If the ground area is hard and difficult to penetrate then you can go for an alternate solution.
How to install temporary roof supports.
I did not interpret your question as whether to support the open web joist on their top or bottom chord.
The ideal height of such beams should be 2 10 foot longer than the distance between the roof and the ground.
Install another bracket in this position.
I would never install temporary shoring of open web joists on their bottom chord.
The temporary support that you use will need to have a firm footing in order to hold up the weight.
These pieces act as the bases that hold the legs of the support beams.
Make sure that the beams support the roof well and are not left loose or wobbling.
Once it is plumb mark the position on the porch roof.
Use several bolts at the ends of the wider lumber 7x2.
Use 2 inch carriage bolts to secure the posts to the mounting brackets.
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If you have space and there s any structure left near the ends the easiest way is to not replace the existing rafters and joists but to sister a second board on the side of each.
Continue installing porch roof supports.
For a perpendicular support you should use square pieces of 3 4 inch plywood or other scraps of wood that are at least 10 inches wide.
Bring in a sledgehammer and insert some wooden pieces into the ground.
Place the wood on the floor or ground and hold in place with small screws.
Bring in the temporary beams and install them in place.
Stand a post up in the bracket and hold a level on its side.