Rot Doctor


Subject: Deck Repair
Date: Sun, 06 Feb 2000

Hi doc,
Next spring I’m planning a deck repair/improvement project. Your construction note mentions a couple of items I’ve never heard of before: 2-part polysulphide adhesive, and disodium octaborate tetrahydrate solution. Could you possibly explain to me what these are and where to get them?

1) I’m familiar with the polyurethane adhesives that come in a caulk tube. Is the 2-part polysulphide superior to this? Is it a liquid you brush on?

Polysulfides enjoy favor as a teak deck caulk on boats, polyurethanes are noted for exceptional bond strength. Two part polysulfides are somewhat quicker curing than the one part polysulfides. For the project that you have in mind, either product would be fine to use. Just follow the manufacturer’s application recommendations.

2) From your note, it seems the octaborate is a powder to be mixed in water solution. Is this an anti-fungal or anti-insect agent? If it’s a water solution, I’d assume you must wait until the water dries out completely before applying CPES?

Disodium Octaborate Tetrahydrate is a naturally occurring mineral salt commonly called borate. It is the active ingredient in Borax. When diluted 1 lb. borate to 1 gallon warm water and soaked into wood, it is an excellent fungicide and insecticide with relatively low toxicity. It’s biggest drawback is that since you use water to get it into the wood, water can also leach it out of the wood. For it to remain effective you must keep water off of the wood. Of course if you keep the moisture away from the wood, there is no way for the fungi to grow, and so you do not need the borate as a fungicide. It still remains an insecticide.

As far as using borate in conjunction with CPES, you are right, you would want to allow the wood to dry if you use the borate first. My feeling is that any wood that is treated with CPES is not going to be tasty to fungi or insects, so no borate is needed. In addition, CPES restores strength to the wood, while borate doesn’t. Borate is cheaper per square foot than CPES, so I fix the deteriorated areas with CPES first, then treat the surrounding wood with borate if I want an extra degree of insect protection on the wood that is not treated with CPES. By the way, there are some glycol based borates. DO NOT use these, or plain glycol (antifreeze). Yes, antifreeze is cheap, and yes it will prevent fungi and insects. But it does not make a good substrate to apply paint or any other substance to, and it degrades over time. Borate is available through various outlets under various trade names including Penetreat and Tim-Bor. They all are 98% Disodium Octaborate Tetrahydrate, so just read the label.

As part of my deck project, I intend to “seal” the deck (to dry in the patio beneath) by installing a second layer of cedar planking perpendicular to the existing (5/4″ X 4″ cedar). I’m planning to use 1″ X 4″ clear cedar at 3/16″ spacing, sealed with a polyurethane caulk over backing rod between. I’ll use screws from beneath to attach it, so I’ll have no penetration of the upper surface. If you have a better suggestion for the caulk, please let me know! I want to seal the surface to protect it from weather and sun, but not to the point where it becomes glossy. Is the CPES product good for this?

Now, I also want to treat the understructure with something to prevent any further rot. The 2X8 fir joists and 4X8 beams (not pressure treated) below have pretty much survived 22 years of the Northwest elements with minimal degradation. When I replaced the original planking about 7 years ago, I treated the understructure pretty good with green copper arsenate. However, I expect some traces of rot have started at the joist-beam interfaces, and I’m wondering if there’s a way to treat this without disassembling the deck, bearing in mind that after this project is complete, there will no longer be any water reaching these members.

I was once of the opinion that rot would stop if you prevented water from reaching the wood, even though it had already started. However, I’ve recently been told that’s not the case because there is enough moisture available from the humidity in the air to continue feeding the rot fungi. Perhaps you can confirm or refute this? I’d much appreciate any comments you may have on this project.

Gerry M.

I’ll answer the last question first, and we’ll work back to your questions about the second layer of decking. I think that your own experience should answer the person that told you that the wood will continue to decay from the fungi even if the moisture content of the wood drops. If that were so, would the beams and joists of your deck have lasted 22 years in the Pacific Northwest? I think not. If there is not enough moisture in our Pacific Northwest air, then where is there? We regularly deal with log home owners where the inside of the log has rotted so badly that the log is literally hollow, while the outer inch of the log is basically fine. If your source of information was correct, the outer part of the log would've rotted along with the inside. Why does the outer part of the log not rot? Because wind and sun keep the outer inch of the log below the 20% minimum that rot fungi require. The spores will go dormant, and wait for conditions to improve, but not continue to rot. I will ad that there is one kind of fungus that will grow hyphae (straws) to suck water to the wood from anther source, if the wood isn’t wet enough. It will even go through brick to get to water. With that exception though, dry (less than 20%) wood will not continue to rot.

What happens, as you can tell from looking at your own deck, is that most decks arc constructed so that water can wick into areas where the wind and sun cannot dry the wood quickly. These include where two pieces of wood overlap, and where nail or screw holes allow water to wick along their length. Once the water gets into these areas, it doesn't evaporate, it can take it’s time and wick into the wood. Enough of this and the moisture content of the wood stays above 20%, at least in that area. Then you have rot. Rotted wood acts like a sponge, holding moisture for the good wood beyond, unless it is close enough to the drying effects of the air and sun.

So what should you do about the partially rotted wood? Treat it with CPES. Also treat any areas that are not rotted, but allow water to wick in. Once you’ve treated these areas, any water getting there in the future will not be able to get into the wood. You can do this without disassembly since CPES is such an aggressive penetrant. A little CPES in the proper places should allow the support structure to last another 22 years.

So now as to your plan to seal the deck with a second deck on top. Your idea of attaching it from below is a good one. I guess I only have two main questions. One, are you planning on sealing the cracks between the upper decking with polyurethane (which is fine as a caulk) so that no water will sit between the upper pieces? If the water sits between the upper pieces, it will just wick into the wood from the side. If you intend to top caulk your seams, couldn’t you just do that to the bottom layer instead? Two, once you keep the water from going between the planks, where will it go? Do you have a slant to the deck for drainage? If so you will want to rout a shallow groove to the outer edge of the bottom of the deck planks. This will form a drip edge keeping the water from running back along the deck planks underneath.

planks       |
groove ____/\|
drips        .

After you’ve properly prepared the deck, what do you treat the top with? We are currently testing some epoxy-based products, but don’t have enough data to make any recommendations yet. Sikkens and Cabot are the best manufacturers of the traditional oil-based wood preservatives.