I described my problem in this question here

How to fix deck board at edge not lined up with beam?

This is the outcome of that project. There ended up a bit of gap in some area like 1/8" - 3/16". It's the one on the left here:

enter image description here

The reason is that when I cut 2 boards using the table saw, it's not straight all the times. Plus the bottom beam is not flat so it's impossible to join two boards like that without any noticable gap when looking closely.

At the end, the gap is a bit larger but not bad. You can see the beam directly below:

enter image description here

My question is: how to fill the gap to prevent water from sipping in and rot the lumber below over the years? If there is no lumber below, it's easier. But this is always a target for rotting.

I am thinking about filling in the PL 400 completely and wipe it off at the surface using thinner or gas to clean it. Making it like some sort of filling / grout.

What are some options?

  • There are router bits or saw blades that allow you to make toung and groove that can prevent debris from dropping into the cracks. Since you are having trouble with getting straight cuts on a table saw the router may be a better way to go. Ripping some composite's is similar to trying to rip noodles (imo). Looking at the photo the support member looked like regular lumber not pressure treated. If this is the case the small gap area may outlast the other areas.
    – Ed Beal
    Commented Aug 9, 2016 at 7:43
  • It's pressure treated beam below. Just the color / flash from camera making it pale. The boards are already cut so I need to fill the gap in between now.
    – HP.
    Commented Aug 9, 2016 at 16:07

1 Answer 1


The answer that you are looking for is... there is no answer.

You will not keep the rain from migrating under the deck boards. That is the reason the framing for decks are done (or should be) in a rot resistant wood. If you wish to add another layer of protection, remove the deck in the area you wish to further protect. It should be easy since it is screwed down, and cover all the tops of the framing you wish to protect with a "peel and stick" membrane that also seals around any penetrations. This product is usually used for flashing around windows and doors when they are installed in their rough openings. It comes in rolls 6" wide or wider, but you can slice it into 3" widths to cap over 1 1/2" wide tops or use it full width on the 3" backer under the joint in question. The sticky material also seals around any screws you put through it so no water penetrates through where the screws are. Here is a link that shows how it is used on a deck. It is a few pages down on the pdf.

  • Yeah I should have installing the flash membrane BEFORE putting down glue and the decking boards on top. Now it's nearly impossible to pull the boards out. The glue is PL 400 and it's incredibly strong. I may damage the lumber more than trying to adding more protection by pulling it out at this point.
    – HP.
    Commented Aug 10, 2016 at 5:27
  • The pressure treated lumber you have should keep it in place for a long while. From what I researched nothing glues composite decking together or to a substrate. At least not PL400. Otherwise, I have no personal experience about it.
    – Jack
    Commented Aug 10, 2016 at 14:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.