Adding Skirting
If a deck is raised high enough that you can see under it, you may want to cover the framing and footings with skirting. Unless you live in a dry climate, make sure the skirt provides enough ventilation that the area underneath can dry out between rainfalls. If you install skirting made of solid 1xs instead of lattice, leave at least 1/2-inch spaces between the boards. The vinyl lattice shown here is ideal -- it won't rot and it requires no painting.
Making it strong enough
If the skirt will be less than 2 feet high and in a low-traffic area, attach it to the joist leaving its bottom edge unsupported. For a skirt that can withstand bumps from a lawn mower or errant soccer ball, frame along the bottom: Tie the horizontal piece at the bottom to the posts under the deck, keeping it at least 3/4 inch above grade. Use only pressure-treated lumber.
If you want to use the space under a deck for storage, frame one of the skirt sections with 2x4 or 1x4. For occasional access attach the section with screws that can be removed. If you need access often, install hinges and a latch.
Prestart Checklist
Time
About a day to frame and install about 40 feet of lattice skirting
Tools
Tape measure, post level, drill, hammer, circular saw, handsaw, square, string
Skills
Improvising a framing plan, cutting boards, fastening with screws or nails
Prep
Choose a skirt material that suits your needs, estimate materials for framing
Materials
2x4 for framing, lattice panels or other skirt material, 1x2 or 1x4 for trim, 7/8-inch roofing nails, 2- or 3-inch deck screws or nails