Wednesday, December 2, 2009

How To Make a Grow Bed (PART 3 - Setting Support Posts)

Setting Posts for a Grow Bed

There are two ways to set posts for the grow beds. The surface on which you are going set the bed will determine how your posts are set. If you are going to put the bed above a hard surface such as concrete, you will set the posts on footings. If you are setting up the grow bed above dirt or any other unstable type of surface you will need to set up posts with concrete into a hole, just like a fence post

Fencepost Method


For this setup we will be using the fencepost method. Here is what you should have when done with this part.


5 - 4"x4" Posts (See note below to determine length of posts.)
2 - 2"x4" Studs (See note below to determine length)
6 to 10 - 80lb bags of concrete* (your actual amount will depend on the size of the holes you dig. I always buy extra, it's cheap and has many uses)
4 - 3" Galvanized Lag Screws
4 - Galvanized Washers
Few - 2 1/2" or 3" screws

* TRY NOT TO USE POST HOLE CEMENT. If you have no choice or you already bought some, you can use it, but it will not be strong enough for this purpose unless you premix it.

To determine post lengths: Four of the 4x4's you will need to take the desired height of the bottom of the grow bed(or top of the mount) to the ground, plus 18". The fifth 4x4 post and the 2x4 posts will need to be 7" shorter than that. Depending on your length, you can get two or three posts from one 8 foot 4x4 post, so don't purchase more than you need.

Lay your mount where you want the grow bed to be. Correctly place it in position on the ground. Mark the ground where your posts will be according to the image below. Remove the mount and start digging. Holes should be about 20" to 24" inches deep. See image below.

When digging the holes, make sure the bottom of the hole is larger than the top. See image below. This prevents the post from sinking into the ground in the future. This is important because of the enormous amount of weight that will be fluctuating continuously. Once the holes are dug, set your mount back over them and make sure the center of each hole lines up with the mount correctly.

After verifying the holes are correct keep the mount in place and level it by placing blocks underneath each corner. Then add enough gravel to each hole to make them about 18" deep. Place the four corner 4x4 posts into the holes and use the mount as a guide to lineup the posts correctly. Use a level or a string line to make sure all the posts are at exactly the same height. Add or remove gravel to the holes as needed.

Now you need to plumb the posts, this can be a little tricky. Use a post level if you have one, if not you will need to do it manually by adding guide boards on two sides of a post to make adjusting the post easier. See image below.

Once two posts are plumb, put a board over the top of them and screw it into the top of each post. Make sure the tops are still level. Screw the mount into the posts for temporary stability. Repeat on opposite side for the other two posts.

Double check all of your posts for plumb and level. Once you are happy everything is good, prepare the cement according to the instructions on the bag. If you are using post hole cement, DO NOT pour it into the hole then add water to the hole as the instructions will read. Premix it in a wheelbarrow, then it should be fine to use. Do not add too much water. The more water you add, the weaker the cement will be. When it is ready, pour some into each of the four holes with posts, slightly overflowing the hole. Take a stick or something and work the cement down into the holes, removing any air pockets. Once poured, recheck all of your posts again. Make adjustments quickly for ONLY anything that is too much off the mark, but if something is slightly off, don't worry about it. You want the posts to be set firmly in the concrete and moving them around will weaken the setting. Let dry for 48 hours.

Once dry, you can move the support mount up to the very top and level with the posts. Put the washers on the screws. Secure the mount with the lag screws by pre-drilling holes through the mount and one inch into the posts. Make sure the drill bit is smaller than the screw and do not drill too far into the post. An inch will be far enough. Use a socket wrench to tighten the lag screw.

Add the next three posts(4x4 and two 2x4 posts) underneath the 4x4 beam by placing the posts into the holes and secure them to the 4x4 beam. Plumb them then add the cement to the holes. Let dry 48 hours.

This shows the center post after placing
the grow bed on the mount,
which you will do in the next part.

Here is a 3D Model for a clear view of what you should have now.

That is it, you ready for the next part.

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