Building with pallet wood can be a way to save a little money and create useful structures. First you'll need enough equal-sized pallets to complete your project. Industrial businesses guard and reuse their pallets, so make sure that any you find are free for the asking, or offer a small price or barter to get them. Builders throw away scrap pallets used to package doors and windows from remodels, so check home construction sites.
Strip the pallet boards away from the frame of one side of four pallets. Remove or hammer in any nails or staples that are sticking out. Keep the wood you remove intact, because you will use it later. Lay your pallets end to end so that the outside frames form an 2.4 x 1.2 m (8 x 4 foot) rectangle. Cut three 2.4 m (8 foot) two-by-fours to the same length as your combined pallets and place one along each edge of the pallets. Join them together with screws or nails into a single 2.4 x 1.2 m (8 x 4 foot) panel. Trim any boards that hang over the edge of your two-by-four.
Lay two more pallets beside the panel. Attach them to the middle two-by-four with nails or screws. Add remaining two-by-four to the outside edge to complete the panel. Nail the pallet wood you stripped from the other side into place to cover the gaps in the remaining pallet boards. Ideally, they should overlap. Make six of these panels.
Place one panel either on a concrete pad or smooth, level piece of ground to create the floor. Make sure that it has adequate support to hold the rest of your shed, and is completely level. You may want to leave the gaps in the pallet boards open for drainage, or tack two sheets of plywood to the panel to provide a smooth floor.
Place two of your wall panels upright along the outside edge of your floor, touching where they meet at the corner. Do not overlap them. Stand a 5 x 15 cm (2 x 6 inch) board in the outside corner where the panels meet. Angle it between the two panels and toenail it into each side. Make sure to use enough fasteners to provide a strong joint. Add your third wall in the same manner. You will need to tack two-by-fours to the outside edges, angled down to the ground as temporary supports. The open end will be the front of your shed. Drive nails on the inside toenail through the floor panels into the outside walls to fasten them together.
Cut one of your long two-by-fours to fit across the top of the open end from outside edge to outside edge. Fasten it to the tops of the walls. Repeat at the opposite end and evenly space the remaining two to secure the walls. Lay your roof panel on top of these with the pallet boards running from front to back. Raise the front edge by laying a two-by-four under the panel across the open end, on top of the wall brace. Secure the roof with nails or screws and cover with roll roofing. Fold the roofing into the corner between the roof panel and the top of your walls. Tack it to the top of the wall panels, allowing a slight overhang all the way around.
Raise the final panel and attach it as before. Measure and mark an opening just slightly larger than the outside frame of your door. Ideally, place it in the middle of one of the rows of pallets to avoid cutting into the pallet frame. Using a circular saw, cut the opening along the line on the inside measure. Cut two two-by-fours the height of the opening. Attach them to the sides of the opening in the edge. Add a two-by-four header across the top of these. It should reach from inside to inside of the pallet and be attached at the ends to the pallet frame. Nail it to the uprights. Fit and tack your door into position, allowing for adjustment. Check it for proper function before securing it permanently.
Get pallets as close in size as possible. Be careful when stripping the boards off -- they may break easily.
Use plenty of fasteners and add extra bracing to insure the strength of your joints.