Building a Backyard Boulder

Building a Boulder

August 13, 2005:

Last night was the first climbing session on the new boulder. It marks the end of months of work. I put together this article as a resource to those who want to build their own boulder and as a story of what I went through to get mine built.

I made the comment just yesterday that this has been the biggest project that I have ever taken on. I am so excited to have it done. I wasn’t too sure how to organize this article other than to just start writing. There’s a little bit of introduction and then it goes through the stages of construction – 10 stages total. At the end I added some Q&A – if these don’t address a question you have then feel free to contact me through the forum or email me.

I’ve had some form of a climbing gym in every “home” I’ve been in for the past 13 years. It all started with a makeshift wall on the back porch of my childhood home. It looked more like a giant mousetrap than a climbing wall. It was a 14 foot long wall which overhung 75 degrees and was supported on the end by old 2x6s. The thing shook when you climbed on it. We didn’t have manufactured handholds so we made holds by cutting holes in the plywood and forming our own holds out of scrap wood. We had two “real” handholds; a white Metolius horn-like hold and a red Straight-Up hold. We placed the real holds strategically for maximum usage.

First climbing wall we ever built.
First climbing wall we ever built.

As time went on I got better at building and expanded my idea of what home gym climbing could be. I built walls in garages, attics, basements, back-rooms, and under bridges around town.

I knew that I’d always have climbing close at hand if I continued to build my own walls. From those roots I had a dream to one day build something big in my backyard when I finally decided to settle down.

In March of 2005 my wife and I moved into a new house in Norman. The backyard was large – large enough to contain a boulder. And so the wheels started turning. What I wanted was something that was big enough to provide some variability in climbing over time, something that felt close to climbing real rock outside, something you could top out on.  I started to think about the plausibility of constructing such an object. There were many different ways to do it but what I finally settled on was that I had a bunch of lumber and plywood from other gyms and I could reuse all of those materials and cut my cost in half.

So I sat down and began to list all the materials and all the steps that I would have to go through to put together a boulder. I had an idea in my head of some of the features that I wanted in a boulder. I further developed those ideas by sketching them out. I had pages and pages of ideas – sometimes quickly scribbled and tossed, sometimes I shaded them and tried to make them look more lifelike. I thumbed through magazines, guidebooks, watched videos and climbed for ideas. I looked online at the boulders that companies had built and scoured the forums for anyone who might have also ventured into boulder building. But the resources were limited – there were no companies willing to give up valuable information likely in fear that others might start their own business or build their own walls or boulders. The few people that I found were very willing to help and have been valuable resources to me.

After the drawings I had a pretty good vision of what I wanted but I still wasn’t sure how I was going to make it work. So I bought some balsa wood and began constructing a model framework for the boulder. I used foam board for the base and hot glue for the “welds.”

Stick design
Stick design

At this point I was still considering building the frame out of steel – but I didn’t need to do the math to know how much it would cost to buy the steel and rent a welding machine or hire a welder. I’ve done a little bit of welding but I am no pro – and I knew I couldn’t afford to hire a welder. In my model I realized that there are things you can do with metal that you simply cannot do with wood. It may seem obvious but to join wood you have to use screws or nails (screws being much more secure) and there are only certain configurations in which two pieces can be put together – usually involving complex cuts. Whereas with steel, there may be some basic cutting but you can tack and weld two pieces together if they touch anywhere. Also with steel, the framework necessary to hold up a boulder would be less than using wood. With steel, because it is so much stronger, you don’t need the crisscrossing inner support beams – the frame of the structure is strong enough. With my wood frame I would have to build an inner box and work out from there. The box would be anchored to the ground and the walls would be built off the box.

Following the drawings and stick models (I think I built 4 models total, each different) I then built a model out of clay. The clay model more closely resembled the final boulder because it was easier to shape the outside of the boulder with my hands rather than build the frame and imagine what the walls would look like. However, I should tell you that none of the drawings or models had detailed measurements to them. I did determine the size of the footprint that I wanted and the heights of the boulder – but even those things changed when I got started building.

I thought of the project in stages…

Stage 1 – concrete slab

Stage 2 – central posts anchor to concrete, rudimentary box formed

Stage 3 – walls constructed

Stage 4 – plywood added

Stage 5 – wire mesh added

Stage 6 – texture phase

Stage 7 – painting

Stage 8 – t-nut holes drilled

Stage 9 – clean-up

Stage 10 – Add holds and climb

Stage 1

I debated pouring a slab versus sinking posts into the ground. I decided on the slab – though more expensive – because I felt it would offer a more solid foundation and would provide a dry storage area inside the boulder. The plan originally was to pour my own slab but I’d never poured concrete so I contracted it out to the same guys that poured the concrete for our house. It was $300 and well worth it. The slab is 4 inches thick and 6 feet wide by 11 feet long. It took them a couple of hours in the afternoon – it would have taken me who knows how long – and it may not have been right.

Finally in (11) Finally in (12)

Just to back up – before the slab was poured I had a load of gravel delivered. Ideally this should have come after the slab was poured but it didn’t happen that way. In any case, 7 tons of gravel covers about a 40 foot by 25 foot area at 3 inches. The cost was $235. This was actually pretty cheap for a load of gravel. The real work came when I had to shovel every bit of it from the place he dumped it to the boulder area.

7 tons of gravel (1) 7 tons of gravel (2)

OK – so with the gravel and slab in place I was ready to begin building!

Stage 2

I had to purchase some wood for the central box portion of the frame the rest of the wood came from two home climbing gyms. The posts for the box were 4x4x12 posts and there were four of them. I first attached (using a hammer drill) metal brackets to the concrete slab, one on each corner. Then I set the posts on top of the brackets. I attached crossbeams between the four posts. For this you need some help to hold the post while you attached the crossbeams.

housestuff (20) housestuff (19) housestuff (8)

I wanted the boulder to angle down on the top at one end for water drainage and to prevent the look of a big box so I cut two of the posts down some.

housestuff (6) housestuff (5)

In retrospect, this is something I should have thought of beforehand and cut them while they were on the ground. (As you will see there are a lot of areas where I should have done something differently – that’s all part of the learning curve I guess).

Stage 3

After the four posts were set and crossbeams were attached to provide a central box to build from I began to construct the walls.

Even though I had sketches, stick models, and a clay model when it came right down to it I was building using my favorite method – freestyle. It goes against everything in the book – but that’s the way I’ve always built climbing walls so that’s the way I wanted to build the boulder. The idea when creating something like a rock is to avoid symmetry – how many symmetrical rocks do you know of? None. Rock, by nature, takes on erratic and unusual shapes – every rock is different. By taking out a measuring tape I might have tried to match sections of the wall and I didn’t want to do that. Instead, I held up a piece and thought about how the other pieces would meet it and what I would need to do to make it work. In this way I came up with nearly every feature of the boulder.

housestuff (14) housestuff (15) housestuff (18) housestuff (13)

It is difficult to work with the limitations of wood – especially when you are trying to build something like stone – stone and wood are fundamentally different and behave differently especially when you are trying to make one look like the other. Wood (as in 2x4s) likes to have corners – it likes rectangles – it’s cut to make houses, which are very symmetrical. Triangles work better for climbing structures because you can link several triangles together and give the illusion of roundness. But the trick is getting wood to join together to make triangles. This makes for some interesting bevel cuts and unusual joints. Using metal tie-plates is extremely important for these joints because they increase the strength substantially. The metal tie-plates come in various shapes and sizes and are found near the lumber section in Home Depot or Lowe’s.

housestuff (21) housestuff (22)

After the main walls were constructed I designed a door to enter the boulder.

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Stage 4

At this point I had the walls completed framed. I started thinking about water impacting the interior frame so I decided to paint it with Kilz.


As I mentioned, all of the plywood came from previous climbing gyms. I did not have to purchase any of it. If I had to purchase plywood I think I would have had to buy about 15 sheets which would have been about $350. The plywood I used already had most of the t-nuts installed which reduced much of the initial work of drilling holes and hammer in t-nuts. However, I did have to remove and relocate some t-nuts because of cutting plywood or needing extra t-nuts in some pieces. Also, once the boulder was completed I have drilled holes in areas where I wanted to fill in with holds. (Note: You can really do it either way – drill the holes and add the t-nuts before texturing, like I did, or do all the drilling afterwards. The disadvantage to adding the t-nuts afterwards is that you really can’t hammer the t-nuts in once the texture is on because you risk cracking the concrete. Instead you have to have some hold the t-nut in place while someone else tightens each one down with a hold from the outside. It’s time consuming but a safer way to do the job.)

Cutting the plywood to form took several days. There were many small pieces and unusual cuts and some sections required reinforcing after the plywood was attached to the main structure.

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One of the major jobs was reinforcing the top of the boulder. I used 2×6 beams for the top and then used additional pieces to reinforce it even more. In retrospect, I might have designed the top a little better – using additional 2x6s or even 2x8s. I spent a lot of time adding materials after the fact just to ensure that the top would be bomb-proof. Another reason to beef up the top – aside from keeping climbers from falling through your boulder – is so that you can add an anchor on the top if you want to – in case you want to have a top-rope setup for kids.

Stage 5

The wire mesh was a big area of contention. I didn’t know whether I should spend the money and buy enough wire to cover the entire boulder in the stuff or just buy enough to cover the corners and major seams and use concrete glue on the rest. The wire mesh is actually called diamond metal lathe and it costs about $6.80 for a 2.5’x8’ piece – so it ain’t cheap.


I chose to cover just the corners and seams and go with the concrete glue (another mistake).

It took 10 sheets – I was able to cut each sheet lengthwise to create 4 or 5 strips and use those for the corners and seams. I attached the metal to the wood using regular wood screws. I considered using staples but I would have had to use industrial staples and an air-gun stapler. Again, another tool I wish I had.

You cut the wire mesh using a good pair of tin-snips – you’ll know if you have a bad pair because it will take forever to cut the stuff. The wire is extremely sharp – no matter how hard you try not to cut yourself, you will at least once when you start using the stuff. I tried using gloves but it was a hassle so I ended up being very careful and not using gloves – just watch the edges of the wire.

The wire went up with relative ease – I had a lot to cover and I was working alone so it took more time than expected.

Stage 6

The next stage is the texture phase. This was the area I was most worried about and had the most difficult time figuring out what to do. I did a ton of research on concrete, stucco, polymers, texturing methods – I spent a lot of time trying to pin down the best material (based on quality, performance and cost) – but in the end I still didn’t have a solid answer on what to do. If you are building a boulder this is probably where you will have the most trouble. I would add this is also the area of greatest expense. It was an area that I underestimated in every way. But not to be discouraged – there is a way through this rambling forest of briars and ivy. This is the area that will test your dedication and try your patience – but if you preserve the end is right around the corner.

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The texture you use is greatly dependent upon what you want your boulder (or wall) to look like. Being an outdoor wall I wanted something waterproof except for the t-nut holes, of course) and something that resembled the look and feel of rock. Now if I just wanted waterproof and didn’t care so much about rock look and feel I might have gone with a good old paint and sand – it really works well. I even read that you can mix some Elmer’s glue in and it adds a little more durability to the surface. There are also some pre-mixed texture paints available through Metolius, Cheap Holds, Stone Age, Groperz, and Passe Montagne. But I was dead set on something more rock-like which meant I would be in the realm of concrete products or resin systems. I called about some resin systems but they were just way too expensive – and I would need the tools and know-how to do it correctly. Again – I might have hired someone if I had the money but that wasn’t an option. Which brought be to the next cheapest option: cement.

Cement is an amazing substance. Do you know the difference between cement and concrete? The terms are used interchangeable sometimes but they are actually very different substances. I didn’t understand the difference before I started this project but now I know more than I ever knew there was to know about both (BTW, you should look it up on

Ask 10 different people how to make a mixture for a rock wall texture and you’re going to get 10 different answers. Here’s what I would advise people to use: 3 parts sand, 1 part Portland cement, 1 gallon of Acrylic fortifier, stucco fiber, water until thick doughy consistency. You see how I copped out on the water part of it by not giving an exact measurement? Actually, I copped out on the majority of the measurements by saying “part” and then I didn’t give you an exact amount on the stucco fiber either. Well, all of this is for good reason – and it was the same kind of information I got when I was putting my texture mix together. That’s because mixing a batch of concrete has a lot to do with feel. It is very much an experimental science – you really don’t know how it’s going to go until you try it a couple of times. And even then it’s not always going to turn out exactly the same. It is a variable product. Maybe I’m wrong on this but I haven’t measured out my cement, sand mixture exactly – not even once. One part cement, 3 parts sand – eyeball it. That means I mix about  1/3 bag of Portand cement and 1 70lb bag of sand – that’s a little rich on the cement side. The more acrylic you are able to afford the better (I can only afford half a bottle per “batch”). I haven’t used stucco fiber because I can’t find it in Oklahoma – people don’t do stucco here – but if I could find it I would use it. It strengthens the mix a lot and keeps it from cracking.

OOPS – I said the word – cracking. Get ready for this because you are not going to like it  – concrete cracks. Your job is to do everything you can to keep it from cracking. But it will win in the end. Concrete, like everything else in the universe, moves towards entropy. Cracking is concrete’s entropy.

The wire mesh, the stucco fiber, the acrylic – all of these things will keep the concrete from cracking. BUT if you put in too much water into the mix you’ve screwed yourself – it’s probably going to crack anyway. I don’t know how many times I read it or people told me – “don’t add too much water you can always add more later” and what did I do? I added too much water on at least three batches! Those three batches have since been removed and replaced with better concrete.

So there – I’ve given you a recipe – now be a chef – mix the ingredients to perfection and everything will be fine.

Or maybe not…

Because what if what you are putting your texture over is not solid? Wood is not very solid – especially where there are seams and even where there are no seams. Wood flexes, bulges, expands, and contracts. So not only are you trying to keep the concrete from cracking but you are trying to control the subsurface, the wood, so that it doesn’t move and cause the concrete to crack. I wish I had read this somewhere or said it to myself earlier to see how much sense it makes because it would have saved me a lot of effort.

I received some advice that said I could use Elmer’s concrete adhesive rolled or brushed on over the plywood and then trowel on my concrete over this. Now – not to be too negative about it because it DID work – in sections, in fact, about half of the boulder has concrete over the glue without wire mesh. But I do not think that this is a long-term solution. It would have been much better to use wire on the entire structure. The glue is about $21 per 1 gallon bottle and I purchased two bottles, which even for the size of the boulder, was enough. Working with glue is tricky. You brush it or roll it on and then let it get tacky. This glue works just like regular Elmer’s glue, it sets up very quickly – especially in warm weather. So you have to be ready to trowel on your cement mixture within a few minutes and you cannot cover too large an area in glue or it will dry before you get the texture on. Combine the glue with an overhanging wall and a slightly wet mixture of cement and you’ve got yourself a frustrating task. The mixture will drop off in clumps bringing the glue with it.

I found several places after the concrete/texture cured that had separated from the wall and gave a thin concrete veneer. In other words there was a gap between the concrete texture and the wall. A good tap with a hammer and I could make a hole in the texture and crack it away.

On the areas that were like this I cracked the concrete off and covered them in wire mesh. Any areas that I banged on and sounded suspect or showed signs of early flexing or cracking I removed.

I replaced all of these areas with wire and retextured them.

Stage 7

My first instinct was to mix the color in with the cement. You can purchase pigments for concrete and stucco so that’s what I did initially. But the pigment is expensive and you have to use quite a bit of it to hide the natural grey color of concrete. After a few batches of using pigmented concrete I decided to go a different route. I purchased flat exterior latex paint (water-based) and cut it with water. Latex paint is pretty cheap (comparatively) and it is easy to apply. I used a rock-like tan color called Bryce Canyon (cool name for a color) and found some other colors to compliment it. I wanted to add some water streaks so I found a charcoal black color. I used a brush to apply the paint and used a sea-sponge to add highlights with a slightly darker color to give it texture. I used the black paint and poured it from the top of the boulder down the wall to give it a natural water-streak look. (Just as a note – I screwed up the paint a couple of times – went a little overboard on the colors and water streaks so I had to paint over them. But that’s the good thing about paint is that you can usually fix whatever you mess up by painting over it.)

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Stage 8

After the paint was dry I had to redrill the t-nut holes. My method for doing this was to drill a pilot hole from inside the boulder through each of the t-nuts using a 1/8” masonry bit. From the outside I drilled the larger hole (either 7/16” or 3/8”) to the t-nut. Some of the t-nuts popped out when I drilled the larger hole from the outside because of the force of the drill through the concrete.

I went back over several places on the wall and added new holes for t-nuts too. This was really a two-person job because you need someone on the inside to hold the t-nut in place while someone on the outside screws the holds on.

I completed this stage in August so it was about 100 degrees on the outside of the boulder and even hotter on the inside of the boulder.

Stage 9

The clean-up phase included all of the leftover construction debris I had laying around. Also, the gravel landing surface and the brick border keeping the gravel in had taken a beating so I spent some time fixing those things. I’m including this as a stage in and of itself because the clean-up on a project this big can be time-consuming.

Stage 10

This is the best part! Finally, the payoff. Add the holds and climb. Oh, by the way – you’re going to need a LOT of holds.

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Biggest lessons learned – things I would do differently:

  • Use diamond lathe wire mesh instead of concrete glue (you may need some concrete glue for small areas)
  • Do not over-saturated your cement mixture with water
  • Partition gravel area around boulder better from water runoff
  • Apply for the proper building permits

If I had the money what would I do differently or add:

Build the frame out of steel

Use a polymer modified or resin modified cement texture

Add a raised gravel bed and a higher slab

Apply for the proper building permits


How much did the boulder cost?

I know everyone is going to ask me this so I may as well answer it – with a few caveats. Remember that I didn’t have to buy plywood, holds, some of the lumber, and t-nuts. Going through the receipts the amount I spent was about $1500 – but I think I lost some of the receipts. If you are planning on building your own boulder from scratch (of equal size) I would estimate the cost to be between $3000 – 4000, without holds. Adding in the cost of the holds I would guess that you’d have to spend at least another $1000 – $1500.

What kind of texture did you use?

Portland cement mixed with sand and acrylic fortifier.

What are the dimensions of your boulder?

The boulder is 12 feet tall at its peak and angles down to about 10.7 feet at one corner. The base of the boulder is 11 feet long and 6 feet wide. The top of the boulder is approximately 15 feet long and 10 feet wide. It’s kind of mushroom shaped.

How long did it take to build?

I started in February 2005 and finished in August 2005. That’s about 7 months of work. A lot of it evenings and weekends.

Why is the plywood white and why is there route tape on it?

The plywood was from a previous wall, which was painted white and had routes marked on it. I just didn’t bother to take all the tape off since I knew that I would be covering it anyway.

What’s the name of the boulder?

The Thunderhead II (of course there’s a story there).

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