Gunite and gutters

We made a lot of progress last week on our rainwater catchment system. We had given considerable thought to what type of water tank we should install. Most homes in Hawaii that rely on rainwater catchment use corrugated steel tanks. These are probably the most economical. But we didn't really like the look of the steel tanks and were hoping to find something that would be easier to hide. In order to get homeowner's insurance, we are required to have a 10,000 gallon tank - ostensibly for fire protection - although it never made too much sense to us that the required tank size was the same no matter the size of your house or your lot. So we're talking about a pretty big tank.

After some investigation, we decided to go with a concrete tank from Pacific Gunite. They had the most experience on the island with these types of tanks. The cost was twice what a steel tank would have been, but we decided the quality, durability and look were worth the investment. We could bury most of the tank below grade, and with a concrete lid, it would be sturdy enough to build a shop or storage shed on top.

The tank installation was very interesting! I had never seen something like this built before. The hole for the tank had already been excavated last fall when we had the other sitework done. The tank crew placed a layer of fine base gravel, then formed and poured the bottom of the tank. Next, they put up three layers of different sized wire mesh, from coarse on the inside, to fine on the outside. Then they sprayed the gunite (concrete) onto the wire mesh, on the inside and on the outside. The tank wall is about 4 inches thick at the bottom and about 2 inches thick at the top. Finally, they formed and poured the lid. They also formed and poured an interior support column at the same time. This whole process took about six days over a two-week period.

Installing the wire mesh

Wire mesh reinforcing completed

Spraying gunite on the inside

Spraying gunite on the outside and troweling the inside

Smoothing the outside

Forming the bottom of the lid

Additional formwork for the lid (under a rain tarp)

Lid reinforcement

The tank is important, but it's only part of the rainwater catchment system. The other important part is the roof gutter and downspout system. The gutters catch all the rainwater that falls on the roof and send it through four downspouts connected by piping under the house and into the tank. With over 100 inches of rain a year, we should have more than enough for our daily water needs. Maybe even enough for a hot tub one day! The other wonderful thing about having the gutters installed was getting rid of the dripline that just poured off the roof next to the front steps. There was no way to go in and out of the house during a rainstorm without getting drenched!

One of four downspouts connected to the roof gutters

After the tank is completed, we will disinfect the water with UV and remove particulates with filters. The collection piping also has cleanouts so we can remove leaves and other debris that might come off the roof.