The Rot Doctor


Subject: Dryrot in joists and sill plates.
Date: Thu, 06 Jan 2000

I have a 100-plus-year-old house which is balloon framed; part of it is supported by a foundation of structural clay tile and others supported by fieldstones which have been leveled across the top with mortar. One area also has a cellar which was evidently dug after the original house was built, with those masonry walls laid up against the old foundation; at the top of the cellar wall, the joist cavity has been filled with bricks mortared into place against the underside of the floor deck. It is, therefore, not possible to see the ends of the joists where they bear on the top of the exterior foundation wall.

A few years ago, I noticed a fungus on some of the joists. This was apparently some sort of dryrot. This fungus is no long apparent, but the joists have become springy, and I am noticing deterioration of the sill plates (for some reason, 1 x 6 sill plates were used rather than thicker ones).

In some areas, the joist spans are 14 feet (16" o.c., 2x8's). That would be the maximum. Spans run from 10 to 14 feet. Where spans are long, I am installing support beams under the joists, halving the spans.

I am replacing the foundation in some areas, and will tear-out and replace structural members in these areas where access is optimal. In other places, though, I would like to leave old joists in place and place new joists against them, bearing the ends on foundations where possible, and jacking the old joist and fastening a new joist against the old ones where access to the top of the foundation is not possible. I am concerned that this dryrot may go into the new lumber.

Do you have products which can be used on the sill plates to harden them and stop deterioration? Do you have fungicides to use on the weak joists? Will anything strengthen the existing joists and/or stop further deterioration?

Thank you for any information you might have for me.

D. C.

Dry rot happens because the moisture content of the wood is kept between 20-30%. This gives the fungus and bacteria the conditions that they need. Applying fungicides can help prevent dry rot spores from propagating, but moisture can leach the fungicides out of the wood, leaving the wood vulnerable again. If you keep the moisture away from the wood, the fungicides won't leach out, but if the moisture is not there, the wood won't rot in the first place. We like the fungicides mostly because the ones that we use also act as a pesticide against termites and carpenter ants.

If you want to re-strengthen the wood, check to spread of fungus spores, and prevent moisture from getting into the wood, the best thing to use is CPES. It does all of these things with one product. I would thoroughly treat any soft wood (make sure it is dry) with CPES, and treat any places where the new wood will be in contact with the old wood (treat both pieces of wood). Be sure to treat the bolt holes that you use to fasten the new wood to old before you put the bolts in. Look for any places where you might have had a chronic leak, and fix the leak. After all the repairs have been made, I might be tempted to treat all the wood in the basement with disodium octaborate tetrahydrate (commonly called borate) which has been diluted in water. It is available in crystalline form under the trade name Penetreat as well as several others. This would add a measure of protection against termites and fungus.

Any other questions? Please let me know.