Fabric Formed Posts

1. It often helps to pour the base of the post a couple of days before pouring the rest of it. This way, the bottom is held in place while you set up the forms.
2. Dig a hole, about 12 inches in diameter, where you wish your post to stand, whether it be at the base of an existing wall or on its own. The depth of the hole will depend on what type of force will be exerted on the post. For an acrylic concrete roof, which is fairly lightweight, we dig our hole about 24 inches deep. We always make the bottom of our holes bell shaped (wider at the very base), so that it is harder to pull up in the event of an extreme wind.
3. Place metal reinforcement  in the hole, raised up on rocks or plastic (you want concrete under any metal, so that there is no chance it can leach water and rust).
4. Level the metal (both east to west and north to south) and hold it in place with a couple of boards between it and the ground. [If the post is against a wall, attach the metal to the wall if possible, with a gap in between them.]
5. Pour concrete into the hole to just above ground level. Make sure you tamp it down every once in a while to ensure that the concrete completely fills the hole. We use a 3:1 cement to sand/gravel mix.

The size of your form will depend on the size of your post. We will describe an 8 feet tall post, 8 inches in diameter. Adjust your dimensions according to your own needs.

1. Cut a length of vinyl tarp, 8 feet long and 36 inches wide.
2. Place two pieces of  8 feet 1x4s, 22 inches apart (the tarp will stretch a little, so make it a little skinnier than you want the post to end up).
   [Make the plastic thinner if the post is against a wall]
3. There should be about an extra 7” of tarp on both sides. Double this back on itself, to make it stronger.
4. Staple one side of the tarp to one of the 1x4s, close to the inside edge. Put a staple every 2” or so – the concrete will exert quite a force on the tarp, so more is better than less.
5. Staple the other side of the tarp to the other 1×4, checking the distance between the two boards (22” in this case) as you go.
6. Remove the boards that were holding the metal reinforcement in place.
7. Stand the form up and put it around the metal reinforcement of your post, so that the two 1x4s meet, flat against each other.
   [If your post is against a wall, just put each board against the wall on either side of the metal.]
8. Screw the boards together at the top, bottom, and once or twice in the middle.
9. Level the boards in two different plains, and then brace the form using lumber or metal.
10. Pour concrete into the plastic, patting it as you go. You want the concrete a little soupy, so that it fills the form better. However, remember that the drier concrete is, the stronger it will turn out, so do not make it soupier than necessary. Patting helps to compact the concrete and release air bubbles that can make the post weaker. If you pat it too much, the post will start to bulge at that point.
11. When the concrete is hard to the touch, take down the braces and then release the screws and carefully peel off the form, which you can store for reuse. We have found that concrete sets up far quicker in a plastic form – we don’t really know why.

There are all kinds of things you can do to a fabric form to make the end result more interesting. Any wrinkles or restrictions of the tarp will be transferred to the concrete. For example, if you weave string across the tarp when it is empty, the concrete will bulge out around the string, leaving a weaving pattern in it when you remove the form.