Just finished building a printer of my own design. Nothing unique.
The X-Y plane is fixed and Z is up and down. Height is gained by lowing the platform.

I am using a Printrbot Rev D board and every time I start a print the platform goes up about 5mm.
I would like the first movement to be down 5mm.

Using Repetier Host and Slicr and have added G28 to start code.

Do not know G code but have tried to change Z movement in the code to negative with no luck.

Any ideas?

in configuration.h file change
#define INVERT_Z_DIR true / false
then reload firmware

or

change stepper wires
invert plug or swap wires to pins 1 and 2