I am renovating a 100 yr old brick rowhouse in Baltimore. The house has a basement under the front third of the house and a crawlspace under the rest. The clearance between the dirt floor of the crawlspace and the original floor joists varied from about 2ft. to nothing.
I have removed the floor and the original joists and am in the process of contracting out the excavation of the crawlspace to provide clearance to the joists. My plan is to dig out as little as possible to provide adequate clearance for ducts and plumbing and pour a concrete slab, locally known as a “rat slab,” to provide extra insurance against vermin.
Typically this slab is poured directly on the tamped dirt, but after reading FH and a number of threads in Breaktime, I think a vapor barrier under the slab may be a good idea.
I am open to any suggestions as to how to proceed with this part of the project.
Replies
As long as you're digging, don't go for the bare minimum. For not much more money, you can make it a whole bunch easier to work down there. When you have that nice rat slab, get an old skate board, it makes getting around in the crawl a lot easier.
-- J.S.
I don't want to dig too deep because I think the foundation may only go about 5' into the ground. I don't want to run the risk of undermining the footings. The house has stood for 100 years and I don't want to be the one to mess it up.
I am having some test holes dug, so I should know more about how much dirt I can remove fairly soon.
Don't forget to allow for expansion in your rat slab. An expansion joint should be installed around the inside perimeter of your foundation. Have fun with the skateboard.
definitely put a plastic vapor barrier under the rat slab. you may even want to run it up the walls a bit.
Another thing to consider before the slab is drainage. If you don't have any interior drain tile and sump, you might want to consider adding one. It may not be necessary, but since you're already doing some serious excavation under the joists, when not trench some pipes in and connect to a sump pit? Very little cost to you now, but if you skip now and find you need it later...
Jon Blakemore
Thanks. I will look into having that done.
I had not been too worried about drainage because the house sits near the crest of a hill that runs fairly steeply to the harbor. We had some monster rainstorms last fall and there was no significant water problem. But, youre right, it's cheap insurance as long as the digging is being done.