Drilling a 6 in hole through 4 in thick concrete

[Paddy]

Eddie,

My drill is a Hitachi, DH 38 YE. It's a monster drill, speed to drill is not a problem

The hole through the roof is for a 6 inch dia bath vent fan. Our roof was poured and that vent hole was missed.

It's a rubber roof so I will be using a Firestone "peal and stick" primer activated rubber boot. The roof composition is 4 inch concrete, 4 inch of roofing foam, 1/2 inch OSB and then roof rubber. So I will be working down in a 4 inch hole. This makes web breaking a bit more tricky because that has to be done on a tangent to the hole.

I don't care how ugly the hole is.

I think my best strategy is to be very careful about getting the holes perfectly spaced, not to close and not to far. I could use my smaller hammer drill with a smaller bit to gently start a piolet hole. This would greatly increase my odds of getting the holes in the best location. The Hitachi is such a beast that it can jump around a bit before it bites in.

 

[4570Man]

Without renting a core drill the drilling a bunch of small holes and hammering it out method is about the only way. 6” is probably a big enough hole you could cut it to partial depth with an angle grinder to make for less chipping. Another option would be to not install the bathroom vent at all. You can run the vent pipe horizontally in the attic and tie it together with one of the vent stacks that are already through the roof.

 

[KWentling]

Ya, you can rent a core drill machine. It has to bolt to either the ground or wall to use. Uses water

That size hole can be drilled with a hand drill, but has to be a good one. I've drilled 8" with a hand drill. The issue is getting it started, but if he used a common hole saw to get through the rubber and OSB etc. that will guide the core drill. They need to be run with enough water to make a slurry so that might not work for him. I've drilled over 100 holes this year from 1 1/2" to 12" and up to 20" deep so I have some experience with core drilling. Other than the 1 1/2" they were all drilled with a vacuum base drill. No bolts. The concrete has to be pretty smooth for that to work though.

 

[3gunr]

If you are putting a rubber boot around it then cut a square hole with a grinder or circle saw. Or maybe a sawzall with a diamond blade.

 

[Paddy]

If the roof was just bare concrete and not finished inside, core drilling might be considered. But as I have said, there is 4 inches of foam on top and living space below. So no water and no way I know of core drilling free hand. Like most flat roofs there is no attic, just 24 inch tall steel bar joists. Cutting a square hole cut with a saw to a depth of 4 inches would be a huge hole, far larger than the normal boot sizes. The boot I have is made for 6 inch to 1 inch pipe. So I'm at the largest end of the boot with about 7 inches before I get into the "peal and stick" part. I plan to drill a 7 inch hole slightly angling out below.

I will be drilling small holes around the perimeter and then removing chunks. I have done this many times where time and efficiency was not an issue. It is a roof and I need to quickly cut a hole and get the boot on, I don't want a sudden rain.

The original question was if there was any tricks to drilling patterns that allow chunks to pop out. For example; once the perimeter is drilled, drill a crescent shape so the chunk is not captive. then keep drilling lager crescents until I meet the perimeter. Point being, its best to nibble at an edge and then expand.

 

[ponytug]

Thanks for the more detailed and helpful explanation. I wouldn't core drill yours.

In your shoes, I would drill a series of 1/2" holes around the edge, and then use a wide chisel on the hammer drill to break the webbing between the holes. I would be tempted to make a drill guide to help the bit get started on the circumference, e.g. 6" diameter 4" thick piece of wood, with a drill hole on the edge. If you went to a 1" hole, you might be able to get a carbide coated sawzall blade down the hole to cut the webbing. You will probably need it when you hit rebar anyway.

Personally, I hate to cut into flat roofs because of the future leak risk. Is there another viable solution?

All the best,

Peter

 

[4570Man]

Why do you need a 6” diameter vent vs a 4”. A 4” is a lot more in the range of hand held core cutters.

 

[Paddy]

ponytug,

I'm with you, I don't like cutting holes in my flat roof. That is why I have put this off for 10 years! Amazing we have managed with out vents. I did cut a pc of drywall in my ceiling so I can tweek the pipes.

As a side note, removing the webbing between holes using a smaller bit works, drilling diagonally. This works great for straight lines. Curves are tricky, but a circle in a 4 in deep hole, will be difficult.

Plan

I drilled one hole from below so I'm certain where the hole goes. As of now it's just through the concrete. Final step tomorrow will be to drill through the foam, OSB and rubber and see where it pops out.

The rubber, OSB and 4 in foam will be cut at 6.5 in clean circle. I'll do this with a Saws-All, but angling out a bit so I can remove foam to mark the circumference on the concrete.

Drill the perimeter, again slightly outward. As noted above, I'll have 2 hammer drills, one to act as a center punch, the other to drill the hole. This is to control webbing left

Drill holes from 12 to 6 and 9 to 3

The first chunk I will remove will be a rectangle, 3 of the 4 sides will be free. Knock it down

Then drill pc of pie, knock them down

I found that I have an 1-1/2 bit. I now plan to drill the perimeter, thinking bigger is better

 

[Paddy]

I have to thank my wife for choosing the the extra quite bath fan with a 6 inch vent pipe.

 

[3gunr]

If this is only an exhaust fan you should be able to reduce down to 4 inch , I have drilled lots of vent holes in brick houses by drilling small holes in a circle then drill on an angle in both directions across multiple holes , you can pop the whole thing out at once,