I’m in the planning stages of an approx. 12′ deep by 28′ wide shed roof addition over an existing concrete slab, for a screened-in porch. The problem is that, as might be expected, the slab has no footer. It’s about 5″ thick, 11 years old, with nary a crack (which isn’t such a remarkable feat in central TX). I don’t know how much if any rebar is in the slab. The soil around and under the slab is quite shallow and stable – don’t think it’s been disturbed more than 12″ or 18″ down, ever. mostly caliche at that depth.
I’m assuming that it would not be wise to set my posts on the front edge of this slab without somehow reinforcing it. One option would be to temporarily undermine the places where the posts will go, and pour an ex ‘post’ facto footer underneath the existing slab. However, I’m not sure this could be done without creating voids under the slab, which could leave me worse off. Alternatively, a footer could be poured as an extension to the existing slab and mechanically anchored to it – seemingly foolproof, but would require the most work. Or the posts could be set on sonotubes entirely distinct from the slab (this approach would probably be harder to finish off nicely, since it’s going to be screened in).
The addition is planned to be an asphalt-shingled 3 in 12 pitch roof on 2×6 rafters hung from the existing rim joist and supported in the front by 5 or 6 4×4 pressure treated posts anchored to the slab. 24″ rafter spacing. The existing eave would be modifed to connect the roof-planes where a transition from 9 in 12 would change to the 3 in 12 pitch. Most of the exterior walls would just be screens and / or partition framing. I had envisioned the ceiling would just be the exposed underside of the t&g roof sheathing and the 2×6’s, painted.
To recap the questions: how would you all go about reinforcing the slab? Or, given my my brief description of the construction and weight involved, is a footer absolutely necessary? Would a conventionally framed 2×4 wall distribute the weight more evenly and make my life easier? Is this a question of the soil type and composition, and whether or not I want to trust it? Other ideas? Need more data?
I’m going to predict that this is the first in a series of questions on this project. 😉
thanks.
Replies
Dunno about Texas, youse guys do things differently down there, but here in California......
1. We would require the slab to be taken out and footings poured around the perimeter. Wouldn't even get past the drawing stage. The clerk at the window would call you on it.
2. Slab over about 4" of gravel and about 2" of sand and a 6 mil vapor barrier over the sand. 4x4 wire for reinforcing.
3. You'll wanna run electrical and phone and maybe gas lines to the perimeter, so they get sleeved and go under the slab, not through it.
4. Sil Plate on the perimeter of PT with tar paper between the concrete and the sil.
Boris
"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1934
Thanks for the reply Boris. I think I'm stuck with the existing slab, based on the cost and time to tear it out and replace it. So I'm also stuck with whatever steel reinforcement it has (the more I think about it, who would pour a slab without some mesh in it? ... this one probably has some steel in order to have lasted this long) along with a lack of a vapor barrier.
I'm actually thinking of installing some kind of aluminum termite shield for the sill, in addition to tar paper, if I do go with the framed front wall instead of posts.
I once considered a similar project (LA County). When adding a perimeter footing, it was recommended/required to be pegged into the existing slab.
Not allowed to build up footings under existing slab here. To keep the existing slab perimeter meant cutting out squares of appropriate size and pouring new footings from bottom up. For longer spans between columns, you'll be moving up in footing dimensions...
I like the overpour idea. A lot!
I called out some concrete contractors after I worked out a basic design.
brian
I thought I might be able to use the overpour idea on my current project, but alas my client has now decided to look at a two story addition. Existing footings along two outside walls are not going to be sufficient.
Oh well...
brian
Don't be so sure.
I hammered out a slab today (2 actually, one above the other) on a 1924 house because the owner wants a wine cellar beneath his new deck. Not a speck of rebar or mesh and the lower, origial pour ws so rotten I'm surprised it held together.
We're doing fottings tomorroe and rebar; gravel etc and when the forms are gone, we will pit in a proper 4" slab over 4" of gravel and 2" of sand.
I'm NEVER surprised what I find in old houses. But I also don't want to get sued.Quality repairs for your home.
Aaron the HandymanVancouver, Canada
With no snow load, and stable soils, I would not worry about this
IF
it were a frame wall that spread the load all along the edge.
BUT
Since you will be doing this on point loads - the posts - you could indeed have a problem and need to plungepour footing pads under the slab at the post points, especially since you are not certain whether steel was used in the existing pad. Only a local engineer can say for certain.
Here's a couple more thoughts tho
Since this is just a screen porch, if the owner is willing to take the risk, you could chanced it as is and it wouldn't be such a terrible thing to jack and replace later - maybe
If you could design the walls with a wainscot base below the screens in such a way as to truss-spread the load across the concrete edge instead of point loading on posts, you would likely be OK
if existing elevations allow it, you could pour another slab, slightly larger than this one, over it with steel re-inforcing and a thickened edge.
Excellence is its own reward!
It's no big deal to cut a few holes in the slab and drill and pour some footings. If you're worried about how that will look as a finished area, you could be creative and plan it out to set some cheap landscape bricks around the post perimeter or something like that to break up the look between the old cement and the new, but structurally that is how I would handle it. In Wisconsin we have to drill to 42-60" below grade, I wouldn't have a clue about Texas but I bet it's not much, you could probably just dig out the depth with a shovel!
Dog
42-60" !
Around here (west of Austin) solid rock is never more than 18-24" away. other areas have more topsoil, if they are closer to a flood plain or other depository source.
The finished floor will be indoor / outdoor tile of some kind, so cutting into it may be the way to go. But since the posts are going right along the front edge of the slab, I'm going to investigate building the footer underneath it, from the side, without cutting.
thanks Dog
Without doing a trussed framing arrangement you described for load distribution along the wall, I'm thinking that using a conventionally framed wall would probably only marginally decrease the point loading, since the minimum vertical spacing is about 4' before the screens get too small.
That being the case, I'm concluding from the various feedback that some footer is required. I'm curious about the plungepouring idea - can you point me to some information on that process? Correct me if I'm wrong - is that a way to pour a footing under an existing slab without having to cut it? Otherwise I have to either cut holes and pour footings from the top, or build outward, in front of the existing slab, and have the slab float independently.
The owner is a AAA+ bond-holding, risk-averse type. ;-)
building up isn't really an option.
thanks for your thoughts.
Piffins got a great idea,
along a similiar vien you could bore say 4 piers per long side, pour a conc footing for each, set a post on them carrrying a header that can span that 10 or so feet and place false infill coulums in between.
Not sure of your access but you could cut 16"x 16" access hole in the slab if needed and fill later, or possibly drill beside and under. If you have the access, try finding a local guy who specializes in fence post digging, likely theres one around you whom uses a tractor with an auger and could do that sort of angled work.
Just make sure you size your header/beams, columns and footings correctly, and if you can make your columns independent of your slab then even better, you'll minimize cracking.
Just a thought.
Cheers.
Edited to say "so does Mad Dog"........
Edited 5/12/2003 9:01:12 PM ET by CDN_Builder
yes, I'm thinking along the same lines... the question now is whether or not to cut and pour footers from the top, or attempt to the leave the slab intact and come at it from the side (the front side) and build underneath it. I have to wonder if that's a valid approach... a post above says that's a no-go in LA county, and I'm not too surprised to hear that.
thanks
Edited 5/13/2003 12:27:58 AM ET by dumfounder
I would probably dig in under along the edge and pour a continuous footer.
But you are wrong about the full wall or truss wall not changing the point bearing.
Suppose that you have a thousand pounds of load on each 4x4 post every four feet.
if true full 4x4, that is 2250#/sq ft
Now spread that over four feet by 3.5" to get 856#/sq ft - roughly a third as much, and that uniformly spread instead of point loading..
Excellence is its own reward!
But you are wrong about the full wall or truss wall not changing the point bearing.
My assertion wasn't that the truss wall would not change the point bearing - I definitely agree a truss wall could be used to spread the loading evenly. In considering the conventionally framed wall, however, I was neglecting the load spreading of the sill, which I agree is not a good approximation. My point was just that the vertical framing spaced at 4' will not be spreading the load across the concrete as well as a 16" spacing would.... which is true, but it is probably more correct to assume perfect load distribution along a sill (as you do in your calculation). Good reason to double (& stiffen) up the sill.
In any case, your point is taken... I was being overly neglectful of the effect of the sill. Your argument is further bolstered by taking a full 4x4 as 4"x4" (16 si -> 9000lb/sf) instead of 8"x8" (64 si -> 2250lb/sf).
thanks
By any chance have you taken a shovel along the outer edge of the slab and dug around to make sure it was not poured as a grade beam? Could be your lucky day.
Is that the same as a mono pour Mad Dog.?
You could always make your roof a bit bigger and dig 3' foot holes around the perimeter. Like a pole Barn. In texas I bet the depth is less. 3' here in PA. Set the posts in the ground and anchor them. I would Be more worried about wind. "Alright, I'm not from Texas", but don't you guys get windstorms there? But not to worry about the snow. Still 3:12 is fine.Where there's A wheel there's a way, got any wheels?
Yep. It is thought of as a "monolithic" slab, poured as one continuously, but the grade beam provides a perimeter that is say a foot deep, to contain the added point loads of walls and posts.
If this were my job, I would check that first. Then, I would try Piffin's thought of doing nothing, take the risk and see what would happen as a consequence. He said that the slab is roughly 5" thick which is fairly substantial--a fully loaded semi can safely drive on 6"! It's only a screen porch with a conventional roof, and I don't think the loads would be too great. I don't think I would do it here in the upper Midwest because of the freeze/thaw problems that we've got, causing a lot of frost heaving but I'm thinking they aren't getting that in this location because the old slab has no cracks and even the worst part of Texas probably isn't getting much frost.
But I would make sure that the customer (and that it was cleared with building inspector) was clear on this cost-saving risk management, get it in writing and not let him concern me with it again. If there was any hesitancy on either of those parties, I would go the full route and do it right or walk off and let someone else worry about it. Once those posts are in, nobody needs to do any more worrying, thinking or second guessing. It's a done deal then.
dog
I'm going to double check the outer thickness today - it's possible I missed something, but I'm not optimistic.
Also I'm tending towards a regular framed bearing wall instead of posts, based on piffin's thoughts on load distribution - so I think it will be sufficient to pour a mini-footer under the outside rim of the slab.
I'd like to do a more formal analysis of expected loads - can someone point me to a table of typical densities of materials (asphalt shingles, plywood, 2X, etc) to come up with a working estimate of the stress on the slab due to this construction? I'd come up with a per sf weight of the roof, figure the center of gravity based on the roof pitch, add the weight of the wall, and that should be fairly accurate.
thanks again.
A single word explains why we are limited to those two approaches...
eArthQuAkEs
brian
I was having a little (but honest) fun. It's quite that simple.
There are other possible geologic concerns, especially in LA/CA.
My first construction experience was going down to the basement with the old man to raise the house because of a sinking foundation. I also had to watch him demo his retaining walls that were keeling over. Everything built on solid foundation work.
Geology 101 taught me about a phenomenon called "creep", where saturated clay soils on inclined grades will move over bedrock. Okay. There was the explanation.
Turned out that 20 years later, it was discovered that all the homes built in the area were built over an ancient landslide.
The homes were built on unstable soil.
Imagine if one attempted to pour a footing under an existing slab or didn't dowel the foundation and slab together under such circumstances...
brian
creep!
no, not you
I have rebuilt a home that was built with one side on ledge and the other in slide clay. After a hundred years the lower footings had slid about 3.5" to 5" horizontally and 1.75" to 4" vertically.
We jacked and pulled most of her back together again, this time with footings three times as deep..
Excellence is its own reward!
I thought some of you all might like to hear about the progress on this project - as you recall I was gathering opinions on how to deal with a slab without a footer. My initial thought was to raise a 2x4 bearing wall to spread the roof load and hope for the best. I wasn't comfortable taking that risk, however. Also the fact that the slab is 3" out of square with the house over 30' complicated things. Instead I ended up setting 6x6 posts off the end of the slab on their own footers, and will at some point extend the slab out to the front of the post bases (and tile over the whole thing). It can be screened in at that point if so desired.
The construction is simple: a doubled 2x10 beam bearing on 6x6's at 11' spacing, supporting 2x6's that extend into the preexisting roof. 5/8" sheathing, asphalt shingled to match the original. The new rafters are bolted to the old ones. Simpson ties at the post base, beam to post (T-shaped), and beam to rafter (90deg twist strapping). The original roof pitch is 9 in 12, and the new porch roof is about 3.5 in 12. We overlapped the tar paper 50% and used ice and water shield at old-new roof intersection. Hardi-soffit and fascia (great stuff!)
I learned a couple of lessons during the remodel, most of which are probably well known by breaktimers:
- don't assume the existing rafters are set at regular spacings. I thought I'd be able to sister the new rafters alongside and get convenient spacing for the roof sheathing... nope, they varied from 16-26" over the span. And some were sitting well off the perpendicular. sigh.
- take care to oppose the bows in the rafters: we crowned them religiously, but didn't pay attention to the bows, and ended up with ~80% of the rafters whipping in the same direction... cost extra time squaring the whole thing up. may also be a good idea to double the outside-most ones to help straighten them.
- be extra generous for rafter length. I ended up underestimating the amount that would extend into the old roof, and due to the change in plan of moving the posts off the slab, had to extend the tails of the 16' 2x6's to accomodate a 24" soffit. Shoulda just ordered the right size.
The Texas high pressure weather pattern was perfectly timed... we had the roof open for about 5 nights but didn't even worry about it. (or was that a result of the martinis?)
I still need to install some of the ties, wire it, and finish off the soffit & ceiling... but the outer shell is complete and the sun is no longer baking the slab (& us). I also want to add some curved bracing at the top of the posts. other decorative suggestions?
all in all, a fun week off for a bunch of amateurs...
some pictures should be attached. thanks for all the advice
Edited 7/1/2003 7:25:40 PM ET by dumfounder
Do your research as to whats apropriate in your area, your local bldg officials can ably help alot. My suggestion may work where Iam, but may be completly wrong for you.
This forum is a great place to start.
I'll ring in on a different account that no one has mentioned yet.
If I've got this right, you plan to asphalt shingle that roof and leave the sheathing and rafters exposed from the underside. That means you'll also be looking at the tips of all those shingle nails unless I'm missing something. Not the prettiest sight in the world nor the easiest to paint unless you intend to spray.
Maybe you could build a second layer of sheathing on top set with 2x2s on top of the rafters and use the space to help vent off the heat from the roof.
no, you didn't miss anything from my description - good point. So you suggest: rafters, 1/2"(?) t&g sheathing, 2x2 spacers along the tops of the rafters, then 5/8"(?) plywood and the roof. Sounds reasonable to me... I could vent the gable ends of the roof where the 9:12 and 3:12 pitches meet, as well as the new eave at the bottom, to create a decent airflow. This is a western exposure in Texas, so cooling the roof is important.
I wonder if there's a standard approach to create the sort of old fashioned look I'm after (exposed framing viewed from the underside).
Old fashioned around these parts would be 1x4 t&g beadboards for the show part……or those beadboards applied to the bottom of the rafters. Looks great with a ceiling fan or two mounted up there as well.
I'm skeptical about using 1/2" ply for your exposed sheathing unless you can get that in t&g cause the butt edges between the rafters aren't likely to stay nice and lined up……or you may have to install h-clips or something to accomplish that alignment and then that won't look so good. If you decide to go with a ply or ply product for the exposed sheathing, I guess I'd be using something heavier and t&g.
Knowledge is power, but only if applied in a timely fashion.
Standard approach would be to use boards. You could use 1x6 or 8's, skip sheathed with say a 1" space, and lay 3/8" plywood over that, use 1" roofing nails and you're good to go.
Did you say 1/2" tongue in groove? I don't think I've ever seen that.
Wouldn't the screen porch do a good enough job of venting, or maybe the screens do not go high on the end walls? It's hard to vent a shed roof anyway, without using roof vents.
While you're playing around with ideas for the "right" look, I'll throw this out your way, too. If you want something a little more on the old fashioned fancy sort, you could grab your router and profile the bottom edges of those rafters to boot.
Or you could apply the beadboards or whatever to the bottom of the rafters and apply some profile-edged clear 2x2s on the bottom side of that and the rafters. That way your eye/brain has an adequate amount to latch on to and would still trace the raftering, plus you'd have an even larger vent area above.
I've got this same setup on the 12x32 screened back porch I built on this house. The screens here run near to the top plate, but I bet you're going to find that on a hot still day the heat will build up on that roof and radiate to the space below and the occupants. I experienced this on other porches and so I made sure to vent this one. Glad I did. And so I guess I'd vent it anyway I could. Better to light one candle, than to curse the darkness.
Knowledge is power, but only if applied in a timely fashion.
I don't think this qualifies as a "standard" detail, but here is a picture of the project I'm building right now... I am in a heating and cooling climate, so we framed the roof conventionally using 2x8's, drywalled the bottom, blew it full of cellulose, and then applied Doug Fir T&G and these "fake" Doug Fir 3x6 rafters. The hip rafters are 3x8's, and we beaded the underside of the rafters.
I originally was going to do it using the 3x6's as the real rafters and using SIP's above them, but after realizing I would have had to custom fabricate hardware to attach the rafters (because it would have shown, defeating the purpose of the old look), I decided it would be simpler this way.
That's purty, Nick. Nice space with some darn good lookin' material,too.
Knowledge is power, but only if applied in a timely fashion.
yeah, very nice. the 3-by's make a big difference... I'll be sure to do that instead of 2x.
thanks - that's what i'm shooting for!
I could be wrong but I think that you can use a good old fashioned stud finder ( the magnetic nail finder type) and get a feel for rebar or mesh. The newer higher priced electronic type will find rebar. I am not an expert but what would be wrong with using treated 4x4 for a sill with 4x4 posts resting on them. All anchored and bonded, etc. You need to have something to attach screen to and this gives good toe kick protection. What is the rating of 5" concrete even without rebar? Surely a 4x4 sill would spread out quite a load.
I like that idea. Maybe he could use a termite barrier, followed by a treated 2x4 fastened to the slab with wedge anchors or epoxied bolts or whatever, then the 4x4 on top of that to spread the load.
Unless you guys know a better way to attach the 4x4 directly to the slab.