How to Build a Pole Barn Roof on the Ground

Use graph paper to draw plans for the pole barn. On one sheet draw the shape of the barn to scale, specifying its eventual dimensions and sketching in the locations the trusses. Roofing material is typically sold in 8-foot-by-4-foot sheets, so designing your project to use multiples of these dimensions cuts down on both work and complexity.

Design the roof trusses. These are triangular, braced frames that -- once oriented vertically -- give the roof its sloped faces, often referred to as the “fields” in industry-speak. Trusses are typically arranged so their slopes fall to the sides of the building, not toward the front and back; this minimizes the amount of run-off dumped on users of the barn during rain. Arranged that way, the trusses' shape and size are determined by the eventual slope required and the width of the pole barn from side to side.

On a separate sheet of graph paper, draw a plan of the trusses; all will be the same for a simple square or rectangular pole barn structure. Detailing the lengths of all the timbers allows a "shopping list" of required materials to be compiled.

Cut all the lumber to size. As pole barns are usually at least large enough to accommodate several vehicles or pieces of equipment, the roof trusses need to be made from relatively heavy lumber. For this reason, a circular saw or table saw is recommended.

Assemble the trusses on flat ground. Almost invariably the floor below the pole barn should be flat and level; if the leveling part of the process has been completed, use this area to assemble the trusses. Lay out one truss at a time, putting the two top chords -- the pitched timbers that will be immediately beneath the roof -- and the single bottom cord in place, to make the triangular shape. Next, insert as many cantilevered supports as are necessary to make the entire framing structure rigid. Use nail-plates, sometimes called gussets, to secure every attachment point.

Remove the finished truss from the working area, lay it flat or lean it carefully against a support, and make the next. Repeat the process until all the trusses are complete.

Return one of the trusses to the flat area and orient it so it is vertical and occupying the space where the pole barn’s back wall would be. Have an assistant hold the truss in place. Move the second truss into position in line with the first, exactly 4 feet forward if you're using 4-foot roofing panels.

Purlins are the timbers that run front-to-back along the fields, perpendicular to the trusses and horizontal to the ground. Their primary purpose is to support the roofing sheathing, but -- in prefabricating a roof that is later to be lifted as a complete unit onto the top of the support structure -- they can also lock trusses in place relative to each other. Using long timbers as purlins allows several trusses to be joined together, which increases the structure's strength. Use a purlin approximately every 2 feet down the slope of the trusses, overlapping their ends like the pattern in regular brickwork. Once the first two trusses are attached to each other, bring in the third and repeat the process until the entire roof shape is assembled, with all the trusses equally spaced, secure, forming the skeleton of the roof from back to front.

Brace the truss structure further if necessary. Increased bracing between the trusses becomes incrementally more important the larger the roof is; small structures may need purlins only, while large structures may also need cross-braces, lateral braces, wind braces and stringers doubling as braces.

Set the stringers, the lateral beams that run between the trusses from front to back, at the bottom corners. Stringers would typically be attached to the tops of the poles once they have been concreted in place, and the roof is then built on the stringers. For a project where the roof is fully assembled on the floor and then raised to the tops of the poles, the stringers are preinstalled to give the roof structure more rigidity when it is lifted into place.

Fix the sheet roofing material in place. Follow the manufacturer’s instructions mandating the extent of the overlap where each sheet joins the next. Screws are more resistant to gusty winds than are regular nails. Save sealing the spaces between the sheets until the roof is in place; a small amount of warping is to be expected as the assembled roof is first suspended and then settles onto the poles, which would likely compromise the sealant.

Install the roof cap. This is most conveniently done using prefabricated galvanized steel panels available from most home improvement warehouses in a variety of angles to suit different roof pitches.