Foundations

Assembling the Foundation: Pre-drill a 6″ long x 3/8″ pilot hole through near each end of the two sides. Then use a 1/2″ diameter bit to drill through the first 4×4 only, leaving the inside 4×4 with the smaller pilot hole for the lag bolt to thread tightly. Treat any cut ends of the PT lumber to guard against rot.
Once the 4 by 4’s are bolted together and are in place, use a 4′ or 6′ level determine your lowest spot and decide if you need to raise or lower the foundation to ensure it is perfectly level. It works best if the 4 by 4 is mostly buried in the bed of crushed rock, so that the door is just slightly above grade.

The greenhouse frame will be secured to this base with pan-head stainless steel screws. The frame should not actually touch the PT lumber as the preservatives can corrode aluminum, so it sits on a cedar strip 1 ¼” wide by 9/16” thick. Don’t skip this step: it’s important for other reasons as well

It is important that the foundation be level and square. A long builder’s level is useful for this, and you can see if you are out-of-square by measuring across on the diagonals, as in the diagram. Corner to corner measurements should be the same. If one measurement is shorter by, say 3/4″, tap (rack) the longer corner by 3/8″. Check again to ensure equal measurements.

Securing the Foundation to the ground:
Once the foundation is in place and you are happy that it is level and square, you will need to secure the foundation to the ground, particularly in exposed, windy sites. On the longest 4 by 4’s, about a foot from the ends, drill pilot holes, not straight down but at about a 45 degree angle, then drive an 18 inch piece of ½” diameter re-bar through the 4 by 4 and into the ground. (An angled pin is much stronger than one that goes straight down into the ground.) The hole at the other end should be drilled at the opposite angle thereby securing the greenhouse strongly, no matter which direction the wind is blowing. Four pins on an 8 by 6 ft house are adequate, more on a larger one. For 128 and 148 houses, we usually add a third pin in the middle of each side.

What Are the Dimensions of my Foundation?
See the chart below for the model of greenhouse you have. The chart shows the outside dimensions of each greenhouse and the outside dimensions of the 4 by 4 foundation are on the right hand side of the chart below. The dimensions for the foundation will allow you to place the frame more or less in the middle of the four by four, with about an inch of extra space all around.

Once the foundation is in place (and did I mention that it has to be level and square?) and the frame of the greenhouse is bolted together, position it on the foundation, then lift it up and place the 1 ¼’ by 9/16” cedar strips underneath the sill. Next, use 1 ½ inch pan-head stainless steel screws to secure the sill to the foundation. Each bay of the greenhouse is two feet wide. Pre-drill through the aluminum sill and place a screw in the center of each bay, but offset the holes slightly towards the outside edge, so that when you put on the glass, the panes do not sit on a screw-head. Screw down the gable ends first and then the sides, and make sure there is no bowing in the greenhouse sill. Use a long straight-edge or board along the sides to see if they are straight before you screw them to the foundation. Note: the holes that you drill through the sill plate are the only ones you have to drill for your greenhouse installation.