Pouring concrete floor in dirt floor basement

[AndyF]

I have an older home, >175 yrs, and this is the year I'll be taking care of some major maintenance items. One of the items I plan to take care of this year is finally pouring a concrete floor in my basement. The floor is now dirt with the furnace, hot water heater, water softener and well pressure tank mounted on individual concrete slabs or on 12X12 concrete pavers.

The basement is relatively shallow - ~6 1/2' - 7' from dirt to the bottom of the rafters and has concrete/rubble walls. I'm thinking of digging out 6-10" of dirt, putting in a few inches of gravel and then pour a 4" slab. Is this an OK plan, or are there better ways to do it?

What is the best way to ensure drainage? With the dirt floor, I have standing water in one or two places every spring and the sump pump runs fairly often. Should I run some drain tile to the sump, or will the gravel do an OK job?

[egon]

Just some items that may/may not pertain.

How deep are the wall foundations. Would it be logical to try and pour underneath the foundation a little section at a time using rebar and mesh.

Are there any post supports in the basement or are there places some could be placed for future stability and adjustments. If so these spots should be picked beforehand and a good footing poured.

Will the basement dirt floor be sloped to a location for the sump pump. This should aid in drainage.

Drain pipes set in a radial pattern from the sump pump location would be of benefit. These could be plastic pipe small od diameter with slots or holes drilled in them. They could even be deeper than the what you plan for the final dirt grade. The sump pump could be set in a deeper well that goes well below the final dirt grade. The drainage pipes could also be extended past the bottom of the basement wall. Preferably vertical pipes around the outside of the basement that conect to the horizontal drainage pipes may be considered.

Will special pads be poured for utility components to sit on?

Has consideration been given to to plastering the inside basement wall using wiremesh and concrete? Done properly this could improve the quality of the wall considerably.

Suggest that the final clearance be adaquate for stand up for regular sized people.

And last but probably most important. How will the changes you make affect the surrounding soil conditions. Ie: will it dry up and shrink causing settlement:

It's a job that will take time and labour. Think of all possible senariors and try and have answeres for them before hand.

Best of luck.

And please note, all the above comments are made without knowledge of surrounding ground contours, soil types and soil water content.

Egon

[Stoneheartfarm]

I guess I'd have to agree with Egon. How deep are the footings? Are they deeper than the depth you intend to dig? With a 175 year old house, you may not even have footings the way we think of them. What are your basement walls made of stone, or cement?

Steve

[AndyF]

The foundation is laid up stone/rubble with a stucco type finish on the inside wall. Its almost 2' thick. I haven't dug a test hole yet, so can't say how far down below the dirt floor it goes.

[BrianP]

The approach I'd suggest is to dig a 2' diameter hole a couple feet deep in a suitable location and position an upright concrete pipe into which you've pre drilled with a few dozen holes. When you move the gravel into the basement, make sure you surround the walls of your concrete pipe with it, and put a few inches of gravel in the bottom. This is your sump pit. You put a sump pump in there with a discharge away from the house. That will take care of the occasional standing water and any floods which might occur. The slab should slope very slightly towards the pit.

Place a few inches of gravel in the basement. Depending on how cold it gets where you are, you might want to put 2" of styrofoam (you lose heat through the slab). Even if you don't have insulation on your basement walls, placing the styrofoam now will help if you ever decide to insulate the basement (which saves heat, makes the main floors more comfortable, etc.

I also suggest 6 mill plastic on top of the styrofoam or gravel. You have to use plastic on top of styrofoam or the slab will crack like crazy. The plastic significantly reduces vapour movement and dampness. Again, even if your basement walls are now damp proofed you can do that later, but you'll never get a second chance to dampproof the slab. A damp proof basement also makes the main floor more comfortable and slows down rotting in the structure.

You will need to install a compressible border around the slab (looks like 1/2" cardboard). Then pour a 4" slab. I believe in 6" mesh, but then my basement floor is 100% living area
(and as comfortable as my main floor).

Even if you never plan on using the basement for anything, in a cold climate or a damp area, insulating/damproofing is good value for your money. The additional cost is modest compared to the cost of the slab.

[Pat]

You have received some pretty good ideas but you need to be carefull that you don't disturb the original foundation or you could be in for some nasty surprises. Conventional concrete foundations could be underdug in sections and have new concrete perimeter added beneath with rebar to tie to coming slab. This is a dangerous undertaking for a DIY project on your old foundation.

I too suggest you consider walking headroom for normal adults. If concerns for disturbing the old foundation are preventing you from digging down to the required depth to put in a proper layer of gravel, insluation, and slab with full standing headroom (at least 7 ft) then consider not digging to the full depth all the way to the foundation but stopping short by a distance at least as deep as you dig (or as is recommended by a soils engineer.)

While drain tiles won't hurt, in my opinion if you properly grade the excavation and dig radial "ditches" pitched to fall toward the sump pump location, line these with geotextile, fill then with gravel and then cover with geotextile you will have good long term drainage. If per chance you are on a hillside or have enough slope to be practical, draining the basement underfloor drains to daylight is far superior to sump pumps. Oh, I almost forgot. I would circle the perimeter with a gravel-geotextile drainage ditch that would connect the outer ends of the radial drains.

If you have radon concerns, you need to bury a section of perforated PVC drain pipe in the gravel in one of the ditches and use a reducer to conect to a 2 inch PVC pipe to vent this to the outside of the building envelope.

There are a lot of different vapor barriers available on the market in various thicknesses. I recommend Stego Wrap but not the thin stuff (get the stiff heavy stuff). You need the "special" seam tape to make it gas tight. There are installation instructions to follow to get a good barrier. Not hard but you need to pay attention to detail.

Rigid foam under the slab is good but be careful not to go too light in the density or the slab will crush the foam and not all at once or evenly which can crack the devil out of the slab.

There are "blankets" you can buy which insulate and act as a vapor barrier at the same time. The ones I have used are aluminum coated bubble wrap. They are cheaper than rigid foam and do a good job. They reflect most of the heat back at the slab. They need to be taped carefuly at the seams. The tape sold for use with StegoWrap is a good choice. Duct tape isn't!

You need to consider conditioning the basement space after finishing the slab. You can insulate the outer walls to improve thermal performance. DO NOT use a vapor barrier behind the sheetrock or whatever you finish out the walls with as it will collect condensation and promote mildew and rot.

There are products such as "Cactus Board" that you should conside using. It would probably cost about $200 or so but would be worth it. It is a clever plastic product that GUARANTEES dry basement floors. It intercepts any leaks in the walls and diverts the water under the floor to be handled by the below floor drain system.

Oh, by the way... With proper insulation, the additional expense to heat and cool the basement is so small as to be unimportant. A litle A/C in the sumer will help keep it dry down there and a little heat in the winter will do the same. If yo choose to not condition the basement with heat and A/C at least circulate some fresh air through it.

One really simple thing I have seen done was rerouting the central heat and air systems return air through the basement. The return air for the system is drawn from the basement where convenient and a transfer grill or a louver door for the door leading to the basement allows air to circulate through the basement when the unit is running. Actual heating and cooling loads from an insulated basement are quite small but the benefit of circulating air through the basement is high.

Good luck as a tunnel rat,


[img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img]

[greggol]

My project involves digging deeper a basement dirt floor and pouring concrete. My house has a basement in two sections. In 1965 the former owner (a skilled mason) attached a 70x32 addition to the 12x12 original house dating from 1880's. In doing so he left he 5.5 ft deep dirt floor basement. He built new foundation walls on two sides then broke through with a doorway. So I have three steps to go up from the new basement to the old basement where the circuit breaker and pump are. It's cold, damp and smelly.

In the dirt basement, there are two support poles under the beam holding up the kitchen floor. The poles rest on 12x12 piers. The water tank is on a slab. I have temporarily suspended the pump from the ceiling.

I only have to do a 12x12 area. I have it dug down but along the perimeter stopped 6-8 inches short of the original footers. I'm not sure what they are. Above the dirt, it is cinderblock. I don't believe the footers are simply stone but cannot be sure until I dig. The soil around here is clay. Water comes in and I will install a French drain under the slab and direct the water to the sump in the new basement (drilling a hole through the double foundation walls). I will insulate and use a vapor barrier.

My question is whether the following plan seems sufficient to provide lateral support for the footers. I will dig along the perimeter but only down about 6 inches. I will bend the horizontal rebar which will underlie the slap upward at the end. With 10-ft rebar, I can bend up a foot at each end and overlap the rebar under the slab. I will have a basket-like shape. I will install forms to make a perimeter retaining wall about a foot high on the inside but only about 4 inches deep over the perimeter dirt against the footers. The wall will be about 10-12 in thick at top, 4 inches thick below the perimeter dirt. When finished, it will be like a short-walled pool. Will the short but thick perimeter walls keep the footers and walls from creeping inward? Do I need joint pads between the wall and the footers?

Thanks.