The frame may be made of wood, concrete, brick or stone. The most common material is wood, although concrete is more durable. Of the woods, locust, cedar and chestnut make the most durable frames. A common plan is to use either locust or cedar for the posts, and chestnut or other less durable woods for the sides and crossbars. The frame may be of any desired length, and wide enough to accommodate the sash. That is, the width of the frame at the top should be about inch less than the length of the sash.
The upper or north side of the frame should be 6 inches higher than the lower or south side, in order to give the proper slope. This can be easily accomplished by using boards of 6 and 12-inch widths. Usually one 6-inch and two 12-inch boards or planks are sufficient for the upper side of the pit, and two 12-inch pieces for the lower side; the boards should always extend to the bottom of the pit. The posts should be made of 2x4-inch lumber, or heavier. It is usually necessary to remove some soil at the sides of the pit where the posts are to be placed. The best frame will be made if the boards are not more than 12 feet long, and posts driven at the ends and midway between.
Crossbars or slides are not always provided, but they possess so many advantages that frames should seldom be made without them. A 2x3-inch piece is about the right size, and should be sound and surfaced on the upper side. Some gardeners prefer a 1/2-inch strip in the middle of each crosspiece to prevent binding of the sash—a provision that is well worth while making. The sash can then be handled with much less annoyance.
When placing the crossbars, great care should be exercised, for if they are too close together the sash will bind and greatly annoy the attendant. If the sash are 3 feet wide, the distance from center to center of the crossbars should be at least 1/2 inch more than 3 feet.