Re-engineering a moisture-damaged floor built on top of bedrock

Back in May I got an email from a contractor named Cam. He had a problem with floor framing in a basement built over bedrock. He wanted to talk.

A few minutes on the phone and I knew exactly what the issue was. Basements built over bedrock can be incredibly damp during summer because the cold rock floor cools the air that creeps in from outdoors. Years of near-100% relative humidity for weeks on end each summer had caused creeping rot to set in on the plates. “The punky wood doesn’t extend all the way across the plates yet”, Cam explained. “The joists are still resting on a couple of inches of good wood on the inside edge.”

After talking about how ventilation during hot weather is actually the source of damaging moisture in basements built over bedrock, Cam and I got down to business – how to replace those plates.

His plan was to jack the floor frame up a bit with beams resting on the underside of the joists. Then he’d rip out the rotting plates and jam new ones in before lowering the floor frame on top. “That sounds like a great way to start swearing”, I predicted. “You probably won’t be able to raise the floor frame enough without damaging something, and even if you could get 1/8” clearance, the new plates probably won’t go in.”

“Yeah, you may be right”, said Cam. “It’ll even be worse because I was planning to put some Blueskin on the underside of the new plates to protect them from dampness wicking up through the hollow sections of the block wall.”

We talked back and forth for half an hour, bouncing ideas off each other until we came up with a good plan. Cam would support the floor with jacks and beams, rip the old plates out, then fill the top part of the block cavities with mortar. I advised against concrete because the aggregate needs vibration to settle. Mortar is easier going that way.

“The Blueskin probably isn’t going to make enough of a difference to stop rotting of the new plates”, I worried out loud. “You need to get the homeowner to use a dehumidifier whenever it’s hotter outdoors than in the basement. You should also use pressure-treated wood meant for foundations. It’s way more rot resistant than regular pressure treated.”

Together we settled on a plan where Cam would use a benchtop planer to shave 5/16” off the thickness of the plates, giving him room to slip the new plates into the old space. Wedges driven in under each joist and locked in place with a shot of PL Premium would fill the gaps before the jacks came out.

As I hung up the phone I realized how powerful an engineering tactic it is to bounce ideas off each other. Cam reached out to me, but I’m sure we came up with a better plan because of tag team engineering. If you’ve ever done it with the right person, you know how well it works.