Proper way to pour concrete pad

Question

I tried to research this but I could not find any information on the proper process to pour a concrete pad against a house foundation.

There were three holes drilled in the foundation and rebar was inserted. Then concrete was poured. The concrete wet for 4 days to help with strengthening the pad.

location is Houston, TX

Edit: the pad is for a generator

Question:

Should there be an expansion joint between the pad and the house foundation?
Will vibrations of the generator propagating into the house be a problem?

The foundation goes below frost line, presumably, the base of the pad does not. Expect issues with the pad moving and the part of the pad attached to the house not moving...unless you put a footer below frostline under that pad. Pads are intentionally separate from the house structure for exactly that reason. — Ecnerwal, Jul 28 '22 at 02:44
Should there be an expansion joint between the pad and the house foundation? — amrog, Jul 28 '22 at 04:42
The expansion joint doesn’t really matter at this time. You’ll eventually get cracking at the joint because the two pieces are likely to move differently. — Aloysius Defenestrate, Jul 28 '22 at 13:03

score 4 · Answer 1 · answered Jul 28 '22 at 13:26

A pad should not be attached to a house foundation at all, unless the pad is sitting on top of an extension of the house foundation complete with footers below frostline - at which point it's not a pad. There's generaly no reason to even abut the house foundation - pads are frequently placed a short distance away from the foundation. They support ancillary equipment that is connected via means that has some flexibility built into it.

A pad floats - it moves with the ground as the ground moves. In any freezing climate, what you have built will be wiggling or flapping up and down (at least one flap per year) while hinging on the point where you've attached it to the house foundation, which will be staying put. That will cause damage to both the pad and foundation.

In a non-freezing climate, you might get away with that, and the house foundation footer might not be much deeper than the pad base. In a freezing climate, this is a disaster that will take about one winter to start happening.

Makes sense. We live in Houston and rarely have a freeze with the exception of the unusual winter storm that hit us in Feb 2021. At this point it is poured so my options are limited. Is there something I should do? — amrog, Jul 28 '22 at 15:08
AC compressors are set on similar pads .For both of my pads ( about 3' X 3') they were precast. Set on level ground several inches from the foundation. No problem of any kind in years. PS; there is no real frost line in Houston, possibly a legislated one. Several of my neighbors have generators; none appear to touch the house foundations. — blacksmith37, Jul 28 '22 at 16:08

score 2 · Answer 2 · answered Jul 28 '22 at 13:26

2

The size of the slab does not need an expansion joint. If the slab was a 10'X10 then perhaps maybe, if larger than that, yes. If it was poured between 2 walls definitely, and no rebar tying it to the wall, defeats the purpose of an expansion joint.

answered Jul 28 '22 at 13:26

Jack

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Thanks for the clarification Jack – amrog Jul 28 '22 at 15:04

score 1 · Answer 3 · answered Jul 28 '22 at 15:59

I don't know what kind of generator you're putting there, but if it vibrates at all, you'll feel it and hear it in the house. I had to mount a large commercial compressor in my workshop on anti-vibration mounts because the vibration when direct mounted to the slab was so objectionable.

For all the reasons stated in this and the other answers and comments, do yourself a favor: get out the sledge hammer and break up the pad, cut the rebar and relocate the forms a few inches away from the foundation. Pour yourself a new pad. This is the only chance you'll have to make it right quite so easily -- without a generator on the pad!

score 1 · Answer 4 · answered Jul 28 '22 at 16:09

Frame Challenge

The generator should not be on a pad attached to the house (what you have now).

The generator should not be on a pad next to the house (what others have suggested).

The generator should be far away from the house.

This is primarily for safety with respect to carbon monoxide (super dangerous) and exhaust in general. For example, Generac says:

18 inches (1.5 feet) from the house
60 inches (5 feet) from doors, windows, and fresh air intakes

I got a "portable" generator and it actually says you should run it 20 feet away from the house. That is a bit extreme (but with no good place between 5 feet and 20 feet, I am actually putting it 30 feet away where I have a good spot for it). But you definitely don't want a generator right next to your house.

In addition to carbon monoxide and exhaust, the farther away from the house, the less the noise will bother you. No back up power system is quiet except a battery backup (e.g., Tesla Powerwall). Some generators make a lot more noise than others. Of course, a bit of noise for a few hours is an acceptable trade-off for having your refrigerator, computer and air conditioner running during a utility outage. But a few feet will make quite a difference.

If your generator is going to be powered by natural gas, then distance does affect installation cost. If it is going to be powered by a separate propane tank or by gasoline then the only down-side of moving it away from the house is a one-time cost for a longer electric cable.

Proper way to pour concrete pad

4 Answers4