Construction Corner is a Q&A column committed to answeringreader-submitted questions regarding construction and development. Inquiries maybe sent to [email protected].

Q: I have heard about cameras that use existing 120VAC power lines for theirvideo signal and power. What are the limitations to these cameras? How many canyou have at a single site?

Jerry in El Paso, Texas

A: These cameras are great for locations that are impossible toreach without trenching. However, the technology is not yet perfected. From atechnical standpoint, you can use one camera per electrical phase. So, on anaverage site, you could use two cameras; but realistically, one camera per siteis all that is recommended.

I have only seen these cameras in a black-and-white version.Their recording quality is on par with an old analog system. Again, they aregreat for being able to see that back door or dumpster that isnt accessibleby other means, but they are not an alternative to getting a regular coax videosystem.

Q: We are adding cameras and a security siren outside our of. ce, running allthe components across the roof. My nephew is installing the equipment, and I amconcerned about him making holes in the top of my building. What is the properway to run wiring across the roof and penetrate into the structure?

Becky in Carlsbad, Calif.

A: The answer to this question is related to the type of roof youhave, but I can give you a few guidelines. First, I recommend all wiring go through conduit along theroof to protect it from the elements, animals or the occasional HVAC repairman.When running conduit across the roof, dont use nails or screws to secure itto the roofing material. Instead, secure 12-inch pieces of 2-by-4 block to theroof with mastic or tar at intervals of about 10 feet, then secure your conduitdirectly to the blocks. This will prevent leaks from screw or nail holes.

When making your penetration into the building for the wire,use a sleeve with a metal flashing that fits snugly over the conduit. Use a lotof mastic or tar around the flashing. Finally, secure a waterproof box to thetop of the sleeve, as this is where all of your roof conduit will terminate.

Tony Gardner is a licensed contractor and installation managerfor QuikStor, a provider of self-storage security and software since 1987. For more information, visit www.quikstor.com.