Free 8' Half Pipe ramp Plans - from xtremeskater
These plans are for an 8 foot high and 16 foot wide halfpipe.
These plans can be adjusted to suit. Make sure that you have plenty of room to build it. It is possible to build it outside, but make sure it can be protected from the weather. .
Free Mini Ramp Plans - from xtremeskater
Plans for an indoor mini ramp halfpipe. This halfpipe can be separated into three pieces so that it is possible to move. The height of this ramp is 3' 5", you can make it any height under that. The width is 8' and can be easily adjusted.
How to Build a Ramp for Your Shed - from associatedcontent.com
This is a good description of how to build a ramp for a shed even though there are no drawings or photos.