How far in from the edge can I drill into concrete?

[Michael Donahue]

I'm going to build 2 sets of railings that will be surface mounted into pre-existing concrete stairs. How far in from the edge do I need to be to safely drill out my holes without cracking the concrete? I'm thinking 2" would work. Does that sound about right?

Thanks!

[John Keeton]

Michael, I don't have a lot of experience with this, but the problem that I have encountered in the past is the aggregate. Depending on what is used in your area, drilling in concrete can be a real unknown. If you hit a good sized chunk of gravel, it can break out quickly. However, if the holes are relatively small, or the aggregate small, it may not be as much of a problem.

[Michael Donahue]

Michael, I don't have a lot of experience with this, but the problem that I have encountered in the past is the aggregate. Depending on what is used in your area, drilling in concrete can be a real unknown. If you hit a good sized chunk of gravel, it can break out quickly. However, if the holes are relatively small, or the aggregate small, it may not be as much of a problem.

The holes will only be 3/16" pilotholes for tapcon screws. I don't really know what to expect for aggregate though :(

[David G Baker]

Michael,
I have a small barn that I replaced all of the sills with pressure treated 2x4s. I drilled around 40 holes in the concrete at the center of the 2x4 which puts the hole approximately 1 1/2 inch from the edge. I didn't have any breakouts or any other problem. The concrete was around 60 years old. The project you are doing may be different because a railing will have a different type of movement than a sill but if the holes you drill are around 1/2 inch you should be fine with 1 1/2" to 2" from the edge. A good carbide bit should drill through the aggregate in most cases but there may be one or two that won't cooperate.

[harry strasil]

It depends on the type of fastener, expanding or not, and the size and the applied forces. When I was Blacksmithing for a living and doing church railings, I used expanding anchors and usually drilled them 3 to 4 inches in from the edge as the steel railings have to stand a force of 250 lbs from 6 ft away striking them, with the small holes you are using you should be ok at 1.5 inches. the real stress points that are the most vulnerable are at corners. FWIW, normal carbide bits are sharpened differently from hammer drill bits.

There are always problems with hard large stones in the cement and of course rebar. if you get to a hard stone, you can alternate between rotation and hammering to get thru them most of the time, rebar is a whole different animal tho.

http://img.photobucket.com/albums/v81/irnsrgn/smithing/luthrail1.jpg

http://img.photobucket.com/albums/v81/irnsrgn/smithing/luthrail3.jpg

[Bob Lloyd]

Michael

You should be fine at two inches, the Tapcon screws should not create much of a lateral force; much closer and you run the risk of spalling the concrete off.

Bob