This is an excerpt from Outdoor Survival Guide by Randy Gerke.
The lean-to is a versatile shelter that can be used in any season and in a variety of locations with great success. A lean-to derives its name from its appearance—a collection of materials leaned against a framework. Lean-tos can be built in various sizes and configurations, including closed or open, depending on the need. A properly constructed lean-to is a simple shelter to use in a wilderness environment. Try to locate a site that is well protected by natural features such as trees, rocks, or land formations. Enlist as much help from nature as it has to offer. Choose a flat area for the floor. Plan the opening of your shelter away from the prevailing wind if possible. Place a windbreak or fire reflector made of dirt, rocks, or logs parallel to and 2 to 3 feet (60 to 90 cm) from the shelter entrance for added protection and heat retention.
A closed lean-to is an efficient and easy shelter to construct (see figure 2.1) using the following steps.
1. Create the shelter framework. Locate a strong stick to use as the main supporting member for the roof structure. It should be a little longer than your height or that of the tallest occupant. One end of the roof support will rest on the ground, forming the rear of the shelter. The other end will become the entrance and should be elevated 2 to 3 feet above ground level. The elevated end can be secured to a tree, rock wall, or dirt brim. If natural features are not available, make a tripod using the elevated end of the long support and two sticks of adequate length to form the entrance. Keep the top of this opening as low as possible while still affording access into the shelter. A low entrance retains more body heat within the shelter and allows less to escape to the outside air.
2. Construct the roof and sides. Start by laying several strong sticks perpendicular to the main roof support and sloping down to touch the ground. Spread them out evenly over the entire width of the main support. If the shelter is for a single person, it need not be much wider than 3 feet; allow about another 3 feet for each additional occupant. If sufficient material is available, try to place the roof poles no more than 6 inches (15 cm) apart.
3. Add layering. When the framework is complete, begin gathering smaller sticks and laying them in the spaces between the framework. As the process continues, use bark, leaves, pine needles, or any other natural forest debris to create insulation. Completely cover the roof and sides with this material. The thicker the insulation is, the more effective your shelter will be. Conifer boughs, bark, and plant material work well for the layering material. If a sheet of plastic, rain poncho, or tarp is available, place it on top of the shelter and hold it in place with sticks to provide added protection from the elements.
4. Finish the entrance. After placing insulation on the ground, cover the bulk of the entrance, leaving only a small opening into the shelter.
An open lean-to can be constructed by placing the main roof support between two uprights (see figure 2.2). Space the uprights so that the area between them is large enough to accommodate the intended number of people lying sided by side. The main support forms the top of the entrance into the shelter. Keep the top of this opening as low as possible, just high enough to afford access into the shelter. To make the roof, place strong sticks, long enough to accommodate the height of the tallest occupant, perpendicular to the main roof support and sloping downward to the ground. Spread them evenly over the entire width of the main support. To close the sides of the shelter, lean smaller sticks from the ground up to the framework at a steep angle. Add layering and insulation as described in step 4 of the instructions on page 19 for the closed lean-to. Sleep with your feet at the closed end of the shelter.
This is an excerpt from Outdoor Survival Guide.