Thoughts on reinforcement vs. ease of screeding for hillside stairs?

I skip the center runner every time beacuse fighting that 2x4 with a mag float is a recipe for a messy finish. Instead, I rely on heavy-duty kickers on the outside of the stringers. If your side forms are 2x10s or 2x12s, just drive some 24-inch steel stakes every couple of feet and use a diagonal brace back to a 2x4 block in the dirt. It keeps everythign from bowing without ruining your screed path.

For the riser boards themselves, I usally rip a 45-degree bevel on the bottom edge. This lets you get your trowel right into the corner of the tread below without the wood getting in the way. If you’re worried about the risers bulging in the middle on a wide set, try using a bridge—basicly a 2x4 that spans across the top of the side forms, with a vertical cleat holding the center of the riser. You can unscrew it and lift it out once the mud starts to stiffen up.

I've found that using Simpson Strong-Tie screws instead of nails for those temporary braces makes life much easier when you're stripping forms. It’s all about keeping the path clear so you can get a nice, even pull with your screed board across the entire face of the step.

I acutally swear by the center runner, expecially if you are dealing with a 4-foot or wider span on a steep grade. If you rely only on side kickers like the previous guy suggested, the middle of your risers will almost certainly belly out under the weight of a 4000 psi mix. I usually rip a 2x4 down to a 2x2 and suspend it about an inch above the finish grade of the treads using cross-cleats. This gives you the rigidity you need to prevent bowing without blocking your float entirely. When it comes to screeding, I do not try to go side-to-side across the whole tread. I work from the center out. Having that center point fixed gives me a consistent grade to pull from. I use a shorter 2-foot screed board and just work each side of the runner. Once the concrete starts to set up a bit and the risers are holding thier own, I unscrew the cleats, pop the runner out, and fill the small void. It adds twenty minutes to the finishing process but saves you from having a "pregnant" set of steps that look amateur from the bottom of the hill.

Have you ruled out the risk of the whole form assembly floating or lifting once you start vibrating the mix? Even with those heavy-duty kickers, I’ve seen steep runs shift upwards becasue the wet mud wants to find level. I’m also curious if you’ve considered the slump you’re planning to pull. If you’re running a 5 or 6 slump, no amount of center bracing is going to save those risers from bulging or the bottom steps from overflowing. Have you looked at going with a much stiffer mix to take the pressure off the forms in the first place?

I'm still relly new to this, so I'm trying to follow along with the 'bridge' idea vs. the 'floating' risk. If I go with the bridge idea to hold the center of the riser, how do you know exactly when it’s safe to unscrew it? I'm terrified that if I pull it too soon, the whole middle will just sag or pop out, and if I wait too long, will the wood be stuck in the drying concrete? Also, since someone mentioned the 'slump,' how do I even check if my mix is too wet for a steep slope like this?

While the previous mention of a bridge cleat is technically sound for minimizing lateral deflection, you must ensure your riser boards are actually dimensionally stable; if you are using standard SPF lumber without checking for crown or twist, your treads will never be truly plumb. I personally favor using 3/4-inch plywood ripped to the exact riser height and reinforced with a vertical 2x4 'stiffback' screwed to each riser center, wich effectively resists the hydrostatic pressure of a 4-inch slump without the intrusive footprint of a continuous center runner. This configuration allows for an unobstructed screed path while providing enought rigidity that you won't experience the catastrophic bowing that typically occurs when the vibrator head gets too close to the formwork.