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Concrete Pad (slab)

I am planning on purchasing and then housing a portable generator in a suncast utility shed I purchased. I need to assure both are level. I plan on placing the shed, housing the generator, on a concrete pad I'll have to build. This project is to be a DIY. I plan on using Quikcrete Crack Resistant Concrete mix and a mold to create it. My concern, before I even begin, is how to provide some kind of drainage for this small slab and maintain a level surface at the same time. I don't know how necessary drainage will be for an approx. 4' x 6' slab sitting in a backyard; but, I would like to avoid mould growth and also keep the generator area relatively dry. Any thoughts or sugestions?

A. Spruce
Re: Concrete Pad (slab)

A perfectly flat slab won't hold water, it just won't shed it as quickly as a sloped slab. For the size you're talking about, don't worry about it, just set your forms level. When you screed it off, DO NOT over-screed it, the more times you drag a screed across it, the more concrete you'll remove, and the more likely you'll create a divot that will hold water. Also, make sure you crown your screed (bow up ).

You should also keep the top of the slab higher than the surrounding dirt, the amount will be dependent upon the slope of the surrounding grade and other landscape details.

Since you're going to put a shed over the slab, you have to worry even less about water issues/drainage, if you're really concerned, you can size the slab to the shed and use Z-flashing around the perimeter to prevent water intrusion, but again, for a generator, not an issue. Also, make sure this shed is well ventilated and that the generator is exhausted outside of the shed, both of these things will prevent toxic gas build-up within the shed, whether the generator is running or not.

Re: Concrete Pad (slab)

If you have never worked with concrete, read/watch demonstrations on youtube first, before you start.

For a small slab, there is no need for drainage. You just lay a 6mm plastic sheet before you pour, then bring the plastic up and secure it on the walls, under the exterior of the shed. The shed interior will stay dry like this. Just make sure the finish grade of your slab is 4" or 6" higher than the ground.

Re: Concrete Pad (slab)

Watching some good YouTube videos is a good idea. Watch the ones from the QuickCrete company.

You can make the exact footprint of the generator the high spot and the rest of the slab sloped at 1/4" per foot or more, OR you can make little legs for the generator OR just make it flat and don't worry too much about it.

Re: Concrete Pad (slab)

You've got some good advice already to watch videos from DIY projects using Quikcrete. Try to watch actual DIY videos if possible, even if it is the product being used by a paving contractor. At least you will know exactly what you are getting into, and you don't have to wonder if it is just a promotional video from the Quikcrete company.

Here is a video selection just from doing a quick search on youtube. Watch the one's that best match your project.

DIY slab with Quikcrete

Good luck!

Re: Concrete Pad (slab)

Concrete is never perfectly flat, so if puddling might be an issue I'd add some slope- it won't hurt the generator to sit on a 1/16 or 18/ inch per foot slope. The warning to place engines on a level surface is to discourage the less-intelligent from trying to run them on a hillside where the slope is too large. If the slope still bugs you, use disposable bondo paddles to shim the machine to perfectly level. Only on a house slab or foundation do I aim for perfectly level because dampness is going to be a bigger enemy that levelness in most cases. In your case I'd slope towards the door from all directions, just keep it slight.


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