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A porch is a porch of course of course..
January 6, 2014 4:19 PM   Subscribe

We are creating a new front porch at our house. The existing porch is a three inch concrete slab set on foundation walls. A previous owner then overlaid really ugly sandstone flagstone on the top and sides. The new porch is going to be smooth, hard troweled tinted concrete with a bench along two sides. Here are my two questions.

1. I have gotten three bids on the concrete. One bid says they have to remove the existing slab before doing any new cement work, basically demoing the entire porch. The two other bids feel there is no need to remove the slab, it is in perfect shape and after removing the flagstone there will be two inches of new concrete on top of the slab and are confident that there will be no cracking later.

Of course the bid that includes the removal is significantly higher than the other two.

I personally feel the porch will be fine built over the old slab, but of course I am not a cement contractor. Any thoughts on this greatly appreciated.

2. Regardless of the cement work, I would like to have metal support uprights for the bench which is going to be on two sides of the porch. I will set bolts in the concrete to secure my supports for the bench. I could use wood, but would like to investigate using metal. So I am trying to locate pre-existing or price fabrication of a "T" that would bolt into the cement wall below and then secure the seating rails along the top. If I use metal it needs to be something that will not rust and discolor the cement, maybe even stainless? More or less flat stock "T" with approx. a two foot bottom and a 10" cross member to accommodate the seating rail.
posted by silsurf to Home & Garden (7 answers total) 1 user marked this as a favorite
For the concrete work, did the contractor recommending removal give any rationale? I don't know why you would want to remove concrete in perfect shape.

I think you want a welder/fabricator for a quote on your seat supports. It will be expensive (and even more so in stainless rather than painted steel). Wood will be much cheaper.
posted by ssg at 4:38 PM on January 6

The issue with the existing slab is whether the new slab on top will crack or not. The one that wants to remove the existing slab says anything put on top of the old slab will crack, the other two contractors feel it is not an issue and removal is unwarranted.
posted by silsurf at 4:47 PM on January 6

2" is too thin for regular concrete. Should be 4" thick minimum and have at least welded wire fabric reinforcement.
If you went with topping cement, 2" would work, but like stated above, you'd be dependent on the old slab not moving.
posted by rudd135 at 4:52 PM on January 6

Don't use steel or stainless metal if you're worried about discoloration. I'm not envisioning your setup, exactly, but you may be able to fab something using aluminum extruded shapes at Home Depot.
posted by notsnot at 5:47 PM on January 6

I realized since I wrote this that aluminum is probably the best bet, thanks
posted by silsurf at 7:06 PM on January 6

You'll want to make sure that the alkali content of the concrete won't corrode the aluminum.
posted by Monday, stony Monday at 7:36 AM on January 7

My guess is that the one contractor that wants to replace the slab thinks that the cracking would be due to the existing slab flexing a bit, since 3" is indeed a bit too thin for a slab (especially if there's no reinforcing in it, which may well be the case). I'm not sure if his concern is legitimate, but you can test it yourself simply by checking out the condition of that existing ugly stone. If it (and the mortar between the stone pieces) has a bunch of cracks in it, then so will your new concrete topping. Just to be clear, in any case the "cracks" we're talking about probably wouldn't be blatant, but would be more like hairline cracks.

By the way, whether you go with the existing or a new slab, you will always get at least some cracks over time. It's just what concrete does. But by adding control joints (scoring the concrete), the cracks will almost always occur only at those joints, so you don't notice them. Presumably your contractor would know this, but maybe not.

To build on Monday stony Monday's advice: If you have bare aluminum against bare concrete, they most likely WILL corrode each other. To avoid this, at least at the point of contact, one or the other (or both) should be painted. I'm not sure if even that would be effective. A bituminous coating (on the aluminum, at the point of contact) is a more reliable solution, as would be a thin separator of a neutral material (e.g. plastic) between the two. With stainless steel you wouldn't have to worry about any of this (and I disagree that it would cause any discoloration).
posted by Mechitar at 10:55 AM on January 7

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