Rot-Resistant Beams

Wood is many things. For microbes, wood is food, though only if wood gets wet and stays wet. This is why you need to build to keep water away from deck parts or let them drain and dry quickly and thoroughly. The hotter and damper the climate where you live, the more benefit a rot-resistant design will provide. Even pressure-treated lumber or naturally rot-resistant species such as cedar will rot if the various parts of the deck stay wet. Good materials never make up for poor design.

One common style of deck support beam is especially prone to rot: built-up beams made by simply nailing together three to five planks (usually 2x8s, 2x10s or 2x12s). These are called “composite beams,” but it’s more realistic to call them “compost beams.” 

When water gets between these boards after a rain or snow melt, it sits there for days or even weeks. If your climate is warm and wet, these areas may never dry out. When nutrients are brought into the gaps and cracks between boards by bugs and their droppings, you’ve got all the ingredients for powerful composting action that speeds rot and deck failure. 

One way to avoid the problem of rot-prone composite beams is to use solid pressure-treated beams – 6x8s, 8x10s or such – instead of planks spiked together. Although beams this large will form cracks along their lengths as they dry, this will not weaken them structurally. All else being equal, a solid beam will last longer than a same-sized composite beam in outdoor conditions. These large timbers, however, are heavy, difficult to handle, and sometimes hard to obtain.

There is another, very simple solution. You can build composite beams from regular, pressure-treated construction-grade planks, but you need to use spacers to ward off rot—similar to using spacers to separate a wood frame from a concrete wall. By building in spaces between the planks, water isn’t trapped. Water can drain and the wood can dry quickly. Watch the video up next for more about rot resistant deck beams.

Spacers for beams are strips about ½" thick, 2" wide and as long as your composite beam lumber is wide. Make the spacers from rot resistant material. I’ve made them from pressure-treated plywood, slices of composite lumber or pieces of all-plastic decking material. A pointed top on each spacer helps shed water. Slip these spacers between pressure-treated planks every 18" to 24" as the beams are assembled, to maintain a space for drainage and drying. If you want to be really thorough, apply a bead of polyurethane caulking around both sides of each spacer before assembling the beam. Be sure to use galvanized carriage bolts – not nails or screws – to hold your composite beams together. I use ½"-diameter hot-dipped carriage bolts. 

How to cut big beams

If you opt to use 6x6s, 8x8s or other big beams for your deck substructure, you’ll need to cut them. One way is to use a hand-held circular saw and cut all the way around the perimeter of the timber. This will leave an uncut portion in the center of the beam because the saw blade isn’t big enough to reach all the way through. That’s okay. Slip a sharp handsaw into the slot left by the circular saw, then continue cutting until you’re through. You can use a specialty tool, such as the Prazi beam cutter, to cut thick materials in one pass, but it’s not worth getting one if you’re just working on your own deck.