There is no standard spacing it depends on what size of rafters you want to use and the span of your shed.
Gable roof shed rafter spacing.
After they re up you can snap a line to mark and trim the tails in a nice straight line to make up for any ridge warp or marking cutting and fixing errors.
Moving to 16 on center you could span your spf 2x4s up to 6 for heavy loads and 9 for light loads which would still work with a 12 12 gable shed.
Rafters for a shed roof.
If your project had a rafter length of 8 or less a safe and strong method is to use 2x8 framed 16 on center.
Spacing your 2 4 shed rafters at 24 on center limits the width of your roof.
If you get premium kiln dried lumber then you could span 10 5 with the same size of lumber and rafter spacing.
Gable roof framing calculator plan diagram with full dimensions for a good straight fascia add an extra inch or 2 to the tails of all your rafters total length.
For light load roofs 24 o c.
2 4 rafters can span up to 7.
For instance 2 grade 2x4s at 16 on center will span just over 9.
As a general rule roofs that have a high human occupancy rate should be framed with rafters every 16 structures such as sheds or barns can be frames with rafters every 24 again make sure to consult local codes.
For standard shed roof rafter spacing see rafter span chart.
Width rafter width rafter boards usually 10 15 cm.
Gable roof rafters spacing layout.