The answer is, it can depend on your climate, your furniture materials, and how often you use your outdoor space. If you sit on your covered patio often, and move your furniture in and out, your cover may be protecting it from UV damage and debris. Of course, in hot, humid conditions, covering outdoor furniture can trap moisture and cause more problems than they prevent. Here is how to decide what makes sense for you.
Situations where covering patio furniture makes sense
Much of the time, summer heat is accompanied by the sun. If blasting, intense rays strewn your patio, covering outdoor patio furniture can extend its life by protecting it from direct sun. This is primarily true for wood furniture which is not naturally weather-resistant, any outdoor patio cushions that use fade-prone fabric, any painted or powder-coated outdoor patio metals that may blister with prolonged sun exposure.
The sun does most of its damage around 10 am to 4 pm, and wood, cloth, and painted patio furniture are all most likely to be damaged during those times. Having patio furniture covers on will prevent prolonged and continued damage to outdoor patio furniture.
Geography plays an important role. In the Southwest, where UV is extreme and humidity is low, covers mitigate moisture concerns in the sun. This also applies to climates with summer frequent thunderstorms, and dust storms; as well as those with coastal locations where the salt air can be very corrosive, and the overall storms can be damaging.
Your usage patterns also impact the decision. For instance, if the only usage of the patio furniture is during weekends, then it would be logical to cover it during the week. This reasoning also applies to vacation periods, and in situations where you have a second, mostly unused, set of furniture during summer. Additionally, furniture that is positioned close to a pool or grill would also benefit from the covers as chlorine and greasy spillage can compromise the finish and details of the furniture.
When you can skip covers
Some materials really do just fine in summer. For instance, dense hardwoods like teak and eucalyptus resist rot without covers. Teak will develop a silver-gray patina over time if left uncovered and untreated. This doesn't affect the wood's durability, but if you prefer to keep the original golden-brown color, you'll need to either cover the furniture or apply teak oil periodically. Aluminum is rust-resistant, though it can oxidize or develop pitting in coastal salt air environments, especially if the powder coating gets scratched. Furthermore, synthetic wicker is made from high-density polyethylene, which handles UV exposure well and dries quickly after it rains. Other metals like stainless steel and wrought iron, as long as their finishes are good, also hold up well in most summer conditions.
If your patio has a pergola, large umbrella, or roof overhang, you already have shade and you are getting much of the benefit a cover offers. Adding a cover on top of shade does not usually provide much difference. The same logic is true for furniture that is situated on a covered porch or on a screened in area where the furniture is protected from the direct sun and rain.
Daily use of a patio is also reason to skip covers. If on a daily basis you have breakfast on your patio or you spend most of your evenings outside, the trouble of having to remove and replace covers gets old quickly. People in this kind of situation eventually stop using their covers, and the money spent on the covers usually ends up being a waste.
Moderate summer climates and the occasional rain usually do not require covers on patio furniture either. The patio furniture dries from the breeze, and the furniture does just fine with the conditions.
The moisture trap problem
Covers have consequences in humid conditions. If you have a cover for your patio furniture and you place that cover over your furniture on a hot, muggy day, moisture gets trapped underneath the cover. The airflow is isolated and there is a warm, damp atmosphere for the mold and mildew to survive.
This issue is especially prevalent along the Gulf Coast and any place that has sticky and humid summers. It is common that after removing a protective cover on outdoor furniture, one might discover a thick, fuzzy layer of mold has developed on the cushions and a musty smell has taken over, which is hard to get rid of. With conditions like this, even metal that is supposed to be rust-resistant can develop rust spots. Wood can also warp over time or develop mildew stains that penetrate the wood grain.
You would think that people would see covers that protect from summer rains and understand how to use them without creating worse conditions than the rain would have created. Covered furniture keeps moisture in and constantly damp while uncovered furniture dries out.
If you use furniture covers in humid regions, you have to be very mindful and strategic. Do not cover furniture that is damp, try to leave covers off long enough to air out, and after it rains, make sure to leave the covers off until everything is dry. Many find it much easier to fit cushions in a storage bin and bring the cushions inside than to deal with cover management.
Quality covers with built-in air vents can help reduce moisture buildup. Most mid-range and premium outdoor furniture covers from brands include vented panels on the sides or back, often with rain flaps to keep water out while allowing airflow. These vents also prevent the "ballooning" effect in high winds. Budget covers without vents are more prone to trapping moisture, so if you live in a humid climate, it's worth investing in a vented cover rather than the cheapest option available.
You do not have to cover all or nothing. A popular practice is to protect cushions, the most vulnerable part, while leaving the furniture frames uncovered. Cushions can be stored, standing them on their side against a wall helps them dry off, or cover the cushion with a waterproof cover.
This cushion-only method is effective as long as you have an aluminum, teak, or quality synthetic wicker frame, as these are able to endure sun and rain with little upkeep. It is the cushions that fade, mold, and deteriorate quickly. Leaving the frames exposed and covering the cushions gives you 80% of the protection with 20% of the work.
Another solution is the use of furniture protectant sprays. These add UV resistance and water repellency to the fabrics and woods without having to cover the furnishings. They do require periodic re-applying, usually 1-2 times a season, but that's significantly less effort than having to deal with covers everyday. While some of these sprays work with various materials, others do target specific surfaces, either fabric or wood.
Moving furniture around can help, so can moving it under existing shade, angling pieces so cushions go face down, or rotating cushions so they don't wear down too quickly.
For less frequently used furniture, a hybrid option can work well too: cover it when you are going to be away for a long time but leave it uncovered when you are home for a period when you might want to use it.
A few things to remember when using covers
More than anything else, size matters. Oversized covers that are too large flap in the wind, which can create annoying noise and scratch furniture surfaces. Covers that are too tight, however, create stress points that rip within a few weeks. Be sure to measure your furniture and purchase covers sized to those dimensions instead of going for a generic "large" size.
Make sure to secure your cover properly with straps, ties, or weighted edges. An adjustable strap or drawstring hem works better than elastic, especially if you are expecting heavy summer winds.
Covers that are light in color reflect heat better than those that are dark, which means less heat build up underneath. This won't eliminate the moisture trap issue, but it lessens it. Lighter colors also show dirt more easily, reminding you when covers need to be cleaned.
Before you cover your furniture for an extended period, be sure to clean them first. Dirt, pollen, and bird droppings can be trapped beneath a cover. This can stain and damage your furniture over time. A simple wipe-down can be done quickly and saves you time in the future.
Make the decision based on your situation
There is no right or wrong answer to this question. Someone in Phoenix, for example, with daily-use teak furniture will make a vastly different decision than someone with painted wicker in Houston that they only use regularly on the weekends. Having a covered patio also changes the scenario entirely.
Consider your climate and how frequently you are outside. If covering the furniture after use becomes a chore that diminishes your overall enjoyment of the patio, then the trade-off of protection is not worth it. After all, your outdoor furniture is meant to be used and enjoyed, and some weathering is to be expected when living outside.