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Before installing your spiral stair, or even choosing a place to install your spiral stair, one important factor to take under consideration to ensure the long-term stability of your stair is that of the foundation. How solid is the ground directly underneath where the stair will be? If it’s the only place where your stair can go to suit your purpose, what steps can you take to ensure the ground is suitably solid if it isn’t already? And if there are steps to be taken to make it solid first, what steps can you take on your own and which will likely require a professional?

This is a series of questions you should go through when looking at where to place your stair, because you’ll want the chosen ground completely ready before the day the stair arrives. Otherwise your project will present much more of a headache than it needs to.

One way to figure this out easily is to beak potential spiral stair location down by simple categories then figure out which have foundation issues and which don’t.

If your spiral stair will be indoors then it’s highly unlikely that you’ll have to make any adjustments to your floor space before installing your spiral stair.

One location where you may consider making adjustments is if your stair will be on a concrete floor in the basement.

The concrete will be more than stable enough. But it may have some cracks or crumbling in it due to age and maybe even water flow through some storm doors and across the slope of the floor toward the sub pump. Especially if it’s an older house with an unfinished basement.

If this is the case then taking the time to fill these cracks with a quick concrete mix and trowel job should be sufficient. Let the concrete repairs cure then install as planned.

If you’re spiral stair is going to be outside then there are a lot more dynamics to consider. Will this stair be on a deck, patio, driveway, or just a concrete pad? Or is there any type of finished floor at all where you plan to install the stair? Each of these will have different factors to check before knowing the stair will be secured.

If you’re mounting a spiral stair directly to your deck then you’re likely safe. Especially if the deck is of newer construction. If you’ll be installing a heavier outdoor stair such as a galvanized steel stair instead of an aluminum stair, then it might be prudent to install extra support into your joist system beneath the floor boards just under where you’re putting the spiral stair. Galvanized steel is almost 3 times denser than aluminum, so the added precaution is a good idea. Depending on how high your stair is you may just reinforce the joists, or (space permitting) add a post. Installing a footer (see below) beneath the post if your deck is raised or flush with the underside of the decking may even be necessary if your stair is large and heavy enough.

If your driveway is blacktop, concrete, or any other hard surface with a sufficient depth to it, then your surface is stable enough for installation. Just check how long the bolts for your base plate mount will be so you know you have a thick enough surface to work with. Just keep in mind the same principle as with the basement concrete floor. Any cracks or crumbling should be repaired first to keep your ground level and less likely to deteriorate underneath the stair.

If your driveway consists of pavers, however, then don’t automatically think your driveway is stable enough just because the surface is hard and smooth. Especially if the pavers aren’t mortared together.

Anything like a gravel driveway will obviously first require the installation of a concrete pad or at least a footer.

The same principle applies to patios. If it’s made from pavers that haven’t been mortared together then you’ll need to first install a footer. You may need to teach yourself/lookup how the cut a circle into your current pavers so you don’t have to lay new ones down and so you don’t interrupt the flow/pattern of your pavers. Once you’re circle is cut, you have space to pour your footer.

Concrete patios need the same treatment as any other concrete structure—just repair any cracks or crumbling first so you don’t have a surface that might gradually wear away from underneath the stair’s base.

Concrete Pad:
A monolithic pour concrete pad is just like any other; make sure it’s in good shape prior installing a heavy spiral stair atop it. Repair cracks and crumbling. And make sure it’s of a sufficient thickness before bolting your stair base into it.

Another factor to consider when it comes to a concrete pad is that, should there not be solid ground where you wish to install your stair, a monolithic pour concrete pad is relatively easy to do and cheap to pay someone else to do. It’s a quick solution to your stair foundation problems.

Taking the time to pour your own pad specifically for a stair foundation also gives you the opportunity to really customize your outdoor space.

One issue that immediately makes for a precarious foundation is any kind of slope. Whether it’s how your land lays or if your concrete pad is sinking on one side. If you have a problem with the ground sinking where you’d like to pour a concrete pad then you need to do some direct filling prior to pouring a pad. If you have a pad and one section of it is sinking then you need to either teach yourself how to raise it or pay a professional to do so.

One thing you cannot have is a stair on a slanted plane. This is unsafe and is sure to create excess stress in the structure. So check how level your desired installation location is prior to erecting your spiral stair.

One of the simplest, non-invasive things to do in your yard to create a workable platform for your spiral stair is to install a concrete footer. This is essentially just a concrete tube that goes below your ground a little deeper than your local frost line. This is a common foundation for deck posts. It’s a very simple process that can be carried out by any DIYer.

A footer can be installed in bare ground, just beneath a deck, just beneath a patio, pretty much anywhere outside where’d you’d like to erect a spiral stair.

There are other solutions to foundation/groundwork problems you may encounter in the course of selecting a location for installing your stair. But so long as you take the steps to ensure your foundation is solid and secured you’re good to go with your project. Just take the time to inspect so when you climb your stair you’re standing on solid ground.

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