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I just purchased a 11'x23'x8' aluminum pergola from Wayfair that will arrive in a couple weeks. It weighs roughly 750lb. In some reviews, it says the concrete screws that come in the kit are kind of weak so I'm looking at beefing them up a bit.

The pergola has 6 posts and each post has 4 pre-drilled holes on baseplate. The predrilled screw holes are about 5" apart. Research tells me I should have at least 10x distance of fastener diameter between holes so 3/8" sounds like the widest I can go. I plan to get a rotary hammer and drill some of the holes 5" from concrete edges.

I have a concrete slab that is 4" thick. No footers. Been searching the internet for past few hours trying to find proper guidance on recommended drill depth and cant seem to find anything.

There are a total of 24 concrete anchors/screws to support the 750lb pergola. I live in md and we do get gusts up to 45-50mph. I'm leaning towards Tapcon+ 3/8" diameter concrete screws to not put too much outward pressure near concrete ends(I hear wedge anchors do this).

Now after providing backstory and details of my project, here are my big question I'm hoping someone can help with...

  1. What is the recommended depth to drill into my 4" slab? (For 3/8" fastener)? I was thinking drilling a 3 1/4" hole and using these 3" Tapcons.

  2. Or should I go with the 2.25" depth fastener and drill only 2.5" into the slab?

I know there is reinforcement near center of slab, (chicken wire and some rebar) so some spots my not be able to go deeper than others.

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  • I hope that was re-mesh, not chicken wire in your slab. I suppose chicken wire would work, but...
    – FreeMan
    Nov 10, 2023 at 15:03
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    Yes sorry i call it chicken wire because thats what it looks like but it is re-mesh
    – Cristian
    Nov 10, 2023 at 18:05

2 Answers 2

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You need to drill at least 1/2" deeper than the screw. Concrete debris accumulates in the hole quickly and would prevent full penetration of the screw. All that's in the instructions, though:

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Your overall strategy seems sound. If you weren't using four per post I'd consider something with more grab, but I can't see these pulling out if they're installed properly. You could also consider adding epoxy to enhance hold.

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  • +1. For clarity, are you suggesting epoxy in the holes to help hold the screws, between the post bases & concrete, or both? I could see any/all of those as valid options. (I tend toward overkill, so my reading of the epoxy suggestion would lead me to the "both" option.)
    – FreeMan
    Nov 10, 2023 at 15:05
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    Only the screw holes. Flat joints like under the bases don't usually perform well as they're in tension rather than shear. I suppose it can't hurt, and may reduce moisture penetration.
    – isherwood
    Nov 10, 2023 at 15:31
  • So if i use the 3" screws i would need to drill 3.5" into 4" slab. Is that too close to bottom? Wondering if i should go with the 3" screws or the 2.25". If i go with the 2.25" i would only need to drill 2.75". Thanks again for all your advise.
    – Cristian
    Nov 10, 2023 at 18:21
  • What do you think happens if you drill all the way through? (Nothing.)
    – isherwood
    Nov 10, 2023 at 19:02
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Replace the shipped concrete screws with screws of equal or greater diameter and equal or greater length.

How big of a deal the anchors are depends on what they're doing. If your pergola uses moment resisting connections or bracing like knee braces, then the column anchors are there just to prevent the structure from sliding as a rigid body. The toppling of columns is handled by a lateral system like moment resisting connections or bracing.

Alternatively, each of your pergola's columns could be designed as fixed against the concrete base at 90°, where the strength of your slab provides the lateral system to the pergola. In this scenario, obsessing over the column's base anchorage is warranted.

My intuition is that a company distributing pergolas isn't going to design it so that the bending strength of your concrete is relevant to the design strength of the pergola. Such a pergola would impose difficult design constraints for a supporting wooden deck, for instance. And if people are pointing out how weak the shipped fasteners are, then that's pretty strong supporting evidence that your pergola has a lateral system independent of your concrete.

I suppose wind could cause some significant uplift if plants fill the gaps of the pergola, so that uplift would be a good reason to worry about the anchorage quality.

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