In the most basic form the door is planks of wood nailed onto a frame. Hinges are attached and then the door is hung on your shed or garden building however you will find the weight of the planks of wood pulling downwards by gravity. This is why it is crucial that the frame has proper bracing designed into it to stop this happening.
There should be a minimum of three ledges, timber batons which are horizontal, and in between them there should be two diagonal braces. The braces help to carry the load of the shed door from the outside of the shed door to the side where the hinges are. The planks of wood will make up the shed door are nailed onto the frame and hinges attached. The weight of the door is then transmitted down the edge of your garden building's door frame ensuring that the door will not drop. These diagonal braces should go upwards away from the hinged side so the load on the shed door is transmitted correctly.
Without the diagonal braces your shed door WILL DROP as all that is stopping this happening is a few nails through the planks into the ledges. On most of the cheap sheds you will find they do not have any diagonal braces at all and some only have two ledges. The only thing stopping your shed door dropping is the nails inserted through the planks into the ledges which will NOT stop your garden shed door dropping. Absolute madness!! For the sake of a few pounds these cheap sheds jeopardise the overall effectiveness of your garden shed.
The absence of any diagonal braces on the cheap mass marketed garden sheds would not normally be apparent unless you are aware of the importance of them. ALWAYS take a look at any garden shed you are thinking of, BEFORE buying, to ensure they have these braces on the doors. You can tell quite easily by looking at the inside of the doors whether there are any diagonal braces in place. If you do buy a garden shed without bracing be ready to accept that the door WILL drop and that any lock you put onto the door will need adjusted on a regular basis. You will also find a gap at the top of the door away from the hinges as well as the door sticking.
There are normal shed or garden building doors, which have three ledges and two diagonal braces, and you have joinery made doors. The main difference is that the ledge and braced door, if made correctly as above, will perform year in year out with no problem, with the strength of the shed door coming from the diagonal braces. With a joinery made door the frame consists of thick timber which has proper timber joints so that the frame itself does not need any diagonal braces as the timber joints provide the strength for the door. The planks of wood are again nailed on the outside of the frame but as the jointed frames is super strong the planks of wood do not have to provide any structural strength where as with a ledge and braced door the planks of wood are crucial to the shed door.
My advice is based on 36 years in the garden sheds business and you can find more information about garden sheds and summer houses doors on