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A structural engineer who designed our earthquake retrofit instructed us to put 3/4 all-threaded-rod (ATR) through the chimney at the rafter line on either side of the chimney, back them with Simpson BP3/4-3's on the outside, and anchor them with HTT5-3/4's on the inside of the rafters to 4x6 perpendicular blocking.

The chimney was built in ~1950 has 3 flues. I believe the chimney is a single-brick-width all the way around. The roof is a slick metal roof and we would prefer to minimize working from the roof.

Question: What would you suggest for procedure and tooling to accomplish this?

  • How do you keep the holes on the inside and outside at the level with respect to the other?
  • There are many masonry bits available, is there a certain type you would suggest?
    • Drill from the outside and inside, or drill through with an extra long bit/extension? (The chimney is ~28" thick, IIRC.)
  • For 3/4" ATR we were thinking to use a 7/8" bit to provide ~1/16" space around the ATR.
    • Is this the right size bit diameter?
  • What kind of compound would you use to seal around the ATR?
  • Safety considerations?
  • Other considerations?

The Chimney:

Approximate drill points are drawn in green:

chimney anchors

Engineering Drawing:

You can see the 3/4 ATR drawn in dashed lines at the top and bottom of the chimney. Note that detail 4/S7.03 is not relevant to this SE question:

chimney, top view

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    If only everyone would include this kind of detail in their questions... Well asked!
    – FreeMan
    Aug 24, 2022 at 13:01

1 Answer 1

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You need to drill this from the inside, because that’s the critical location for the tie-back to be connected. Level and inline with the joist will be close enough by virtue of following the bottom of the joist. If it becomes really easy to drill after a few inches, it means you’re in the flue — just keep sideways pressure on to not wander too far into the emptiness.

Use a rotary hammer drill and a drill bit that’ll do it in one shot. 36” bits can be found for surprisingly low prices these days. Don’t push too hard on the last inch or you risk a big blowout on the exterior brick. 7/8 would work, though you can probably get away with 3/4.

Any silicone sealant will work to seal the holes. Use plenty.

Be sure to be safe at heights: either a fall arrest system or scaffold or both would be prudent.

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