Generally speaking, shed doors are the first problem you'll have with your shed (other than Mother Nature). It makes sense as your shed doors are the only moving part of your shed and are opened and closed many times throughout their use.
If you purchased your shed from one of the big box stores, you will have shed door problems sooner than later because of the way their doors are built.
The sheds the big box home improvement stores sell are shipped by the manufacture on tractor trailers and all the parts and pieces of the shed must fit into a nice box so they are lighter and take up less room to cut down on shipping costs and make them easy to load and unload.
Quality wise, the item that suffers the most are the doors. They need them to be as light as possible and fit flat into the box; thus many shortcuts are taken giving the shed buyer a weak set of doors that will fail sooner than later.
The shortcuts include; using weak trim pieces as the actual frame structure of the door, only using staples to fasten the door sheeting to the trim and not using a complete 2×2" or 2×4" frame on the inside of the doors for strength.
As your shed ages and settles into its footprint in the backyard, the shed floor becomes un-level resulting in hard to open doors. The shed owner must use more force to open and close the doors which loosens the staples and nails that hold the doors together. Note: the shed builder is trained to add screws to the factory built door for added strength but seldom is this actually done and if it is, they use too much force and spin the screws which result in no holding power at all.
As the staples and nails loosen, the seams and trim in the door start pulling farther apart making it even harder to open and close the doors. Even more force is needed and before you know it, the doors are falling apart.
Most big box sheds come with a warranty and you can get the doors replaced. Most warranty claims are not scheduled appropriately and it will take many calls to get your new doors. The squeaky wheel will get the grease in this case so be diligent in your complaining.
The bad news; the replacement doors will be the exact replica of the existing doors and the same problems you're having will happen again. There is no warranty on the replacement doors so be prepared to spend around $500.00 for doors from the original manufacture.
The good news; you can buy replacement doors on the Internet that are better than the doors that came with the shed and are easy to get and install yourself.
The first step is to measure the vertical and horizontal door opening (also called rough opening) of your shed. Use your measuring tape and measure from the bottom of the header to the floor. This is your vertical measurement. The header is the piece of wood (usually a 2×4) that is above the door opening. It could be a 2×6 if you have a larger shed. Then measure your horizontal width from the stud (usually a 2×4, also called a jack stud) on one side of the opening to the stud on the other side. This is your horizontal measurement.
On cheaper sheds, the jack stud could be a 2×3 (which is okay) or the door opening may not have 2×4 studs on both sides. If this is the case, you can easily add a stud(s). Measure the length needed cut to fit and toenail the new stud in.
Once you have your measurements, you can shop online and find the best deal. As of this writing, there are only 2 companies that build and ship wooden replacement shed doors. Both companies are reputable, make a superior shed door to the one you'll replace and both ship UPS. All you have to do is find the best deal.