The strong back is built with a top and bottom layer of OSB and a frame between them. This yields a very stiff structure that will maintain flatness. This picture shows the beginning of the process. There are several braces inside the perimeter frame. Sorry about no pictures of the next part.

 

The process took most of the day. I spent some time making sure the panel was flat before I fastened the panels down. 

 

Once the strong back was done I set the forms on top of it. That’s when I noticed a serious error.

 

If you recall, I made 10 forms. There are only 8 on my strong back. It is 17 inches too short – TARNATION! 

 

Back when I took all the measurements I set a form at the back edge of the door. It is an excellent reference line. I call that one Form 0. The form locations in front of Form 0 are given a positive position while the forms behind Form 0 are given a negative position. When I built the strong back I only looked at the location of the most forward form (#6) and set the strong back length to that. There are 3 forms behind Form 0 and they cover 17 inches. 

 

I was fairly disappointed at this discovery and called it a day. 

 

The next morning I built an extension to the strong back.

 

The other surprise I found at this time is – this thing is heavy! It is also bulky at 32”x75”. I’m guessing the beast is about 40 pounds. 

 

The next step is mounting the forms. Total hours are up to about 64.