In the not too distant past, hot tub covers and spa covers were mostly flat and would easily get waterlogged and sag. Because it was flat it would not shed rain well. It was frequently made out of materials that did not last very long when exposed to the sun, weather, chemicals from the water, and heat from the tub. Fortunately, today's hot tub cover is generally much better constructed.
Today's covers normally have a sloped foam core. This simply means it is thicker at the hinge in the middle, and when viewed straight on, has a slight pitch like a roof. Some typical measurements are a 4" thick middle, tapering to 2" at the edge, or 5" tapering to 3". In addition to giving it additional strength, the pitch also keeps rain and water from melting snow from standing on the top.
The foam core is usually wrapped with a 2 to 4 mil thick polyethylene plastic, and this is the main barrier against the core absorbing water. Some manufacturers seal this plastic with an adhesive tape, which can pull loose over time allowing water into the foam core. The better hot tub cover manufacturers heat seal the polyethylene. Also, some manufacturers will double wrap the foam which makes it even more resistant to getting waterlogged.
On top of the plastic wrap is an outer covering of a heavy duty marine grade vinyl. This will be mildew and UV resistant and in the better quality tops will be double-sewn with a commercial thread that is UV resistant as well.
Many tops also use a hinge seal gasket to prevent moisture loss and hold in heat.
To protect from curious children, look for tie downs with fasteners that have a child protective plastic keyed lock. This might take a bit more time to remove when you want to use your spa, but if it is located outdoors the added security and peace of mind will be worth the small amount of time required to unlock the hot tub cover. As an additional benefit, the fasteners will help keep the top in place during high winds.
To keep the neighborhood kids from having a party in your spa while you are on vacation you'll need to invest in a set of steelcore security spa straps. These are a separate purchase item, and will serve to keep all but the most determined teenager out of your hot tub.
"Walk On" Hot Tub Covers
If there is any chance someone is going to be walking across the top of your spa, or you have children that might climb onto the top, you need a hot tub cover that is made to withstand this weight. Also, if you live in a climate that gets lots of snow each winter, and you have an outdoor spa, you should consider a hot tub cover that can withstand the extra weight of a couple of feet of snow on top.
In what are known as "Walk On" hot tub covers, there are fiberglass sheets laminated on each side of the foam core. These add extra strength so that a normal weight adult can actually walk across the top of the hot tub cover without it collapsing.
WARNING: Do not ever place any weight on the top of a hot tub cover unless it is designed to carry that load.
A "Walk On" cover will have a reinforced middle hinge and some extremely heavy duty covers do not have a hinge at all, but separate into 2 parts. Because of the weight of the materials the 2 halves make it safer and easier to remove.