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Discussion Starter · #1 ·
I live in SE Wisconsin, and have a large cement slab on the back of my house. I've had 2 contractors out to give me a bid on building a 12X12 pergola on it.
The pergola would be freestanding on the cement slab, not attached to the house. They are both giving me quote for cedar.

What puzzles me is that one contractor is telling me that he will need to cut out and set the posts in cement footings, because he does not want to risk cracking the cement. (he says the pergola would be heavy).

The other contractor directly contradicts that statement, and wants to build the pergola on the slab, and just attach it directly to the slab. Who do I believe?

Both contractors have an A+ rating on BBB and come recommended by Angie's list.

I'd really appreciate anyone who can help me sort this out!
Thanks!
 

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This work might require a permit. Regardless, what does your building department say? I would be suspicious of a proposal to build on the slab, especially if the contractor doesn't know how it is constructed.
 

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I would go with the guy that wants the footing.

Make sure the Building Inspector has a copy of the print for the footings before approvals.
 

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As lenaitch noted, building inspector may not give you an option. Make sure contractor pulling a permit is part of the quote.

Guy with independent footings is probably spec'ing a better build. Interesting that you said the first guy wants to put posts IN the footings. More commonly they are ON the footings. He may be looking to set the posts in concrete for the extra rigidity that it will give you. Wood in concrete has the risk of rotting, but my old pressure treated deck had wood in concrete, and after 15 years the wood below grade was no worse than the wood above grade. For a structure where all the weight is 10 feet above the ground, the extra rigidity may be worth the risk.
 

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I've seen enough 10x10 shed on concrete slab, no foundation, to believe that the inspectors may pass direct build on the slab. I'm in NJ and frost depth is 3'. Does this pergola have a wood floor? 15 yrs is not enough time to test a life of pt lumber buried in concrete. Problem is wet/dry cycle of exposed lumber that speeds up rotting. Treated lumber that constantly remains wet will look new after 30 plus yrs. If your floor is slab and pier is finished flat along with floor, the post will rot where it sticks out of the slab. If the pier is sloped and post is flashed there, post will last longer. 12" gravel drain under the pier.
 

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Here in Michigan you would not need a permit for something like that under 200 square feet
As to whether you need footings I am no expert but I wonder how thick the slab is. I have been known to park 10 tons of cars and trucks in my driveway with no problem.
 

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Discussion Starter · #8 ·
The contractor definitely needs to pull a permit - I had checked with the Village and they do have some rules in place for these types of structures. However, I don't see anything about footings.

As far as the cement slab itself, I don't know the thickness.

I will say this - we did have to have mudjacking under 2 of the slabs because they are next to the house and had a negative grade (another one of my worries - don't want water in the basement!)

I believe you are probably right, SPS-1, that you install on top of the footings, that was probably my misunderstanding. I went back and looked at the footing description, and it mentioned that they'd dig the holes, pour concrete to grade, and then install the 6X6 posts on that.

I guess I could ask the building inspector what he thinks?
 

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The contractor definitely needs to pull a permit -

I guess I could ask the building inspector what he thinks?
Again in my area if one needed a permit one would submit a plan for approval. Don't you need to submit a plan? That is where the building department will either want to see some sort of footing or not. Best to ask them up front.
 

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Since a typical pergola has 4 or more posts carrying a point load to the ground, the safe route is to cut the slab and place concrete piers. As for setting the posts in concrete, it would depend on the design of the pergola and if there is anything else providing diagonal bracing.

You might get away without too much cracking or movement by placing it directly on the slab. However, it depends on the condition of the slab/ground and the size/weight of the pergola.

Personally I wouldn't be too concerned about cracking and would go for the simple route of attaching directly to the slab. I would add bracing to the pergola to add rigidity if needed. It wouldn't take much.
 
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