We’re probably going to have out patio replaced this month. It’s a poorly poured, patched-with-asphalt, cracked, ugly concrete thing we inherited with the house.
We’re looking at having someone come in to remove it all with a bobcat and then repour
new slab, properly sloped, slightly above grade, control joints, etc. The slab will be 3-4″ thick, 6-bag mix with fiber and a curing agent (at our option…we may stain it, so we may forgo that.)
Anyways…the question: At some point, we may add a pergola above this. We don’t want to attach it to the house, so it’d be free standing on 6 posts. Can these 6 posts use the slab as the ‘foundation’ or do I need to think ahead here and poor 6 piers now, and then poor the patio around that? the front piers could always be added in front of the slab, but the posts in back would be sitting on the slab itself, unless I decide it’s worth the effort to pour concrete piers now, and pour the slab around them, but I’m not sure if I’d reall need it since I see things like Garages built directly on slab these days.
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this is a function of the underlying soil, groundwater, and frost depth conditions.
look at it this way, if the ground heaves and the slab tilts a quartter inch, you may not notice it much underfoot, but if that tilt plays into the column at ten feet high, that could result in a 3 or 4 inch lean, depending on how everything is tied together.
If you have decent drainage and well compacted stone/gracvel mix under the slab, you are probably OK, but if water will be puddling right around there and possibly freezing, then you are likely to have a problem.
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Thanks, piffin. So it sounds like it's not really a structural issue (ie, taking the load of the pergola...which souldn't be that much) but rather your typical how-well-is-it-poured issue.
We don't have good drainage (pretty much solid clay) and so I will be entrusting this person to put down a good base layer, as well as plenty of slope. Part of the reason for the new patio is due to lack of slope on the old one, so we've had some water problems.
This also depends on the design of your pergola. Many designs don't have much lateral stability since the structure is pretty open without much in the way of diagonals. The pergola might have long posts sunk into the ground a few feet to help keep it upright. In that case, you won't need concrete piers.
Al Mollitor, Sharon MA
If you can lock down the design of the pergola today, yeah, I'd incorporate additional concrete below the slab where he pergola posts will go. I'd actually be less concerend with the slab moving, and more concerned with the load that the post will carry transfering to the slab cracking the slab.
Easiest, and maybe the safest bet? Just pour the slab and be done with it. Then when the pergola design is set, use a diamond blade to cut a small hole in the slab. Use a post hole digger to dig down, then either set the post deep for lateral stability, or pour a concrete footer, add a simpson bracket, and attach your post to that. That will keep the structure and weight of the pergola isolated from the slab.
Well, that'd work...I guess I was curious as to how easy it'd be to cut a 6" piece out of the center of a concrete slab (neatly, that is...).
Tho not inexpensive, you could hire a concrete cutting co. and have them core drill whatever size you want. A round hole with a square post. Or, if you are sure of the location of the poles, form and expansion those locations now.Remodeling Contractor just outside the Glass City.
Quittin' Time
For a 6" hole you have a point. Even if you got a clean hole you'd have trouble excavating down with a post hole digger. Probably have to use a poaching spade or a trowel.
I was thinking larger, 10" square or so. That'd give you wiggle room. Diamond blades on a circ saw or on a grinder work well.
Calvin has a good point about forming out the locations before you pour, but again, if you know the exact location of the posts now, then I'd just dig the base out now and pour integral footers when you do the slab. Also, you can oversize the integral footers (laterally) to again give you leeway when you eventually site the posts.
I just built this pergola last week. I set the 6 x 6 treated posts in 3' holes filled with 8" of concrete at the bottom and dry quickcrete 2" from the top of the hole.(2 bags per hole)
Plumb add water and let sit over night.
The next day I through bolted 2- 3x10 Crosses on each side. Then I notch the 3x8 to keep the whole assembly from twisting. I toe nail 3x3's on top of that.
"Rather be a hammer than a nail"
Bob
Pro-Dek...your work is always impressive. Nice stuff!
OK, good advice from everyone. Since the pergola, itself, has a 20% chance of being built, I don't think I'm going to pour the footings now. Instead, I'll just block out the spots for the rear post footings along the house, pour the slab, and then just fill those in with concrete so they're separate from the main slab. Then, if/when we do the pergola, I can pry those up/out and and poor the footing there. The front columns we can simpl put outside (in front of) the slab, so that won't be a big deal there.
Thanks again for the suggestions!
-Darrel