http://builderswithoutborders.org/PUBLICATIONS/PUB6.HTM
The Pallet Truss: A Low Cost Alternative Roof Structure
by Alfred von Bachmayr – Tesuque, New Mexico
In the past several years, I have been working in the border area around El Paso/Juarez and have been amazed at the number of uses people have devised for wooden shipping pallets. I have seen everything from full houses to an incredible variety of fencing designs, furniture, and even kids’ toys. I realized that the low cost and availability of pallets makes them a reliable and versatile resource. At the same time, I was struggling with what seems to be a universal challenge—low-cost structural members for roofs that allow for thick roof insulation. The typical 2×4 or 2×6 (38×90 or 38×140mm) rafters work structurally in some areas with low live loads, but the joist depth does not allow for sufficient roof insulation. The idea of a truss made from pallet parts seemed like it could be the answer to these challenges.
With the help of some local carpenters, I began trying different truss configurations and joint connections. We looked for pallets that had full length 2×4(38×90mm) members and avoided pallets that had been cut out to allow the fork lifts to access them from all sides. After several prototypes we developed a simple way to construct trusses that also proved to be structurally sound. I had a structural engineer model the truss in a computer and give me the required number of nails for each joint. We decided to use glue at each joint to achieve the extra capability needed, and the nails were put in from both sides to tighten the joints. Any configuration of truss could be developed, assuming the length of the pallet parts is not exceeded in the design.
The first challenge was to find a way to disassemble the pallets. Pulling the nails was almost impossible without destroying the parts. We found that by cutting the nails with a reciprocating saw, using 8-9in (20-23cm) metal cutting blades, the pallets would come apart quickly with no damage to the parts. We then had to create a way to standardize the construction of the trusses so they were dimensionally stable and consistent in strength. For this, we built a jig out of 2×4 rails nailed to plywood or OSB that mirrored the shape of the outline of the truss (Fig.1). We drew all the parts on the jig so everyone knew exactly where all the parts went to construct the trusses. We cut 12in(30cm) long gusset plates and the diagonal braces from the slats and also squared the ends of the struts on a chop saw. Then using the jig, we began building the top and bottom chords. We put 12in(30cm) gussets in the jig at each joint of the 2×4 struts (Fig.2), then glued (liquid nails or PL 400) the gussets, and put down the precut 2×4 struts/chords and clamped them to the rails of the jig. Then, we put glue on the contact point of the chords and finally the other gussets on top. We nailed through all three pieces with six “8d” nails on each side of the butt joint. We then flipped the chord over, reclamped and put the same number of nails on the opposite side (Fig.3). We did this for all the chords needed for the truss.
With all the chords constructed and using the same jig, we put down the bottom diagonal braces, glued on top where they contacted the chords (Fig.4), then clamped and glued on the top and bottom chords (Fig.5), and finally added the top diagonal braces. We then nailed through all three pieces with 8d nails, three per side. Like the chords, we then flipped the whole truss over in the jig and put in the same number of nails on the opposite side (Fig.6). It is important to verify that each joint has the required number of nails and glue, and that the holding power of the nails is not compromised due to a split in the wood. Finally, we had a completed truss (Fig.7).
Other discoveries. Sometimes the wood in the pallets was so hard it required predrilling all the nail holes. When the strut and the slats were both hardwoods, we used a 7/64in(2.8mm) drill bit, drilling well into the strut but not the full depth of the nails. When only the slats were hardwood, we used the 1/8in(3.2mm) bit, drilling only through the slat and not into the strut. If you find pallets made of soft wood, you can use a nail gun and compressor to speed up the process, but this should only be done with skilled operators.
It is important to make sure the trusses have sufficient strength for the loading conditions. The local building department or structural engineer should be consulted to find out the required design loads. If possible, find an engineer with a truss-modeling program to help you with the design. At a minimum, I design for 20 pounds live loads and 10 pounds per square foot dead loads when using about an R-40 insulation. This is in conditions where there are no snow loads or extreme wind loads. It is possible for you to test out the strength of your truss by simulating the loads using people. First calculate the amount of weight your truss will have to carry and build one. Using the appropriate number of your friends that approximate the loading placed uniformly along the member and with the truss set on blocks where the wall bearing would be, verify the amount of deflection experienced at the center of the member when fully loaded. This deflection should be no more than the length of the span in inches divided by 180.
The truss shown in the diagrams is a parallel chord truss that is 24in (61cm) deep. We used 36in(91.5cm) struts for the top and bottom chords and cut 36in(91.5cm) slats for the diagonals. We found that we could consistently find pallets that were at least 36in(91.5cm) square, so we designed around those sizes. At times, you may find larger pallets to use so your design can utilize the longer chord and diagonal lengths. Make sure the pallets are not so badly damaged that the parts are unusable due to cracking and splitting. Many different truss configurations are possible. Possible designs include a gable truss, one with a flat bottom chord and sloping top chord and non-symmetrical shapes. Try several layouts of the diagonal braces to find the one that places the braces directly over wall or beam supports. The spacing of the trusses will affect their loading. Reducing the spacing between trusses increases the loading capabilities of the truss.
In areas of the world where goods are shipped on pallets, discarded or economical ones can be located for building purposes. For this reason I feel the opportunity to make structural roofs out of them is a very viable alternative in many developing areas of the world. In addition, we have considered the idea of facilitating economic development in some areas by empowering skilled individuals to form a small cottage industry to manufacture trusses. In so doing the variables could be better controlled and the adequacy of the members assured.
Architect Alfred von Bachmayr designs low-cost, energy-efficient and sustainable buildings. He also invents devices to aid in the construction of alternative buildings, and actively assists in philanthropic building projects through Builders Without Borders and other organizations.
