Joists sag because they are
One of the best ways to check for sagging is to scan the deck or floor surface with your eye at deck level. The other is to bounce while standing on the deck. If you discover a sag, try to determine the reason. You should have a rough idea of what size lumber on what spacing can span what distance. Look for increased loads such as piles of snow drifting or shoveled off the roof onto a deck. If there is access under the structure, look for damage and probe the joists for rot. Watch for missing or ineffective beams.
Sagging floors on decks, entrances, porches and balconies are common, but don't mistake a floor sloped to give good drainage for a sag or settlement problem. Porch floors are exposed to driving rain and snow. As a result, the floors are typically built with a slight slope to drain water off the porch away from the house.
Porch floor boards were traditionally tongue-and-groove, one-inch by four-inch planks, and most builders knew that water would run off the porch better if the joints ran with the water rather than perpendicular to the water flow. Water flowing across the joints between boards finds its way into the joints. Water running along the joints tends to drain out and the wood dries better. The porch flooring lasts longer.
If the porch floor boards are laid properly (perpendicular to the wall of the house) to allow drainage, the joists that support the floor boards must be laid parallel to the house wall, so that they are perpendicular to the floor boards. The beams supporting the joists are usually perpendicular to the wall at either end of the porch.
Many porches extend the full width of the house, and since porch floors do not typically have a huge live load, the joists often span considerable distances, running parallel to the front wall of the house. The porch floors often sags as a result of these long joist spans, especially under concentrated loads from furniture, for example.
When looking at older porches, watch for sagging in the joists as a result of overspanning. In some cases, makeshift intermediate supports have been provided. These often settle, if they don't have a proper foundation and footing system.