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I’ve got a client very set on having there basement finished. The catch however is the slab to bottom of the floor joist is only 6’11” which after finishing brings it down to 6’9.5″. Does anyone have any Guru Ideas for dealing with this. Is ripping out the slab and digging down a real option? How about raising the house? Is that a pipe dream or what. They stand to gain another 1000sq’ so it’s worth asking. If I go down it would need to to be at least 5″ to get a nod from the city who follows UBC with a minimum 7’6″ height. Any info would be helpful!!
Thanks,
Jared
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There are several things to consider here.
First of all, where is the house? If it is in a location where overall height is not restricted, raising might be a possibility. People raise stuff all the time, realistically, it is an expansive challenge to do with anything bigger than a one story cottage. Anything else gets expensive and disruptive.
If you are in an urban area, the popular choice is to go down. There are two techniques for this, underpinning and shelving. Both require decent solid conditions to avoid nasty surprises during work. This really is a good opportunity to make friends with a structural engineer.
Underpinning means you rip out the slab and replace footings a small section at a time. The footing gets dug out in say three to four foot sections and recast at a lower height. As you can imagine, this is time consuming and expensive. It is like building the foundation with the house in place above you.
Shelving is a little less work. When shelving, the slab is removed but not dug out too close to the footings. Depending on the existing soil, a shelf of soil and slab are left near the edge of the footing to support it. The center area of the basement can then be lowered. There really is a bunch of math and experience involved in designing this. The execution, however, is raw labour. Until, of course, you get to drains and other under floor inhabitants.
I have seen a successful DIY shelving project. In this case the homeowners spent the money on the engineer's time and then blew their summer smashing and digging. You might be able to help your clients co-ordinate the engineering and some grunt labour and save yourself a miserable task!
*Rein,You gave an excellent response, correct on all accounts, however; Jared is only going down 5". Assuming even a minimal footing, he will not be excavating bearing soil. The only thing to consider is if the top of the proposed slab ends up below the top of the footing (and it will) he will have to fur out the walls or live with a step around the perimeter.Jared, don't be fooled, this is a lot of work. If it's a walkout, you may be able to get a skidsteer loader in there, if not, rent a conveyor and have lots of help.Good Luck,Jerry
*Thanks to both Rein and Jerry!a)Do you guys think I should even bother looking into raising the house? It's a single story and the only foundation/building connection headache would be the brick chimney @ the fire place. Wireing and plumbing would be redone anyway? I like the idea but it does seem pretty invasive!!b)If the top of the proposed slab doesn't fall below the top of the existing footing would you still suggest getting engineering done on it?Thanks again!!Jared
*Jared,a)Look into it. It's certainly a possibility, but the chimney will be a major consideration. Being on site, only you can determine the most cost effective route.b)You don't need engineering. You will not be removing bearing soil. A simplistic way to look at it is: (looking at the footing in cross section)draw a line at a 45* angle downward and away from the bottom edge of the footing. This area under the footing bears it's weight. If you don't dig in this area, you don't need to worry. You have 6'11" now. Remove the slab (which proboly rests on the top of the footing and is 4" thick) and you have 7'3" to the soil. Remove 9" of soil (a lot of dirt!) and now you have 8'0" and if you have an 8" thick footing you will be 1" below the bottom of the footing (don't worry). Place a 4" slab and you're back up to 7'8" which leave 2" for finish. I've done this recently; any more questions, fire away.Good Luck,Jerry
*Jerry,You've been tremendously helpful! Exactly the info I need right down to the spec's!! ThanksNext question would be did you sub all or some of this out? and if only a portion, what did you decided to take on yourself?yours,Jared
*The only thing I might add is that if you are going to dig, you might as well go for more than 5". Sure the engineering may need to get done, bit with all that labour already in the job, why not get more for your client?
*Good Point Rein,It turns out the footin is only 6" deep which means the top of the purposed slab would be 4"below the bottom edge of the footing. I've never had to get engineering done. Is that gonna be a chuck of change? Jerry and words?Jared
*Jared,I hope this doesn't sound like a stupid question, but are you sure this is a code issue? If it is an older home modern code may not apply (grandfathered in) and in some areas attic & basement space are considered differently than primary living space. Obviously, more is better and 6'9" is tight, but I live in an area of small 1920's bungalows where basements are finished off all the time with that height and at times, considerably less. When done with taste, these areas can be pleasantly "cosy"good luck, and if you have to add height, my choice would be to dig down.-Randy
*Jared,Are you sure your footing is only 6" thick? Anyway, your numbers don't add up or my assumptions are wrong. From my post 2.1.1:You have 6'11" now. Remove the slab (which probobly rests on the top of the footing and is 4" thick) and you have 7'3" to the soil. Remove 9" of soil (a lot of dirt!) and now you have 8'0" and if you have an 8" thick footing you will be 1" below the bottom of the footing (don't worry). Place a 4" slab and you're back up to 7'8" which leave 2" for finish.Even with a 6" footing the dirt should end up 3" below the bottom of the footing, and the top of the 4" slab will be 1" above the bottom of the footing. If the above is correct, you still don't need an engineer. If you want one to give you his blessing, and you draw up what you want to do, it shouldn't cost more than a couple a hundred.A couple of things to think about: Is there drainage tile on the inside or outside?Water problem in the area? (you're going deeper)Do add a vapor barrier.Maybe add radiant heat!9" X 1000sq.ft. =~ 28cu.yds. (not counting the 'fluff factor').BTW, I did everything, but do whatever you're comfortable with and sub the rest.Jerry
*Jerry,Here's the scoop on the measurement. I punched a small hole threw the slab at the footing and measured down to where I could start to scrap dirt horizontally beneath the footing. This is how I decerned the 6" thickness. I live in Tacoma WA where frost heave is not a huge concern. Current code requires footing depth of at least 6". At the time the house was build (1940's) who know's what code was. The slab is not level but sloping from three corners down to the floor drain in the "Laundry space". The lowest Ceiling height is 6'11" the heighest almost 7'2". Where it is 6'11" the footing is not visable. Where it is 7'2" it is almost 4" below the top of the footing. So from the base of the footing to the bottom of the joist I have 7'4" I'll need to dig down 4" below the bottom edge of the footing to get the 7'8" I'd like. Hope that clarifies the situation?Again thanks Jerry!Jared
*Randy,Thanks for the advice. I've already met with the city plan reviewers and had a curtesy Inspection. Unfortunately there aren't any grandfather clauses in this regard in this area. Bummer!Jared
*Well Jared,That's what I get for assuming ;-) I think that clears it up, except for one thing. If the bottom of the footing is at 7'4" and you want a 4" slab, you need to dig down 8" (not 4") below the bottom of the footing to get 7'8" to the top of the slab. You need room for the slab or did I srew up again? That puts the top of the slab 4" below the bottom of the footing. You may need to loose some more interior space as Rein said previously (sorry Rein). You will have to pour a short reinforced wall where the slab meets the footing and you may want an engineer's blessing, even if you don't need his stamp. Sorry if I misinterpreted your original post.Jerry
*Make everyone remove their shoes.
*Thanks for all the help Jerry,JaredI'll let you know what the engineer says!
*Jared, I hope I'm not too late but I have been thinking about this hardly [that is very hard]. And two camps have evolved - raising the house and lowering the slab [basement floor]. But I would like to propose a third option, which, since I thought of it, is the best. I don't know what kind of span your floor is above the basement but it is probably supported by joists of the proper thickness, maybe 2" X 10". And you currently have 6' 11" and must have 7' 6" which means you must gain 7" + some ceiling finish. The easiest way to do this is to cut 7" off the bottom of the joists. Now all the other experts would say "Whoa! Sally" and they would be right. A 2" X 3" does not make a very satisfactory joist. At least not if it is made out of wood. But if you use steel, I have a hunch that a "2 X 3" steel bar would have similar stiffness to a 2" X 10" wood joist. Or you you try 1" X 3" steel bars and space them at say 8" O.C. In fact, my first impulse was to just replace the sub floor with 1" thick trench plate. And you could do that. And it's not only termite proof, but .50 caliber machine gun proof too. So now you have three options: 1) You can raise the house 7 or 9 inches [which is a lot of work]. 2) You can lower the basement slab [which is a lot of work]. 3) You can minimize the thickness of the interface between the basement and the room[s] above [wood is a lot easier to work with than concrete]. -Peter
*Peter,Interesting Idea. I must admit it sounds a bit, well a bit far fetched but if your serious and have stats or can refer me to any I'd certainly look into it. All options are open at this point.Jared
*I still think it would be easier to just make everyone take off their shoes.
*Being a new homebuilder, my first suggestion is sell the house thats too small and buy or build one thats the right size for you. I you hire a moving company it is still affordable and no labor to you.....If thats not an option, dependant on house size,and whether it had an attached garage, I'd raise the house and add a 1' knee wall. That way the utilities are not a problem. We have people here that will raise and block the house and set it back down when you're done.
*Whenever I get into these big structural problems, I spend the client's money on engineering first. Doesn't cost me anything, gets their stamp, I have less liability and don't have to worry as much about a house falling on my own head. Sometimes it's overkill, sometimes they save everyone time, money, headaches and probably trips to the hospital.Seems like a great way to find out how serious a potential client is, also.
*So how high are your shoes Luka?
*Jared, How wide is the span? Are post [and beams] in the basement acceptable? What is the allowed deflection? What is the loading? Where is Joe Fusco? Are there walls in the upper room (which can be used to suspend the main floor, if necessary)? Have you heard of a "tension box" or maybe "torsion" box? The idea is basically the same construction as a hollow core wooden door. Rather thin, very stable and an efficient use of materials. In this case, you would cut each joist down to say 3" and then plate the entire bottom surface with maybe 1/4" plywood - glued and screwed [and the butt joints secured with gussets plates glued and screwed for continuity]. This is essentially a form of "H" beam laid on its side and the flanges extended enough to meet each other and form one continuous surface. -Peter
*I don't wear shoes. I have to use my long toenails to dig into the mud to keep from falling on my ass when bringing the groceries up the hill.
*I was going to resist this but was in one finished basement where the homeowner had "cleared" a large central area of the basement "room" of wires, pipes, etc. and carefully "finished" joists and bottom of subflooring, including adding molding to the "coffers" created by joists. I wondered if it would have been easier and no more expensive to cut and install drywall or some sort of paneling against subfloor rather than sand and paint but to each their own. It definitely added a sense of greater heigth to the room. Edges or border - 4-6 feet all around finished space - was at bottom of joists.
*Bill Conner has another good point - eliminate the drywall shims to gain added height. [Or eliminate wasted clearance.] I would also suggest looking into steel studs [heavy gauge]. You still haven't posted the span.?! What is the thinnest possible floor? [for a given span.]-Peter
*Peter,the biggest span is 12'.Jared
*Jared: For a 12' span, I graphed out a span table for 2" X 4" at L/240, 20 LL, 10 DL. I AM NOT AN ENGINEER. Anyway, if you space 2 x 4 joists at 1.24" - that should work. On the other hand, the curve does show a slight upward tendancy, so maybe a solid slab of wood 3.5" thick should work. Eliminate the dead load of drywall. You didn't state the thickness of your current floor. Are there any engineers out there who would take a crack at this? What is the thinnest floor that can span 12 feet? How about aircraft "honeycomb" contruction? In commercial construction, the use a thick, corrugated steel with the corrugations about 4" deep and then they pour about 4" [?] of wet concrete on top of that and it holds up for at least 12 feet. I still feel that the easiest approach if the slim downthat floor.
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I've got a client very set on having there basement finished. The catch however is the slab to bottom of the floor joist is only 6'11" which after finishing brings it down to 6'9.5". Does anyone have any Guru Ideas for dealing with this. Is ripping out the slab and digging down a real option? How about raising the house? Is that a pipe dream or what. They stand to gain another 1000sq' so it's worth asking. If I go down it would need to to be at least 5" to get a nod from the city who follows UBC with a minimum 7'6" height. Any info would be helpful!!
Thanks,
Jared