Can I build my own water catchment system?

Or do I need to buy a rain barrel?

Sure you can. It depends on your purposes. Are you okay with 55 gallons or would you like 250 gallons? Most would choose the latter. Do it right the first time, so there's less upkeep. You can continually fill vessels as your catchement continually replenishes itself by rain showers.

Check this config out.
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I don't know how the homesteading world would run without 5-gallon buckets and IBCs!

Nice setup, BTW.

Yeah.  The 5-gallon bucket is the best use yet for plastics.

Build away! 

Use whatever you have available and whatever you can get your hands on.  Post pictures.

http://oasisdesign.net/water/storage/ferrocementtanks.htm

www.paceproject.net/Userfiles/File/​Water/​Ferrocement.pdf

http://oikos.com/library/water_storage/index.html

I've purchased several poly water tanks, the largest 3000 gallons.  But the poly tanks are expensive! 

i;m working on a system with 10-15 55gal drums on it,

i put together a lil playlist of videos i found on youtube of others systems.

http://www.youtube.com/playlist?list=PL8332738338820328