Just saw another tool for straightening bowed deck planks. I thought I'd volunteer the method I used to install my cedar deck planks last fall. I had used the nail method for my first deck where you place fat nails between the boards to determine the gaps between the planks, but the gaps turned out to be too narrow, and clogged up with pine needles very quickly. So I thought up a quick method that I could use to get 1/4" gaps pretty quickly.
Mind you, my method isn't going to give laser-straight decking, but who but me will notice anyway? My method requires a few bits of wood and a bit of string, all essentially free. No fancy expensive tools.
First, I cut four wedges out of some scrap, very slow taper, maybe
1/8" at the skinny end, and 1/2'' wide at the fat end. I struck chalk lines across the joists for every third plank. Then I proceeded to install the deck planks in groups of three.Facing in the direction of the already-installed decking, I would place the first plank of each group of three good side up, with the concave edge facing me. I justified one end of the plank to its mark and screwed it down. Then I moved to the next joist, justified the plank to its mark, and screwed it down. Because the plank was bowed toward me, I could use my legs as levers to push the plank forward to justify the plank to each mark.
After the first plank was installed, I took the next two planks, concave side toward me, and placed them in the gap between the first plank and the finished deck. Using the wedges, I adjusted the spacing of them at one end so that the gaps were equal, then screwed them down to the first joist. I proceeded from joist to joist, spacing with the wedges, until they were all screwed down.
That's it. The installation went pretty quickly, and I did it all alone. The wedges provided a lot of side force, so I was able to install even badly-bowed boards when necessary. They can even be used for the first board installation if necessary. A few times I placed a couple of 2x4 scraps in the gap between the installed decking and the first plank of a group, and wedged against them to straighten the first plank out, and at least once I screwed a temporary scrap on the other side to wedge against.