VILLAGE JOURNAL Building a Farallones Style (Dual Batch) Compost Toilet - Part 2
In finishing the slab we have to fashion the top of each chamber floor into a drain pattern to direct the excess liquid effluent to run down the slab and into the drain hole without pooling in any place. A fall of about 10mm is recommended. If for some reason you don‘t get this exactly right at the time of pouring the slab, don‘t panic, you‘ll get a chance to correct this later, after the block-work is completed.
EnviTE Green Team participants Cayle, Josh and Sally get tactile with the mortar laying. Ordinarily a pointy trowel is used in preference to fingers.
5) Block-work. The next step after letting the slab cure for a few days is to lay the courses of besser blocks. In this case we use the half width blocks (90mm wide) as they do the job well enough at about half the cost. But if you want an even sturdier structure and can afford it then go for the full size blocks. The other thing to note is that we‘ve opted to modify the plans and go one course higher (to 6 courses) to allow for a slightly larger chamber. There are a few tricks in doing blockwork. Firstly you‘ve got to get the mortar mix just right, can‘t be too runny or too dry. We use a General Purpose (GP) as opposed to General Building (GB) cement powder and mix 1:3 with brickies loam. As with slab pouring it‘s good to get a bit of momentum up, and dampen the face of the blocks about to be laid and use a rubber mallet to tap down lightly. Very important is to constantly check the course is level horizontally and vertically along the outside wall. We opted not to use string lines as they can get in the way and the advantage of having a few people on the job is that someone can be specially assigned to regularly check with short and long spirit levels and adjust where necessary while the rest of the team steams ahead with the blockwork.
There are a few things to include between our courses. We place between 1st & 2nd and 5th & 6th courses some 8 x 125mm gal coach bolts sticking out around 75mm head first. These are later to hold the raised metal floor and baffle. We also insert some tie wire into the back corners of each chamber allowing enough out to later hold the vertically mounted slotted 100mm ag pipe which provides for extra air circulation in the chamber. At the front end we sink some sections of continuous 8mm threaded rod, protruding out about 40mm, to later be used to attach the form-ply chamber doors to.
It‘s very important that you remember to put these fixtures in place as you do the block-work as concrete when set can be very unyielding. The walls have to be absolutely straight and even more importantly the front face of the blocks has to be really flush so that when you later attach the chambers doors they seal nicely all around.
The blocks come in full, ¾, ½ and corner shapes (see plans) with the layout changing according to the course (1,3,5 or 2,4,6). It‘s good to do a dry run before you start the mortaring to get an idea of how the block arrangement works. Another important tip is to make sure you keep the wall that divides the two chambers, in the centre, running perfectly parallel to the other walls, as the chamber diameters need to be the same to comfortably fit the steel grated floor and baffle that is to come.
Hopefully, by the time you‘ve finished the top course, all your walls are straight and the blocks are level at the top. You can now leave it to set and start preparing for the next stage - the suspended concrete floor.