An Inground Trampoline Design

A note for my regular readers, like the much-acclaimed Bercomac snowblower post, this post is intended to help out the many people googling "inground trampoline" every day. Since I really knew nothing about this before embarking on this endeavor, and I started with a bit of googling and found almost nothing useful other than a bunch of people trying to sell me kits, I'm taking the time to document this in the hopes that I can be of some help to others in the same predicament. That is, people who have a bit of the do-it-yourself streak but want a starting point from which to get the creative juices flowing.

Of course, it wouldn't be Me, CherkyB, if there weren't some color commentary along the way. I'm no Norm Abram.

A little background: when we bought our "wonderful" house here in Fort TomCollins, CO, it already came with an inground trampoline. However, like most of the exterior landscaping features of the house, the design came up a bit short on the longevity scale. There was a hole dug in the ground, and a retaining wall of sorts was built using 3/8" plywood for about a foot in height, and then two rows of Trex decking material. I'm guessing this material was chosen as it could be bent and formed into a circle easily. It certainly wasn't chosen because it would stand up to the pressure of the earth. It was, at the time of purchase, largely buckled inwards, though the 12 support posts were holding. Thus, the earth was slowly sinking around the trampoline, washing away a little more with each rainfall. The final straw for the design was when I was jumping on the trampoline with my son, and one of the springs tore off. Unable to find a replacement trampoline of the exact same dimensions, I really was forced to redo the whole thing.

This design is one that developed while I was chatting with my neighbor about what to do. It's a good deal of work (about 6 man-days), and has been dubbed by my loving and always supportive wife as "the most over-engineered inground trampoline ever," but I think it should hold for 10-15 years at least. Keep in mind that, since I already had an inground trampoline prior to this, I'm skipping step one: dig hole. I'm not sure what kind of fancy design needs to go into that step, but mostly, I think it involves digging a hole that's about 2 feet deep at the edges and 3-4 feet deep at the center. That's a lot of digging for a shovel, but your local rental center will rent you any number of different kinds of mini-excavators that will do the trick, though I hear it may be a better deal to hire this part of it out.

Before we get to the details, here are the materials and tools lists. There is nothing exotic in either list. Both can be had at either Lowe's or Home Depot. I spent about $350 on the materials, but I'm in a medium-high priced geography. I managed to get by without buying any new tools for this project, though I did rent a spiffy laser level. You could do the job without a laser level, but it was a lot easier to get the levels marked on the posts with one.

Materials List:

• 6 4x4x8 pressure treated lumber, each cut in half to 4' lengths
  
• 6 2x4x8 pressure treated lumber
• 18 2x8x8 pressure treated lumber
• 1 4x8x3/4 sheet pressure treated plywood
• 96 3.5" coated deck screws
• ~100 2.5" coated deck screws
• 12 60lbs. bags of concrete
• 4 4'x8" tubular concrete post forms, cut into thirds (16" lengths)
  
• 12 wooden stakes
• Mason's line
• 6' rebar
  
• Landscape fabric
• Landscape fabric stakes
  

Tools List:

• Shovel
• Post-hole digger
• Plumb bob
• Tape measure
• 4' level
• 1' level
• 12" compound power miter saw (or 10" sliding compound miter saw)
• Speed square
• Miter saw protractor
• Post level (optional)
• Electric drill/driver
• Saw capable of cutting 4x4 posts in place (Sawzall, chain saw, hand saw, big circular saw)
• Handheld Saber/Jigsaw
• Router
• Roundover bit (1/4" or similar)
• Laser level
• Circular saw
  

Whew. That seems like a lot, but most people who would be undertaking such an endeavor probably have most of these tools already.

Now, here a few drawings explaining the basic design. First, a top view. I'm building a 12-sided polygon (dodecagon) around the trampoline. I've chosen 12-sided in this case because most larger, circular trampolines have 12 legs, and this lines up nicely. I've decided not to go circular like the previous owner did as I am not convinced anything other than corrugated steel can easily be formed into a circle and also hold up to the pressure of being a retaining wall. The 12-sided nature of the design also means the spans are slightly under 4' each (on a 14' diameter trampoline), so I can buy 8-ft lumber. 8-ft. lumber is easy to handle, and it wheels well on the cart in the home centers, fits nicely in the pickup bed, and can be loaded into the miter saw by one person without assistance. It's a very convenient size.


The angle between each post is 30 degrees, as there are 12 such angles in the 360 degree circle (360/12 = 30). We get to use something called "the law of cosines" to determine the straight-line distance from the center of each marker stake to the next, as labeled "C" in the above figure.

What this says is that for whatever radius you have chosen for the stakes, the straight-line distance from the center of one stake to the center of the next is 0.517638 times that radius. OK, so what do you do with that?

Well, for starters, you're going to want to sink a stake into the center of the hole. I used the 6' rebar for that.

Then, pick a radius that is large enough to be outside the hole. For instance, if you are putting in a 14' trampoline like I am, the radius of the trampoline is 7' (radius is half the diameter). I'm going to want to pound some stakes into the ground to mark these angles, so I'll add a foot to that to clear the lip of the hole. So I have an 8' radius for the marking stakes. Tie a piece of mason's line (mason's line is string that does not stretch or sag - available at your home center in the aisle they sell plumb bobs and chalk lines and the like) to the center stake and mark 8' from the center stake on it. Stretch this out to the ground past the hole and pound one of your 12 stakes into the ground at the 8' mark.

Now, rotate the mason's line until you are exactly 0.517638*8' (4.14ft = ~4' 1 2/3") from the center of the previous stake. Pound in the next stake. Go all the way around until you have all 12 stakes in the ground. If the distance from the last stake to the first one is much bigger or smaller than the distances between all the others, somewhere it is messed up. Go back and check if everything is uniform. Make adjustments as necessary, but don't go nuts on this. You can handle some non-uniformity. You just don't want it to look like you were drunk when you laid it out.

Now that you have 12 stakes in the ground, run a mason's line from the center stake to each of the 12 stakes high enough above ground so that they are more than 6" above the grade. This is so as to not interfere with the posts in a later step, as the posts will initially be too long and will be trimmed later. Also, try to make them all about level with one another (you can use a line level or laser level or just eyeball it- the exact measurement comes later). You'll end up with something that looks like this (though, for accuracy's sake, I actually tried to do this by measuring the angle between lines when I did it, and it didn't work. So these directions are the new, improved directions.):


Alternatively, you can cheat by using the frame from the trampoline and marking where each leg is. That's a whole lot easier.

Now, maybe it's time for me to explain exactly what you're trying to build here. The key to this design is that the walls are keystoned in place between the posts. That's a fancy way of saying that they are trapezoidal, and thus they cannot actually move forward once they're put in place without pushing the posts out. But the posts can't push out since that would push the next section of wall, and so on all the way around the dodecagon and back to the start. Thus, the design could be considered "self-supporting" in that any pressure from the earth serves only to tighten up the wall connections, not to loosen them.

Here's a side-view of the wall. I'm using a 2x4 plus three 2x8's to achieve the height I want. I could have used 3 2x10's, but my 12" miter saw won't cut a 2x10. If I had a slider, it would. But a 12" slider costs a fortune, and a 10" slider can't cut 4x4's.
Your next step is to set the posts. This is the biggest pain-in-the-behind step if you have clay soil like I do. It helped that it was over 100 degrees every day I worked on this.

In the previous step, you established a web of mason's lines radiating out from the center of the hole. Now, you're going to want to take a Sharpie pen (or something similar) and mark on the mason's line the distance from center you want the posts. I suggest that you place the inside edge of each post 1" farther than the radius of the trampoline. So, for example, if you have a 14' trampoline like mine, place the inside edge of the post at 7'1" from the center of the hole. Then start digging.


Use your plumb bob from the mark on the mason's line to make sure your hole is in the right place. Keep in mind you marked the front of the post, not the center, so the center of the hole should be half the width of a 4x4 (3.5"/2 = 1.75") farther back than the plumb bob. Dig until you're 16" or so below grade.


Now, drop a 16" long, 8" diameter post form into the hole, take a 4' 4x4 (which you made by cutting the 8' ones in half), and drop it in, too. I like to put a post level on mine, which is the red thing on the post two pictures up - a level that attaches to a post with an elastic strap and gives you plumb on both sides simultaneously. The post form will hold about 60 lbs of Quickcrete, which I dump in dry and then add water, stirring with a piece of rebar. You don't have to worry about getting each post at exactly the right depth, as we're only using 42" of the 48" post (the rest to be trimmed to fit after the wall is in). You should put the "factory" end of the post down, as that end is soaked in the pressure treatment, whereas your fresh-cut end won't fare as well in the ground without you coating it with something. It'll be OK up in the air, though.


Once you have all the posts in and the concrete is set (overnight if you didn't buy the quick-dry), it's time to get serious about the level. I rented a laser level for this step. $10/hr. You could also take your 4' level and mark level on each post all the way around the dodecagon. But I wanted to play with a laser level. It doesn't matter exactly what height you mark, just that you mark the same height on all the 12 posts.


Now, figure out the height you want the wall, and mark height of the top of the bottom board on each post. Do this by measuring down from the laser-level line a fixed distance on all posts, and use your speed square to make sure the measurement is transferred to both sides of the post without wandering. You should expect that your concrete footings are not all at exactly the same height, so you should start out with whichever post has the highest concrete, as measured by the distance down from your level line. You can't put the bottom slat of the wall any lower than the highest concrete footing unless you plan to do some notch cutting in the slats to clear the concrete.

This next part requires a modicum of precision. Get yourself a miter-saw protractor like this one:

You're going to be using this to mark the angle between the wall and the post. You didn't get every post in at exactly the same angle, so going forward, every cut is custom-fit.

Lay your 4' level (or a 4' straight edge) across the top of two adjacent posts in line with where you want the wall. Mark the top of the posts with pencil along the side of the level, as you'll be coming back and forth on this a couple times. Use your miter saw protractor to mark the angle between the left post and the level.

Then, go to your miter saw and duplicate the angle. Dear God, please unplug the saw before you do this, as it involves laying the protractor along side the saw blade and adjusting the tilt until it matches the protractor. Only a moron touches the blade of his miter saw with it plugged in. Don't be a moron. Now, write down the angle from the saw indicator once it is dialed in. You have 4 cuts to make at this angle, so you'll have to return to it three more times. Cut the end of the 2x4 to this angle.

Go back to the posts and use the protractor to measure the angle on the right post to the level, placed in the same place it was before as marked by the pencil lines. Also, hold the freshly cut 2x4 in place and mark the length with a pencil, transferring the markings to the face of the 2x4 with the speed square so you can see it when you cut.

Now, before you make a cut, measure the distance between the fronts at the tops of the posts, and then measure the distance between the fronts at about the center of the height at which you will place the wall slat. Are the tops closer together than the bottoms? If so, the distance you just marked off on the 2x4 is too short, and you should add to it the difference between your longer measurement and the measurement at the top of the posts. Are the bottoms closer together? Again, adjust the marked line by the difference, though it's easier to make a board shorter if you cut it too long than it is to make it longer if you cut it too short.

Now, transfer the angle for the right post from the protractor to the miter saw (unplug! first!) and write it down, too. Cut the other end of the 2x4 to length. I like to cut a little long at first and then trim to fit.

You may have to go back and forth a few times to get each board to fit just right. It's time consuming, but it'll go faster and faster as you progress.

Now, if the post is severely out of plumb, you may want to use the protractor to cut a compound miter to match the tilt of the post. I was never off by more than 1 degree, but 1 degree seemed to call for a compound miter.

Once you get the board to fit, put it in place with its top edge on the height line you marked earlier for the bottom board. Before you screw it in, check its level with the shorter 1' level (the 4' level won't fit in the span). Then, "toenail" the sides in from the top with 3.5" deck screws (shown in the drawing above a bit). These screws are not structural, as the wall is self-supporting once fully constructed, but they are needed to hold everything in place while building. Countersink the screws such that they don't stick above the top of the board and interfere with the next board.

Now, do the same for the 2x8's for the next 3 levels between these posts. Use the miter saw angles you wrote down for the first board. Stack them up snug to the board below. You don't have to measure for level as there's nothing you can really do about it if the boards aren't straight unless you want to send everything through a jointer.

Repeat 11 more times.


One thing I forgot to mention is that pressure-treated lumber is toxic when you cut it. Wear a respirator.

This next photo is of me driving home the very last screw of the wall. Note how the 4x4 posts are all different heights. These are cut to length after you build the wall, cuz if you try to sink posts into concrete at exactly, exactly the right height, you're asking for trouble.


Here, I have sawed each post off flush with the wall. I used a Sawzall for this. You could use a chain saw if you feel like sanding afterwards, or you could use a circular saw and make a cut from two sides (as it won't cut 3.5" deep). Or, you can cut by hand. I did that once. Forget about it.


Here's a top-view of one of the posts showing how the wall mates to it. That small gap on top will close when some ground pressure is applied. Ground is on the right and hole on the left in this picture. The deck screws were self-drilling, and they countersunk with just the drill screwdriver. I didn't predrill or use a countersink bit.


The next step is to set in the trampoline frame. My trampoline is 3' tall, and I'm setting it with the top about 2" above the wall. This way, the angle is such that no one falls onto the wall if they fall off the trampoline. Now, for those of you paying attention, the wall is 26" high, and I'm set 2" above that for a total of 28" above grade. But the trampoline frame is 36" high. That means I am sinking the legs 8" into the slope that leads from the bottom of the wall to the lower center of the hole. [Note: I'll add a drawing here soon.]

Sinking the posts below grade gives at least one advantage - if for some reason you experience some surface erosion on the slope, this won't undermine the footing or level of the trampoline. You need 8" of erosion before you need to worry, and that much erosion simply cannot happen unless you dig the center of the hole many feet deeper than the wall. Which you won't do because it's both a massive amount of work and completely pointless.

I used my 4' level to get the trampoline frame leveled out. You could also use the top of the retaining wall, assuming it came out. I double-checked my level vs. the wall.

Almost done. I have the landscape fabric down to prevent weed growth, and I have excavated 16" around the wall about 4" deep to fill with "playground mulch" AKA "tanbark". I've got the trampoline frame about 2" higher than the wall so that any tumbling off misses the wall and lands in the mulch.


This post shows what I did with the plywood, jigsaw, and router. I wanted to draw some diagrams and some more detailed instructions before placing it all here, but I have not yet had the time. If you have questions, leave a comment and I'll respond.