Over the past 50 years I have made 3 ferrocement hot tubs as well as a couple of Saunas. This blog is about the hot tubs.
It is a lot easier than you might think to build a ferrocement hot tub and they are more durable than the fiberglass ones. Just like a fiberglass boat, a fiberglass hot tub has a lot of problems. The first has to do with having hot water in it. It is well know that the longer a boat remains in the water the more moisture that goes thru the porous gelcoat and into the fiberglass backing and the hotter the water is, the worse the moisture penetration is. The water in a hot tub is much hotter than any water that a boat would ever encounter, so the problem. By contrast, the longer cement is exposed to water, the stronger it gets. Now for building a ferrocement hot tub.
All of my hot tubs were rectangular though they could be round or some kind of compound shape. It is just easier to go with rectangular, especially if you wanted to line it with ceramic tile. I tiled one and painted two with boat paint. Here is my procedure. I first make an upside down male mold in the shape of the hot tub out of plywood or particle board. Next I cover it with a roll of .003" plastic I got at the hardware store. I stapled it down all over with a manual stapler. Next I covered the mold with two layers of 1" mesh chicken wire which I also stapled down in a few places. Next I cut pieces of 12 ga fencing wire as long as required and place them in a rectangular pattern with about 2" spacing all over the mold and stapled them down just enough to hold them in place. At all corners, I bent the wire so that it provided strength around the corners. Next I added two more layers of the 1" mesh chicken wire and stapled it down well this time. This completed the basic mold.
Next I figured out where I needed plumbing holes for jets and water in and out of the tub. I measured each of my plumbing fittings and found "bean cans" slightly larger than the fittings. I used wire cutters to trim holes in the four layers of chicken wire but did not go thru the plastic. I next took the appropriate "bean cans" with the bottoms still on and put them thru the holes and screwed them to the mold. I next tightened the chicken wire using a mallet and a pair of pliers. By twisting the layers of chicken wire with the pliers, I could get the chicken wire very tight to the mold.
Next for the cement. I used regular cement mixing 2.5 parts sharp mason's sand that I had sifted with an old window screen with one part Portland cement and 0.25 parts of diatomatious earth that is used in swimming pool filters. The diatomatious earth provides the "fines" needed to assure that the concrete is as dense a possible. I mixed the concrete very much on the dry side.
To apply the concrete I used an old vibrating sander that I installed a sheet of thin sheet metal in place of the sand paper. The sander acted like a vibrating trowel that forced the dry side concrete well into the mesh. As the concrete packed it you could see that it actually had plenty of water. Because the concrete was on the dry side, it all stayed in place. The wire was all covered and the thickness ended up only between 3/4" and 7/8" thick.
I let it dry for a couple of hours and then using a large water spritzer, I spritzed the tub all over. After about 6 hours I covered the molded tub with an old blanket which I had also wetted down with water. I next covered the whole thing with a plastic tarp and for a week, once a day, I sprayed the tub with a garden hose and put the plastic tarp back on. I waited an additional week for the tub to cure and then flipped the tub over and took the mold out as well as the "bean cans". With a small disk grinder, I trimmed off the ends of the staples on the inside.
From here on installation of the hot tub was just like installing any other tub. The beauty of this method is that your designs are almost unlimited and you can custom fit your hot tub into almost any existing space. Plus it will end up costing you a fraction of a plastic hot tub and last last longer!
PS--I got part of this idea from ferrocement boat making and shower stalls that my father had made using chicken wire and plaster years before.
It is a lot easier than you might think to build a ferrocement hot tub and they are more durable than the fiberglass ones. Just like a fiberglass boat, a fiberglass hot tub has a lot of problems. The first has to do with having hot water in it. It is well know that the longer a boat remains in the water the more moisture that goes thru the porous gelcoat and into the fiberglass backing and the hotter the water is, the worse the moisture penetration is. The water in a hot tub is much hotter than any water that a boat would ever encounter, so the problem. By contrast, the longer cement is exposed to water, the stronger it gets. Now for building a ferrocement hot tub.
All of my hot tubs were rectangular though they could be round or some kind of compound shape. It is just easier to go with rectangular, especially if you wanted to line it with ceramic tile. I tiled one and painted two with boat paint. Here is my procedure. I first make an upside down male mold in the shape of the hot tub out of plywood or particle board. Next I cover it with a roll of .003" plastic I got at the hardware store. I stapled it down all over with a manual stapler. Next I covered the mold with two layers of 1" mesh chicken wire which I also stapled down in a few places. Next I cut pieces of 12 ga fencing wire as long as required and place them in a rectangular pattern with about 2" spacing all over the mold and stapled them down just enough to hold them in place. At all corners, I bent the wire so that it provided strength around the corners. Next I added two more layers of the 1" mesh chicken wire and stapled it down well this time. This completed the basic mold.
Next I figured out where I needed plumbing holes for jets and water in and out of the tub. I measured each of my plumbing fittings and found "bean cans" slightly larger than the fittings. I used wire cutters to trim holes in the four layers of chicken wire but did not go thru the plastic. I next took the appropriate "bean cans" with the bottoms still on and put them thru the holes and screwed them to the mold. I next tightened the chicken wire using a mallet and a pair of pliers. By twisting the layers of chicken wire with the pliers, I could get the chicken wire very tight to the mold.
Next for the cement. I used regular cement mixing 2.5 parts sharp mason's sand that I had sifted with an old window screen with one part Portland cement and 0.25 parts of diatomatious earth that is used in swimming pool filters. The diatomatious earth provides the "fines" needed to assure that the concrete is as dense a possible. I mixed the concrete very much on the dry side.
To apply the concrete I used an old vibrating sander that I installed a sheet of thin sheet metal in place of the sand paper. The sander acted like a vibrating trowel that forced the dry side concrete well into the mesh. As the concrete packed it you could see that it actually had plenty of water. Because the concrete was on the dry side, it all stayed in place. The wire was all covered and the thickness ended up only between 3/4" and 7/8" thick.
I let it dry for a couple of hours and then using a large water spritzer, I spritzed the tub all over. After about 6 hours I covered the molded tub with an old blanket which I had also wetted down with water. I next covered the whole thing with a plastic tarp and for a week, once a day, I sprayed the tub with a garden hose and put the plastic tarp back on. I waited an additional week for the tub to cure and then flipped the tub over and took the mold out as well as the "bean cans". With a small disk grinder, I trimmed off the ends of the staples on the inside.
From here on installation of the hot tub was just like installing any other tub. The beauty of this method is that your designs are almost unlimited and you can custom fit your hot tub into almost any existing space. Plus it will end up costing you a fraction of a plastic hot tub and last last longer!
PS--I got part of this idea from ferrocement boat making and shower stalls that my father had made using chicken wire and plaster years before.
