Hot Topics: Ancient "Caulk" Between the Deck Boards

Old oakum exposed between old deck boards.

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They made things differently way back when. A 2x4 was actually 2 inches by 4 inches and gaps were sealed with oakum, not with caulk. Oakum’s what kept the tall ships of the sailing days from leaking between the boards, and plumbers used it to keep water on the inside of a pipe. Apparently, carpenters used it when building porches too.

Original Post: What is this stuff between the boards of my porch/deck, and what do I substitute?

Thangor Member

Here's the situation. I have a completely covered back porch that also might be described as a deck. The main part of it is 20' x 10'. There is a roof extension that covers it and provides about 24" of soffit beyond, so it's protected fairly well from direct rain. The house was built in 1958.

Below the porch is a concrete patio. The ceiling to that patio - which means that it is under the floorboards of the porch - is "finished," i.e., it has plywood attached to the bottom of the porch. There are screen-covered ventilation slots front and rear.

What this means is that the floorboards of the porch need to be as watertight as possible, so that wind-blown rain and snow don't get in between the slats and down into the ceiling below.

The floorboards are 5/4" x 6".

I'm prepping to re-paint the porch with Deckover paint. A couple of the boards are rotting at the edges, so I got some replacements and pulled one up. When I did that, I saw that the contractor who originally built the porch used some sort of fibrous material driven into the gaps between the floorboards. If I was to take an absolutely wild guess as to what it is, it looks like horsehair, though I doubt that's what it is.

At any rate, judging by the good condition of the ceiling below, whatever it is has done a great job for the past 55 years. I'm sure it's not in common use today, but what would be a good substitute? I'd use just caulk, but the gap between the slats is right at the maximum for a caulk span, and frankly I wouldn't trust it. I'm thinking of using spongy weather sealing material to act as a support for caulk.

Many thanks for all info.

I should also mention that the only 5/4" x 6" lumber I could find locally is pressure-treated. I'm hoping the Deckover paint can go on fairly soon with no bad repercussions.

Highlights from the Thread

chandler Forum Topic Moderator

New pressure treated boards will take several months to cure out to where you will be able to paint it with anything. Caulk will most likely not hold up in the cracks as you would want it to. The product placed between the cracks of the boards, based on its age and your description, is probably oakum. It is a rope based tar product, originally used by plumbers when pouring lead connections on pipes. You may still be able to find some at plumbing stores. It was driven into the cracks with a dull pointed flat tool resembling a wide chisel. It may pay you to look it up and replicate it in the new larger cracks.

Thangor Member

Many thanks for all the good info and help. I'm located in the Washington, DC area. Any suggestions for where I might be able to find kiln-dried 5/4 wood around here, something that will be ready to paint? Also, the boards I am thinking of replacing are mostly intact, just a little rotted on a corner in one case and the ends on the other. I've seen construction guys on TV tackle such a problem by re-building the rotted ends of the wood using an epoxy-based material. I'm fairly handy, but I have absolutely no experience with such things. Instead of trying to find new wood, though, should I consider this?

chandler Forum Topic Moderator

Not having pictures, we're working blind. Only you can see the extent of damage. Yes, we use two part epoxy (Bondo) to repair wood that has minimal rot. Let us see what you have.

Thangor Member

Before you ask, yes, the railings are also going to be replaced sometime after I get the "flooring" painted. Also, on the image of the board ends, there was a piece of trim there to hide the end of the plywood, and I have a replacement for that. I plan to paint and caulk everything as much as possible.

The rotted wood on the ends of the deck boards.

In regards to the oakum, it kind of looks like the original contractor put jute rope between the boards and then caulked over it. It might not be real oakum. I'm wondering if something like that would work as a repair, but perhaps using synthetic rope. It would at least not rot. I would put caulk over top of it, and in the close-up image, that's what it looks like they did originally.

chandler Forum Topic Moderator

Yeah, the fibrous stuff looks to be jute or rope, just stuffed in there to keep the wind/bugs from emigrating from below.

I would think an epoxy repair of the end of the board would be in order, especially if you plan on refinishing the entire area. The epoxy won't take stain very well, but will paint very nicely.

Thangor Member

Do you think it would be advisable to use synthetic rope to replace the fibrous material? Now I have to ask you where I can get info on how to do that type of epoxy repair. Thanks again.

Now all I need is some days when both the temperature and humidity are under 90 (wish me luck on that), and I'm good to go. I also got some 3/8" jute rope to drive between the boards, and a cold chisel that I'm going to try to flatten.

chandler Forum Topic Moderator

Try using a 5n1 tool. Smaller, strong and more appropriate for the jute than a cold chisel, IMO.

For more on the pros and cons of pressure treated wood, read the whole thread at: //