I am getting ready to dig out and prep for pouring a new floor in my basement. Long story short, it was previously a below grade garage on a 1916 bungalow. We already underpinned the foundation walls so I could lower the floor.
I have a few questions:
Should I add a layer of rigid insulation under the floor. If so, what thickness? I am right outside Chicago so I am thinking this will insulate my floor, which is going to be my office.
Should I add some kind of base of the clay before pouring? The clay is rock hard. Should I spread a layer of sand to help level it out?
While I am at it, I am adding a french drain to the perimeter of the basement that will feed a sump. What kind of slope or pitch is required on the drain tile?
I’d appreciate any thoughts on this.
Replies
Pino--
How far below final grade is this floor going to be? If it's well below the frost line--about 3-4' in Chicago, I'd guess?--, insulating it probably won't buy you that much extra warmth in the basement--it might even make the place colder as it would prevent any available geotherms from warming up the floor....
You will definitely want to lay a good 6-12 inches of 3/4 washed gravel on the clay before you pour, and more if your budget can stand it. Clay drains horribly, so you should be using an oversized French drain, 6" or even 8". And LOTS of gravel to give the water an easy way to get where you want it to go: into that sump.
I think the slope for the drain ditch should be ½" per foot--but don't hold me to that; CRS strikes again....
Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
Finished floor is about 4' below grade on all but one side. What used to be a sloped drive and garage entrance is now a below grade terrace that my future office will open up onto. Frost line in these parts is always figured at 4' so I am right there, hence my thinking about the insulation.
As far as the gravel, are you suggesting that just at the perimeter drain or over the entire floor? If it is over the entire floor I should plate compact the whole thing?
I was planning on using a 6" drain pipe as there is also drain tile on the outside of the walls. The yard slopes to the middle where this addition is so it takes on a lot of water.
For the floor insulation, you are probably better situated to make that call based on 'local knowledge'. Check with people locally who have done that and find out how it worked out.
For the gravel, yes, over the entire floor at least a foot deep, more if your budget can stand it, and in the trenches you dig for your French drain; minimum 12" under the tiles and another 12" over. Figure that your clay basically does not drain at all (which is close to true), so that gravel is going to be acting to create a flow-through reservoir for ground water until it can drain off into your sump and thence be pumped away somewhere inoffensive, like Lake Michigan.... If the volume of water trying to run under your house exceeds the volume of the gravel you put under the floor, it will come up into your new office, sure as Murphy made green beer on St-Paddy's day just to get ya sick....
Most people do compact the gravel with a plate vibrator. I forgot once on a job where we had poured a good 12-15 inches of gravel, and I worried about that damned floor cracking for two years before I finally forgot about it. Nothing ever happened. I saw it last week after 10 years. Still perfect.
So what do I think? How much more will it cost you to compact? Vs. How much it would cost you to start fixing cracks in the floor in a year or two?
You mentioned a walk-out, below-grade terrace, too. Make sure you've got a good, clear, protected drain in that thing, or accumulated rain water on it could flood up and over the sill of your walk-out access and make a nasty mess every time you get a heavy thunderstorm....Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
the terrace will have a box drain at one end with a slight slope away from the office/basement entrance. I am installing two sump pits. One will handle the overflow should we take on a lot of rain. One of the two pumps will have a battery back up as well. The way i see it I just created a 180 square foot bath tub with this terrace. so the more pumping power the better.
My threshold to the office is actually a half step up. Figured this would help keep rain out in case of a disaster It also provided a nice design element as the wide entrance step up from the terrace and down into the office will be built out of a unique looking granite I incorporated onto my sidewalk.
Thanks for the insights.
You're welcome.
Just had a last thought: it wouldn't be a bad idea to put another floor drain in the floor of the office-to-be, and pipe it by separate piping to your sump. That way if the terrace drain plugs up and water does come in, at least you shouldn't have it build up too high or sit there too long. Cheap and easy to do before you pour the floor; expensive and a major PITA afterwards....
Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
Excellent idea, thanks again.