Every year our school has a sports day during the last week of school. This year I built a water slide down the school's front steps as one of the activities for the kids (and adults) to enjoy.

Step 1: Materials Needed

The exact material list will largely depend on your site. Modify this list to meet your needs. The most important ones for building on a set of stairs are the four Ps...

Plywood - You need some rigidity and a little flexibility. Not too thick, not too thin. Goldilocks would pick this thickness. Somewhere around a half inch should be about right.
Padding - I was very lucky to have interlocking closed-cell foam pads that are about an inch thick. You could use carpeting, carpet padding, camp mattress pads, the interlocking alphabet children's mats... With some time you can find something and with a budget you can buy something. You also may need some padding with some higher friction to get people to stop; like an old mattress.
Pool noodles - small, giant, LED lit, glow in the dark...
Plastic sheeting
Tape
Garbage bags
Stairs
Hose and water

Step 2: Build It - Transitions

The first step of building a water slide is to channel your inner 14 year old and consider the transitions. Well, maybe we should back up and consider our audience first. What age will most of the participants be? What is their willingness to get some air, crash, tumble... build for that.

You want to minimize the chance of injury. Safety first! Support any areas of the ramp that need it (see the padding below the plywood). Use multiple layers if you need to. You may want to use screws to secure the plywood to lower layers. We taped the top layer seams of the plywood to keep things from slipping around. We also secured the plywood so it wouldn't slide down. After over 400 uses it still sat strong!

After you have the ramp built, pad it with your padding material. You might want to go wider than the ramp itself just in case. Out of the hundreds of uses we had from kindergarten through adult, nobody went off the water slide and was injured. But accidents happen!

Step 3: Build It - Bumpers

Tape the pool noodles together to make a long continuous bumper for each side of the slide. Pool noodles work great because they are soft, cheap and flexible. You could use PVC pipe, rolled up carpeting or other things as bumpers too. This helps contain the water and the rider. You'll need one for each side... or three to have two runs next to each other for races!

Step 4: Build It - Plastic

Lay out your plastic on the ramp. Cover the length of the ramp with one edge of the plastic along where you want the pool noodle to be. Think of the profile of the ramp plastic looking like an S. You want the pool noodles to lie in the hooks of the S. (The best way to illustrate this is with a crude drawing.) Lay one noodle bumper on the plastic and fold it over. Lay the other noodle on and fold again. Then tuck the remaining plastic underneath.

Step 5: Use Plastic to Go Faster

We found that by sitting in or on a garbage bag or other plastic that the ride was much faster. It didn't prevent people from getting wet - many didn't have swim suits. But the ride was quite fun this way!

Step 6: Run It Safely!

It is important for the slide to be safe so that everyone has fun. If your slide will be used by many people, take a page from the professionals and have someone stationed at the top and bottom. Be sure the landing area is clear before the next person goes. We didn't expect to slide so far on ours so had to build padding around some fencing. Next time we'll make sure there is more space. We also might make a wider run - perhaps 8 feet wide instead of 4 because some kids really wanted to do spins and experiment with tricks when going down.