We are located in a suburb of Pittsburgh, PA.

Our house is a ranch, that is shaped like a "C". It was buily in the 60s. In the 80s, the previous owner had a patio poured inside the "C", and a roof built over it, tied into the old roof. The patio was 16x19, and it was poured directly on the grass. On the side furthest from the house, there are (3) 6x6 support posts for the roof.

Needless to say the middle of it cracked and sunk (up to 6" in places), and then one side of it also sunk, creating a funnel for water that got on it to pour into our basement. We rented a jackhammer and broke out the old slab.

We want to know how to repour the new slab so that this doesn't happen again. I want to pour the slab so that if we decide later, we can enclose the walls and build an addition on it, considering the roof is already built over it.

I also want to pour a monolitic slab, considering there is no access for a cement truck, and it has to be pumped around the house, and i only want to pay for a crew to come out once.

The house is a brick house, with cement block basement walls. The longest side of the slab that is touching the house will have to rest on top of cement block foundation wall. The sill plate and rimjoist are on the innermost 1/3rd of the cement block wall because the bricks were on the outer 1/3rd. Those bricks have since been removed, revealing 2/3rds of the top of cement block wall.

There are portions of the ground under the old slab that have had the soil wash away, and are up to 1 ft deep. What should be used to fill those areas?

How should the slab be designed? Foundation, thickness of concrete, thickness of gravel/fill under it? How does it rest on/tie in with the long wall of the house? The frost line is 36" here to the best of my knowlege.

What are the fewest amount of control joints i can cut in it? Would thirds (6x16) be enough?

What is the MAXIMUM slope this slab can have and still have an addition built on it later?







Area "D" in this picture:


Thanks for help in advance.

For goodness sakes, people, PLEASE don't post such huge pictures! It messes up the width of the copy, so that we have to scroll back and forth for every line.

Now, for your problem. First, if you plan to enclose the area to become a room, there needs to be foundations extending below frost depth on all exterior walls. These walls need to be insulated in accordance with the building code. Second, do not...repeat...do NOT support the floor slab on the existing foundation walls. That will only guarantee cracks. The floor slab should be supported on the ground only. Third, in the area under the slab, you need to remove any and all organic soils or plant material, and any construction debris or stuff like that which may have been buried in the excavations for the house foundation. Then you need to fill any voids and compact the soil. Place 4 inches of gravel beneath the floor slab, place a vapor retarder, and pour the floor independent of any foundation walls. 4 inches thick, with either welded wire mesh or fiber reinforcing. If the slab will support a future finished floor, it will be best to use 4200-psi concrete rather than 3000-psi.

Thanks for the reply.

I apologize for the picture size, i took them on the smallest dimension/size available in the camera, but didn't resize them any smaller before uploading.

I should fill the voids with clean (non organic/plant) soil and compact it?

Is there a way i can pour it all at once (monolithic slab)? My hope was to dig a 12" wide x 36" deep trench all the way around, put gravel in the middle area, and then pour the concrete all at once. Is this feasible?

If i have to pour it seperately, how do i do this? Do i pour the 12" wide x 36" high footers in the trenches, wait, and then pour the slab? Does gravel go between the footers and the slab? How does the slab not "slide" away from the house (a basement slab is contained by the walls)?

Is it alright that my basement walls go down 8ft or so, but the footers for this only go down 3 ft?

What about the 3 support poles for the roof (if i don't enclose it right away)? Do i have to run those down to the footers and pour the slab around it? Any way to support the slab so i can just run them to the top of the slab?

Thanks in advance,
Stuart

If you pour the concrete as you describe, unless it is very carefully detailed and very carefully reinforced, you will get cracks.

First, pour the footings for the new outside walls. Then build the foundation walls, being careful to insulate them and form a thermal break with the floor slab. Then grade your soil, filling and compacting any voids, and place the 4" gravel and vapor retarder. Finally, pour the floor slab so that it is entirely contained within the new and existing foundations. The roof posts can bear on the foundation walls.

Footing depth is required to extend to undisturbed soil. By definition, the backfill of your existing foundation walls is "disturbed". The footings should extend down below any undisturbed soil for two reasons: (1) disturbed soils can settle, possibly causing cracks in the new foundation, and (2) the loads from the new footings, if they do not extend to the same depth as the existing footings, can partially be lateral loads on the existing foundation walls, possibly causing bowing or cracking of the existing foundation.

Thanks for responding again.

Well, there is no budget/time for 2 cement pours, biulding walls, etc. So I have to do one of 2 things:

A) Go ahead with the single pour, by using the careful details and reinforcement necessary. Is it possible for you to tell me how you'd do this, or do you need more information? From what i've read of what you've written to others, i'd have to thicken the slab to double the width over the last 4 feet or so. I imagine more rebar will be necessary, maybe more wire mesh also.

B) Build a patio with no intention of enclosing it in the future. How would you design said patio? What would you do about the 3 posts?

Thanks for your time again,
Stuart

If (A), basically, yes that would be the method, although in addition to thickening the slab at the new walls, you also need to have a 45-degree angle between the bottom of the thickened slab and the wall. A sharp angle would more than likely cause a crack in the concrete. Wherever in the slab you have an inside angle, stress cracks can happen there also, so you would place three of four rebars at 45 degrees spaced 6 inches apart to deal with that. You probably won't need rebars in the concrete other than those. Use mesh in the slab, turning the mesh back on itself in the thickened part. The thickened part should extend a minimum of four feet back from the wall.

Someone should look at the roof loads that come down on the columns, because your foundation wall won't have a footing if poured monolithically, to verify that the roof loads can be supported by a foundation wall alone.

If (B), you will need sonotube concrete piers to carry the loads from the roof columns to earth. Again, someone should calculate the roof loads on each colimn, and size the diameter of the piers accordingly, taking your soil characteristics into account.. The columns should have a galvanized steel or cast aluminum post base and anchor bolt. The roof-to-column connections should also have galvanized steel hurricane clips of some sort, for both (A) and (B) options.

Thanks again for the reply...just 3 more questions....

I never got an answer to "whats the MAXIMUM slope i can add and still build an addition later?"

What is the fewest amount of control joints i can cut in it? Can i cut it in thirds (6ish x 16 pieces) or do i have to go at least quarters?

and, if i go with B (3 sonotubes and a slab) - how would the slab be poured? 4" of gravel, vapor bariier, then 4" of slab? How do the sonotube piers/slab "interact"? Before the slab was poured right up to the posts that went down a few feet into concrete (like fence posts).

Thanks for all your time and responses,
Stuart

How in the world did you ever get this far along without a real plan and/or permit?

You have already desided what you want to do and now are looking at details to make it work. You are also operating trying to accomdate a future enclosure. This wil rais the end cost, compromise the space and make the next job tougher, all to save an existing roof section and minimizing the access problems.

Richard is right about the slab being a "floating slab" and NOT bearing on the existing. For a 16x19 slab, you should have the recommended base, gravel and vapor barrier. If you cut into 6x16 sections you can expect a random crack across the slab just to save 15 minutes of sawing. - quarters would bw better.

Regarding the slope, any slope that works over 16' for patio will be unacceptable once you try to enclose the area and call it a room. - At least future owners will see it.

Saving the old roof section severely limits you to fit into the old fotprint. This may be necessary if the job is driven by the budget.

I think you have the situation a little twisted.

As i said in my first post, there was a patio there to begin with. The problem was that it was poured incorrectly 20 years ago by the previous owners of this house. It rains a lot here, and within the last 2 weeks the patio cracks had gotten so bad that they were funneling water into our basement for the first time ever.

Obviously, the pressing matter was getting the patio broken up so that it stopped flooding our basement. I didn't need to know how it was going to be repoured to break it up. And i didn't know you needed a permit to do this.

I had no budget for this 2 weeks ago, it is sort of an emergency thing. Thats why i'm doing 90% of the work myself, etc.

I just wanted to know if i could pour this in a way that it could be built on later and stay within a cheap budget - and if i can't, then i just want to know how to pour it as inexpensively as possible, so that it is a normal patio, but doesn't crack. I don't want to redo roofs, etc.

The cutting into thirds instead of quarters is not to "save 15 minutes" - its more asthetically pleasing if its feasible. I just wanted to know if its feasible.

What is the necessary "base" for a regular patio slab? How are patio slabs normally poured (not planning an addition later)? Do i just put 4" of gravel from wall to wall, vapor barrier and pour 4" of concrete? I get the part about not bearing on the current foundation.

Thank you in advance,
Stuart

Richard, if you get a moment, if you could answer this last question, i think i'll be set...

If i go with B (3 sonotubes and a slab) - how would the slab be poured? 4" of gravel, vapor bariier, then 4" of slab? How do the sonotube piers/slab "interact"? Before the slab was poured right up to the posts that went down a few feet into concrete (like fence posts). Do i have to thicken the edges?

Thanks in advance,
Stuart

One additional thing to consider. You might want to consider making the bottom of your new footing match the bottom of your existing footings. I know that it will require deeper excavation, but if you hold the new footings up they will surcharge your existing footings. That could cause additional settlement of the existing structure.

You can pour a patio on a well compacted base. If it is not tied to the existing structure, it will move independently. Do not be surprised to see a frost heave because of your location (PA) and the performance of the existing slab. You have bad soil.

As a minimum, I would do whatever possible to put the posts on footings of some sort below the frost level if they are supporting a roof attached to the house - otherwise you could have eternal roof connection problems.

I would not worry about the minimal sucharge effect on a fooing/slab (just make sure you have good soil and compaction). A Sonotube (Sonotudes can be filled as a DIY project using bagged concrete) would be a better method if you have aready decidied you have desided on a structural system. - From a structura/civil engineer that had California register Structual engineers working for me 40 years ago.

If you ever do decide to enclose an area on a floating slab that ends up moving, you may have big problems when you go to sell without a permit, since the space may be a big bargaining point especially if there is no permit and cannot be included in the appraisal. If you don't expect to sell, let your kids worry about it.

quote:

Originally posted by structuraleng:
One additional thing to consider. You might want to consider making the bottom of your new footing match the bottom of your existing footings. I know that it will require deeper excavation, but if you hold the new footings up they will surcharge your existing footings. That could cause additional settlement of the existing structure.

I've never heard of this happening in a residential application and frankly don't think it's a concern. Think of all the houses with attached garages and basements - those houses have garage footings higher than house footings.

While I believe it may not be an issue, it is something to think about.
When an attached garage is built at the same time as the house then it can be considered in design. That's not the same as adding it years later.
Additionally, it's important to understand that while something that is imposing more load than a member was designed for will not necessarily cause failure, it does cut into the factor of safety.

quote:

Originally posted by structuraleng:
When an attached garage is built at the same time as the house then it can be considered in design. That's not the same as adding it years later.

I know that your the SE here but on new construction, when the foundation for the garage is poured, the adjacent basement concrete is extremely green and the backfill was probably done the day before. I can't believe that this is less of a risk factor than butting up to concrete that has had months, years to set up.

In fact, SE or not I know that statement in false.

As far as design - How many homes do you know that has individual structural design of the foundation. I'm willing to bet less than 1%. In the real world, weather pending - foundation wall is poured one day, 2 days later the floor trusses are installed, the next day it's backfilled and the day after that the footings for the garage are being poured. No individual structural design because it's not necessary.

I really don't know much about residential construction practices. I must admit my ignorance on teh subject. That being said, however, I would imagine that poured foundation wall wouldn't have the forms stripped for at least 3 days. Is that a fairly accurate statement?
If so, then there is likley little to worry about unless you have poor quality concrete.
That being said, however, it really shouldn't be backfilled until it's attained its 28-day strength (which typically happens in 7 days).
Additionally, surchargine the footing is more of an issue regarding the soil bearing capacity and not of the concrete, though as you point out it does surcharge the wall as well. Again, in new construction this would at least be on the radar of the designer (though probably not for a home as few are actually engineered). For an add-on later, it isn't.

Beg to differ...it was, a few posts back...by none other than the ignorant architect who isn't qualified to do structural design.

Wow, didn't mean to start a huge arguement. I know you're all talking about if i add a foundation, etc, but now i need to know about the other "regular" way to pour the slab if i don't intend enclosing it later.

If i go with that option (3 sonotubes and a slab, not going to be enclosed) - how would the slab be poured? 4" of gravel, vapor bariier, then 4" of slab? How do the sonotube piers/slab "interact"? Before the slab was poured right up to the posts that went down a few feet into concrete (like fence posts). The posts are pressure treated 6x6s. And in this case do i have to thicken the edges?

Thanks in advance,
Stuart

Assuming that you are NOT going to enclose this space. This is how I would do it. First get all the old concrete out of there, strip all the black dirt away down to the clay, gravel, or sand.

Next I would forget the sonotubes. Instead I would dig a hole down to the frost depth (width of the hole will need to be calculated based on the width of the required footing). Upon inspectors approval, I would then pour the required depth footing down in the hole. Call back inspector if required. Set your posts and then backfill the holes. The reason for doing it this way rather than using sonotubes is because you are going to be getting wind underneath pushing up on the roof. Posts being buried 4 or 5 ft in the ground is going to give you much more bracing and stability than if they were sitting on top of sonotubes. Pole buildings are built this way - for good reason.

Now that your roof is secure, lets look at the concrete. In these types of setups I like to thicken the edges to 8" but it's not really required. It'll just give you a much stiffer concrete slab. Since you stripped all the black dirt away, you will need to backfill and compact the gravel back up to fill in this area. Sometimes this is as much as 2 ft deep. Remember the thicker the base of gravel, the better drainage you'll get resulting in less heaving in the winter. Put 6" of gravel down, compact, repeat until you get up to your required elevation. If you decided to go with thicken edges you'll need to shape your backfill around the edges to accomodate this. Since the concrete is outside and will stay outside, a vapor barrier is not necessary.

You have to also consider that this slab is going to heave up and down in the winter so you want to this to happen independently to your posts. You absolutely don't want your posts moving up and down with the concrete. I normally just tack strips of 1/2" plywood around the posts where the concrete would have touched it and pour the concrete up to that. Don't nail the heck out of this plywood because you want it move once the concrete moves so just tack it enough to hold it in place. Hold the plywood about 1/2" lower than the top of concrete to give yourself some space to come back and caulk it once the concrete is cured.

Couple more things - hold the top of the concrete about 1 1/2" below the threshold of the door, the last thing you want is frost heaving that slab and pushing the threshold of the door. Oh I almost forgot about rebar, typically we just use #4 (1/2" dia.) @ 24" o/c but it certainly wouldn't hurt to close that spacing some. The more rebar - the stiffer the slab is going to be. Go to your local lumber yard and ask for some rebar chairs. The purpose of these is to hold the rebar off the ground. Rebar in contact with the ground will eventually rust away and will not maintain it's strength. Also since you are not going to enclose this, make sure to slope the concrete enough to drain stormwater away from the house. Hope this helps

Looking for more opinions...

...assuming i don't want to put the posts directly into the concrete, how else could i design this so that it makes sense?

Is there any way i can do this without the wood touching either the grass/dirt or be put directly in cement? The grass/dirt comes up almost even with the slab.

Thanks in advance...