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Old 12-31-2009, 07:31 AM   #1
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Need Patio/Concrete slab advice


All,
I have been receiving bids on the forming and pouring of a two tier concrete patio measuring approximately 800sq/ft (4" thickness). I plan to put flagstone overtop of the concrete slabs. I live in Western PA, so the slabs are a must. Most of the bids have specified that the slabs be anchored to the house, and that the two slabs be ancored to each other (there would be one step down in between slabs). They have also simply decided to mortar the flagstone on the slab afterward. As I mentioned, 9 out of 10 bids are using the above method.

I recently had a bid from a fellow who told me NOT to anchor to the house. He said that there is no reasonable benefit, and that drilling through the brick facade and cement wall of the house might eventually do some damage, particularly if the patio shifts and takes the brick/wall with it. he also mentioned that if there is any settling, water might fill the void beneath that patio causing even more issues. He recommened an expansion joint running aloung side the house. He also did not recommend anchroing the slabs together. In effect, he wants both slabs to be completely floating.

This same man has also recommended setting the flagstone into the concrete itself while the cement is still somewhat wet. The stones would basically be sitting ontop of the cement, maybe 1/4 inch in. He said that he will do sections at a time to accomplish this. He said that the chemical bond between the concrete and stone is fantastic and better than the bond with traditional mortar. He will then mortar the joints afterwards. He also wants to put 3/16" control joints into the slabs and flagstone itself (basically a giant 'X" in both patios). He says they will eventually crack anyway, so cut the joints so you control where the creack occurs). I have never seen/heard of this method in any of my research.

Also, my house sits on damn near solid stone (it took the excavators and extra week to chisel out the rock to pour the foundation). This contractor said that he wouldn't drill up stone in order to put down the traditional gravel/limestone bed for the slabs. He said is doesn't make much sense.

In effect, what he had to offer made sense, but I am somewhat skeptical as no one else has offered this same advice in their bids (especially setting the flagstone into the cement--I have NEVER seen this mentioned). Any guidance here? Thanks.

Roger

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Old 12-31-2009, 07:40 AM   #2
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Need Patio/Concrete slab advice


I like that guy, I agree myself that not anchoring to house a good idea. slabs should stand alone. Pouring in sections and adding slate is fine as long as he can keep section pours flat and in plane with each other.

when they pour basement and garage slabs do they anchor to house?

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Old 12-31-2009, 07:47 AM   #3
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I agree--his not anchoring to the house seems like sound advice, I just don't know why no one else recommended it? Does it help control cracking of the patio and overall stability (maybe at the expense of potential exterior wall issues later on)?
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Old 12-31-2009, 07:53 AM   #4
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they are using the house as a ledger/bearing point. If the slab settles at the front it will create a pivot point at the house with up force. I dont like that. If you have a lot of rock under your soil as you said have them dig down in several spots to create piers that are a contiguos pour with the slab, this will add support
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Old 12-31-2009, 08:09 AM   #5
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Have you seen flagstone set in the concrete and not mortar? That is the one that gets me the most. he's offering to do it this way for a very reasonable price, so my " too good to be true" sirens are going off.
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Old 12-31-2009, 04:51 PM   #6
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Have you considered stamped concrete?
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Old 12-31-2009, 05:45 PM   #7
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I've considered it, but my wife has her mind set on the flagstone
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Old 12-31-2009, 05:53 PM   #8
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I would suggest asking him what his process is and how large an area he is pouring at one time so that he has time to properly set the stone before the concrete cures to much. I think it's doable I would ask to see a job he's done and talk to the owners adout puddling , cracks, flatness, overall satisfaction with work, loose stones, response to owner call backs if any
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Old 01-01-2010, 06:48 AM   #9
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When accepting bids from contractor's they should all be based on the exact same information.

If you gave bidder #10 the same information as the others and are now considering #10 offer I wouldn't.

When accepting bids you should compare all of them and evaluate them I say the majority rules, go with one of the others.

Unless you give the other bidders the opportunity to quote on the same way # 10 proposes. If they do bid it might be cheaper that #10.

Hope this makes sense to you.
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Old 01-01-2010, 07:42 AM   #10
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yes, i provided all bidders the exact same information. #10 called me and said that he would not recommend doing it the way I had speced out. I went back to a couple of the others and asked them what they thought about not anchoring the patio, and they said they would not change their method--they would go ahead with the anchoring. so yes, while I provided the specs (based on a design I had done from a landscaper), this last bidder was the only one to come out and say that he would do things much differently. I'm not a concrete contractor or a mason, so I am not sure what the industry standard method is here. The other contractors could very well have been quoting my specs to the letter, even if it wasn't the way they would do it if they had their druthers.
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Old 01-01-2010, 11:44 AM   #11
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Back to the basics. I don't understand why slabs are essential because you live in western PA. I live outside of Boston, I have a slate flagstone walkway in back, a brick walkway in front of my house, and a paved driveway, none of which are on concrete, and all of which have worked just fine for about 20 years. My walkways are on mason sand, and my driveway is on crushed stone.

If you are planning to walk on the flagstone or drive on them? If you are going to drive on them, there may be some merit to concrete slabs, but I don't think so, 4 inches of properly graded sand on top of an adequate foundation such as rock should work well. You can use polymeric sand to fill in the joints between the flagstones so the sand does not shift around, or you can use ordinary mason's sand as we do, and accept a small amount of sand coming out of the joints every year or so. As for mortaring the flagstones, you can do that too, however the mortar is prone to cracking, and so far as I can tell the only advantage of mortaring the stones is that it eliminates the problem of sand coming out between the joints.

If you are concerned about settlement, you should realize that properly compacted sand will not settle after placement, so neither will the flagstones. There are numerous posts on this site describing proper procedure for flagstone, brick, or stone walkways, check them out.
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Old 01-01-2010, 12:08 PM   #12
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Quote:
Originally Posted by Daniel Holzman View Post
Back to the basics. I don't understand why slabs are essential because you live in western PA. I live outside of Boston, I have a slate flagstone walkway in back, a brick walkway in front of my house, and a paved driveway, none of which are on concrete, and all of which have worked just fine for about 20 years. My walkways are on mason sand, and my driveway is on crushed stone.

If you are planning to walk on the flagstone or drive on them? If you are going to drive on them, there may be some merit to concrete slabs, but I don't think so, 4 inches of properly graded sand on top of an adequate foundation such as rock should work well. You can use polymeric sand to fill in the joints between the flagstones so the sand does not shift around, or you can use ordinary mason's sand as we do, and accept a small amount of sand coming out of the joints every year or so. As for mortaring the flagstones, you can do that too, however the mortar is prone to cracking, and so far as I can tell the only advantage of mortaring the stones is that it eliminates the problem of sand coming out between the joints.

If you are concerned about settlement, you should realize that properly compacted sand will not settle after placement, so neither will the flagstones. There are numerous posts on this site describing proper procedure for flagstone, brick, or stone walkways, check them out.
Dan you the man HOLLA!! I'm with you.
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Old 01-01-2010, 12:08 PM   #13
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Need Patio/Concrete slab advice


whay he said
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Old 01-01-2010, 01:11 PM   #14
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From what all the bidders have told me, the concrete is a must because of the freeze/thaw cycle, and what 800sq feet of patio slab is prone to do given that cycle. Smaller areas like walkways would not be as susceptible to the effects of shifting or settlement, I presume. None of them have recommended a dry set.
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Old 01-01-2010, 01:27 PM   #15
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Freeze thaw cycle makes no sense in this case. Freeze thaw is a problem that applies to silty soils, whereby the silt soil expands when moisture in it freezes. The expansion of the soil causes all sorts of problems with foundations, including shifting, cracking and displacement.

In your case, you said that you are on rock, which is NOT subject to freeze thaw. Neither is the sand you would place above the rock, since it is free draining. If you are on silty soil where you are planning to put the patio, a concrete slab has the same problems (worse really) than a dry laid flagstone patio, since the concrete is subject to cracking and displacement if the silty soil underneath undergoes freeze thaw cycles.

If you are on silty soil, then the contractor would need to excavate the poor soil down at least two feet, and replace with crushed stone. coarse sand, or similar free draining structural fill in order to prevent frost heave damage to the slab. You would need to do something similar, although not quite as deep, if you dry lay the flagstones. The reason you can afford to go less deep with soil improvement in the dry laid condition is that dry laid stones are less subject to damage from frost heave, since each stone can move around independently, as opposed to a slab, which is going to crack if it heaves.

In fact, some people build dry laid on 4 inches of sand directly above frost susceptible soil, and simply relay the stones every few years as they shift. This avoids the need to excavate and replace poor soil.

In short, I see absolutely no advantage to the concrete slab solution, only high cost and potentially poor results. If you are on poor soil, the contractors should have explained their methods for coping with the soil, if you are on rock then you still need about four inches of sand to bed the concrete in, since concrete placed directly on rock is subject to cracking due to point loading of the rock on the concrete.

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