Blocking is important, especially where it’s needed for structural support of guard posts [see “Foolproof Guard Posts,” July/August 2016]. But where I build, blocking is also often a hidden danger that can become a rot area around the base of posts, where they can be weakened at the point of greatest moment.
A little bit of flashing goes a long way toward preventing this problem. In addition, I modify my blocking depending on how the guardrail posts will be finished off. If they are going to get a sleeve or wrap, for example, I will wrap a peel-and-stick membrane around and extending up about 6 inches above the deck height. This protects the wood, especially all the butt joints where water can creep in and stay long enough to turn everything into food. Intermediate blocking is made of PT lumber that is the next size down and is held to the bottom of the joists.
I try not to butt any joists or blocking lengthwise to another joist, and if I have to create a double or triple joist for some reason, it gets a metal or a peel-and-stick flashing over the top. All wood beams get a piece of metal cap flashing over the top.
When I have a picture-frame detail, I find I can often add a joist just inside the guardrail posts. This acts as blocking and allows the picture-frame-to-field-decking transition to happen in a gap, where there won’t be blocking to hold the moisture (see photo, above). I use small 4x4 offcuts to maintain the spacing between posts. This method requires fascia outside of the framing to keep help support the picture rail, but that’s easy enough to do; in fact, I usually install spacers between the fascia and the framing to avoid trapping water there and to give the picture rail a stronger connection.