In 1993, my front porch was built using 2x6's for the decking, mounted perpendicular to the front of the house. After too many years of painting, replacing porch post's and rail balusters, I'm ready to go with composite, but know of two issues.

  1. The composite boards are .94" thick, but the 2X6's are 1.5". Where the boards go underneath the siding "J" channel, and the front door, there will be a .56" gap. Do I add a piece of moulding, shim the joists under the decking, or some other idea?
  2. If I mount the new boards perpendicular like the old, the length will be 6' 2", resulting in some serious waste of composite. I looked at a neighbors porch and they ran one board on the outside parallel with the house, and that looked good. Or, if I tried to run all boards parallel with the house, now I need to replace/redesign the joists so they run in the correct position. Thank you for any suggestions you have!
  • 2
    This question is well within the context of woodworking so it's totally appropriate here, but you might get more/better answers on the DIY stackexchange site, diy.stackexchange.com Jun 3, 2019 at 15:46

2 Answers 2


You could just stick a bit of trim underneath the J channel to make it look nice.

To your length issue, I'd either try to source 20' lengths, or tweak the design to let you use 12'. You could do this by making a picture frame around the perimeter. (This would require a bit of blocking at the house and outside, but that's not hard to do.)

  • Yes. I guess the trim option is the easiest & best way to conceal the gap. But now it seems I just found another issue while reading up on joist spacing. Seems the builder decided that since he was using 2X6's and not deck boards, and the fact he was making it 6 feet wide, he places the joists on 18 inch centers instead of 16 inches (18, 36, 54, 72). Trex (and I assume the other composite folks) suggest 16 inch spacing (and I assume code would require that as well). Oh well, this just fits into my project requirements that I always have an unnecessary wrinkle! Thank you for your suggestion! Jun 2, 2019 at 21:43
  • Doublecheck the joist spacing requirements of your specific decking with the manufacturer. They might say that 18" OC is fine. (And if it came to it, it's relatively easy to add joists between joists. The only thing you'd really want to buy/rent is a palm nailer.) Jun 3, 2019 at 0:12
  • Trex, TimberTech and Fiberon all state that joist spacing should be 16" for perpendicular mounting, and less for diagonal. Looks like I will be either adding joists, or thinking about some underlayment. Jun 5, 2019 at 14:41

How about laying the boards to use least wastage, then laying some t&g on top perpendicularly, to look like the old ones. You get the benefit of a modern substrate, and the look to match the original. If not T&G, then just get half inch plain timber and nail it down to look like the originals. It will take up the gap, and look way better.


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