Things You'll Need
Scissors or utility knife
Soldering iron, 25-watt model with a 45-degree cone tip
Lightweight sandpaper, 200 grit or higher
A plastic shed can crack for a number of reasons, such as shifting of the surface it resides on, expansion and contraction from seasonal temperatures, or even an errant throw from a ball, rock or other projectile. All of these situations, among others, can lead to a crack. Although there are specialized tools -- "hot air plastic welders" -- that are made to repair plastic cracks, they are expensive and are mainly used on a commercial level. However, skillful use of a common soldering iron will achieve the same repair results.
Plug in the soldering iron and allow it to heat up.
Cut some small pieces of plastic from areas of the shed where they won't be missed using a scissors or utility knife. Plastic to look for are extra plastic tabs, overhanging pieces at the top or the bottom or extra pieces that originally came with the shed and are not being used. Conversely, if you know the type of plastic the shed is made of, like PVC or ABS, a hobby shop or arts and crafts store will carry these plastic pieces that you can use. These pieces will be the welding material used to repair the crack with.
Sand the area of the crack with a lightweight sandpaper to remove dirt, debris, surface oil and wax.
Spot weld the crack together to keep it from spreading when you do the repair. Start at the top of the crack, spot weld the bottom of the crack next, then spot weld every inch back up to the top. Do this by melting each side of the crack slightly by applying the heated soldering gun onto the plastic. As the plastic begins to melt, feed in a piece of the plastic welding material that you have snipped to bridge the gap between the crack. The melted plastics will bond together and become solid when the plastic cools.
Melt a V-groove along the seam of the crack, from the top to the bottom. Use the soldering iron tip to impart the groove approximately two-thirds of the way through the plastic. This doesn't have to be exact, but what is important is that you do not melt completely through the plastic at any juncture.
Return to the top of the crack once the V-groove has been made, and begin feeding in the welding material. Melt both the material and the sides of the groove so that the welding material bonds with the V-groove. Don't worry about lumps, bumps or smoothness at this point. Just fill in the groove with melted plastic. Allow to cool when done.
Sand the repair smooth if so desired, using a lightweight sandpaper.