Is this normal and true? We are thinking the open deck area may have less support from the bottom. --Aldie
Answer: Check whether the weight of the porch's walls and roof are carried directly down to the ground. You should see support joists (thick horizontal framing, typically at least 2 by 8 inches) right underneath the decking there, with sturdy support posts beneath those. If the joists don't line up with the walls, other parts of the deck may bounce a bit when you walk on them, sort of like being on the long end of a teeter-totter.
Also, make sure there are enough joists to meet the manufacturer's recommendations. Residential decks, which are designed to carry a load of 100 pounds per square foot, should have joists at least every 16 inches when the decking is one inch thick. Measure the spacing from center point to center point of the joists; if you just want to measure the gaps, they should be no more than 141/2 inches wide. If the decking is 11/2 inches thick, the joist center points should be no more than 20 inches apart.
Commercial decks, built for 200 pounds per square foot, need joists 12 to 16 inches apart, depending on the thickness of the decking. Because of the porch weight, the commercial standard might be more appropriate for your deck. Joists supporting its walls and roof might need to be thicker or doubled up, or the posts underneath might need to be closer together than on other parts of the deck.
It's also possible that the decking inside the screened porch is quieter simply because it's more sheltered and thus isn't expanding and contracting as much as temperatures shift. Composite lumber doesn't shrink and swell as much from moisture changes as wood decking does, but temperature variations have a greater effect.
To compensate, Trex instructs installers to leave side-to-side as well as end-to-end gaps of one-quarter to one-half inch between boards. The gap widths vary according to the temperature when the material is installed. If the gaps are missing or too narrow, perhaps your decking has loosened fasteners as it expanded, leading to the rattling you hear.
If the contractor who built the deck can't help diagnose and fix the problems, call another contractor or a structural engineer. At least an expert's assessment could bring you peace of mind that your deck is as nice to walk on as it is to look at.