I know this has probably been covered in other posts, but just thought I'd post a picture of the storage rack I built from PVC for my three Hobie kayaks (two Outbacks and one Oasis tandem). I used 2 1/2" Schedule 80 pvc even though Schedule 40 probably would have been ok. The additional bulk of the Sch. 80 makes a rock solid rack. The supply house was out of Sch. 80 T-joints, so I wound up using Sch. 40 joints instead. For those who don't know, Sch. 80 is grey and Sch. 40 is white. At least it helps to demonstrate how I put it together. In addition, I bought 4 swiveling wheels so I can wheel it around.
The dimensions I used were 72" for the length (not including the joints) and 42" for the width (not including the joints). The vertical spacing is 24". I could have made it smaller overall, but I have plenty of room and I always think bigger is better if you have the room.
The one suggestion I would make that I didn't think about till I was done, would be to leave the bottom caps (the ones with the wheels) unglued. By leaving it unglued, you will be able to replace/remove the wheels should you ever need to do so.
I'm posting a link to a picture here.http://flic.kr/p/ecsfyN