I would like to cover the ceiling a porch with something.

How about 1/4" T1-11 siding

http://www.lancasterbarns.com/acatal...ge-section.jpg and

http://www.joneslumber.com/plywood.shtml#structural

T1-11 Siding Prem. 6-Patch 8"oc SYP (4x8) T1-11 Siding Prem. 6-Patch
4"oc SYP (4x8) T1-11 Siding Prem. 6-Patch Plain SYP (4x8)
T1-11 Siding CLR WPF 8"oc (4x8) T1-11 Siding CLR WPF 4"oc (4x8) T1-11
Siding CLR WPF Plain (4x8)