Fort Myers, FL
Have a roofer install a pitch pan. That way when it leaks, his phone rings.
JFletcher
Senior Member
- Location
- Williamsburg, VA
:thumbsup:
five.five-six
Senior Member
- Location
- california
And then just run all my conduits through the same penitration?
mgookin
Senior Member
- Location
- Fort Myers, FL
Coordinate that with him. He's going to cut a hole in the roof and install a 4 sided flanged metal box that extends a few inches above the roof deck. You're going to run you're wiring, then he's going to fill it with pitch. Use a chunk of carflex to a j-box below the roof deck and you'll have a raceway for life without ever having to mess with that roof again (or until your carflex rots away).
- Location
- Massachusetts
The company I work for would not even think of doing this ourselves. We would hire a roofer to do it and take on the liability.
JFletcher
Senior Member
- Location
- Williamsburg, VA
Same here. Flat roofs are even trickier than residential pitched/shingled roofs.
I'd have the roofer there before you start core/hammer drilling the roof.
Cow
Senior Member
- Location
- Eastern Oregon
- Occupation
- Electrician
I agree with everyone else. Use a roofer and get their input before you start making holes.
mgookin
Senior Member
- Location
- Fort Myers, FL
Don't make the hole. The roofer makes the hole.
five.five-six
Senior Member
- Location
- california
I have a coring contractor to make the hole... I didn't think roofers did that.
mgookin
Senior Member
- Location
- Fort Myers, FL
My apologies. I overlooked the part about a 12" thick concrete deck.
Make sure that coring contractor is liable for hitting any reinforcement. If it's post-tension and he cuts one, it can get nasty (tendon separates in two and accelerates out each end of the building creating a very dangerous situation).
- Location
- Massachusetts
Can you provide any link to that happening?
I have seen a number of these cut and they have never moved a bit, they are held fast by the concrete.
Now of course they should not be cut and structural engineers where required to design a fix for the reduced strength but I have never heard of the cables moving.
- Location
- Massachusetts
I see, the difference is pre or post tension.
We don't see post tension here only pre tension.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
You may be thinking of pre-tensioned concrete in which the cables are tightened (with the ends supported by special forms) before the concrete is poured.
Post-tensioned slabs require that the cable be able to move in response to being tightened after the concrete has set. This generally involves a tube positioned inside the form to keep the cable in the proper position during pouring and allow it to be tensioned after pouring. That would allow the cable to move when the stress is relieved, like any taut cable snapping under load.
See http://www.concretenetwork.com/post-tension/basics.html
Note that bonded (grouted in place after tensioning) tendons (cables) may not move much if cut, whlle unbonded tendons may move with enough force to dislodge the grouting over the tensioning points.
In either case, an engineer will be required to determine whether the slab is still within safety margins if one tendon is cut. Repair could be very expensive!
mgookin
Senior Member
- Location
- Fort Myers, FL
It happened on a job I was on 25 years ago. I was not there at the time. It was a project of several highrises. I was a 3rd party engineering inspector. It happened on one of the buildings built prior to me being on that project. It ripped through another building when it went flying. Nobody got hurt but there was property damage. The other end landed in a mangrove swamp.
Golddigger nailed it.
Post-tension is in a sleeve where the tendon is pulled in the field after the concrete comes up to design strength. This is most common on your taller highrises where you have centralized sheer wall(s) which commonly bound elevator shafts and utility chases.
With prestressed the tendons are pulled prior to placing the concrete and this is done at a prestressed plant. They'll have beds, usually up to 300' feet long, they'll place & pull their tendons, place the concrete, then saw each slab to a length meeting the customer's requirements after the concrete comes up to design strength. These are the planks you see going down the road on 18 wheelers. They are available in hollow core & solid flat planks, and also as double-T, quad-T, etc. planks like you commonly see in parking garages. These are most common on midrise buildings.
Cold formed (rebar) is when you just form it as you go in the field, place your rebar (and often wire mesh too), place your concrete, then remove shoring after it comes up to design strength. This is common over corrugated galvanized sheet steel and in steel or concrete post & beam construction.
OP says he's dealing with a 12" thick concrete roof deck. That sounds odd. A non-hardened structure would normally not have a roof deck that thick and a hardened (DOD) facility would have a roof much thicker.
- Status