I (novice) am building a shed/playfort on an existing 11′ square concrete slab. Yesterday I checked the level of the slab, expecting it to be out by maybe 1/2″ and expecting to use shims to level a PT 2×4 sill like in the old FH article. It was out by 3 1/2″ over 11′! That’s quite a stack of shims. I see 3 options:
1. Scribe a PT 4×6 to the slab.
2. Rip small blocks cut from a PT 4×4 to the right thickness to put between a double 2×4 sill every 16″ to make the top 2×4 level.
3. Build the wall framing without the top plate, plumb and square it, then cut the top of the studs level and attach the top plate.
Any thoughts? One thought that I am ignoring is that the slab is unstable. Option 2 might allow adjustments as the slab slowly settles.
Paul
Replies
A variation on option 3 should do it. Don't worry about the slab settling, it's a shed/playhouse. Find the lowest corner, this will be the longest stud. Do the math for the remaining 3 corners. You can use a transit if you have one but this is a shed, use a level on a straight 2x4. Do your layout on the slab - top plate and corner studs perpendicular and square, bottom PT plate nailed to the two previously measured and cut corner studs, cut the remaining studs to fit between the top and bottom plates, stand the wall up, brace it plumb and fasten it to the slab. Do the other 3 walls, add the second top plate to tie it together and your frame is done. The top is level and the bottom follows the slope of the slab. If you feel the need you can cut the bottom of each stud at the angle that matches the slope. But why? - It's a shed. You can attach the siding while the wall is flat on the slab or you can do it after the framing is up. Your choice.
Hi Paul.
Both Ralph and RReed gave you basically the same advice....which was"don't bother trying to place shims anywhere, simply use longer stud material where need be to bring everything level. This is excellent advice. You should always strive to have your bottom plate bear its full weight on the slab. Shims create small point loads, and over time your walls may sag or settle underneath these point loads. This sagging cannot occur if plate is in solid contact with the slab.
Since your walls are only going to be approx 11 feet each in length. I think I'd opt with Ralph's suggestion to simply measure and build your walls on the ground, using the top plate as your reference for cutting the intermediate stud lengths.
Here's an easy way to measure your 2 corner elevations. Place a concrete building block at each corner. Use a mason's string line and strectch it tight to each block. Now , using the string as a reference line, measure down from string to top of slab and record your measurement. Do this at each corner. the corner with the larger number (reading) is your low point. Subtract this number from the other corner's reading to find your height difference. (Example...one corner reads 8 inches....other corner reads 10 inches....the 10 inch reading is the low spot...this spot is 2 inches lower than the other corner. You can lay out 4 concrete blocks, strectch a string at all 4 locations, measure these and find out all your elevation info.
It goes without saying, but, install your desired standard stud height at your highest corner elevation, and then from that point, install longer studs to compensate and give you your overall desired height. HOWEVER, This stud method will cause you to use more material. If you simply want to use 8ft stud stock and don't really care whetehr your ceiling height is a couple of inches shorter than planned, then place your 8fter at the lowest elevation corner instead. Now, all your 8 fters will have to be slightly cut down when going from "low end to high end." Hope this makes sense.
RReed's method of wall building will definately work, but would be more labor intensive; especially if you are doing this project by yourself. Having to brace each corner post, plus mark each stud length with the chalk line ( I'd be more inclined to eyeball the chalkline and the stud and then mark it with a pencil...if the stud was not perfectly vertical when rubbing against the chalkline as RReed suggested, your mark will be off...IMHO) will take a lot of extra work and usually requires another set of hands.
Ralph's method is quicker, and should be accurate enough for a 11 foot span wall.
As for sheathing the exterior walls with plywood, sheath it like Ralph suggested; with the walls lying on the ground. Sheath your wall and then cut off the excess that protrudes past your plates. Another option is to sheath your walls and allow an inch or so to extend down past the bottom plate. You can only do this if your wall plates are going to be installed flush with outside perimeter of slab. This extended lip helps with wall/slab alignment and will also help shed water away from the bottm plate.
LOL.
Davo
how thick is the slab, does it have footing, if its a 4 inch slab with no footing, you can jack the slab level and backfill undeer with sand or very wet concrete slurry
I don't know. I'll check.
Paul
Davo, LOL indeed.
This might work better with a stake at each corner. With a water level you can easily mark an equal height above the slab at each corner. THEN measure down to the slab from your marks.
Is the slab flat? Joe H
Joe H.
You're absolutely right, a water level would be more accurate than a stringline between 2 blocks with a line level attached. Course, a laser level with tripod and measuring rod with a sounding remote would work even better( which is usually what I normally use) I thought Paul was building an 11 foot square playhouse for his kids, and thought he didn't want to spend much money or go high tech. The stringline covering a distance of only 11 feet would have sufficed. But, your water level is definately a better choice.
P.S.... from reading some other posts, it appears that Pauls "lil playyhouse" is actually turning into a larger, 2 story structure! So much for discussing the low tech approach, huh?
Davo
Davo, actually when I wrote that I had a picture in my mind of your 4 blocks sitting at the corners of the slab and measuring 8" all around. But yeah, this thing is gonna look more like a silo than a playhouse with a 11' footptint. Joe H
paul, I'm sorry. but ralph is correct and the problem can be overcome.
a faster simpler method is to shoot down your bottom plate just like normal, then nail on the corner studs and plumb and brace them firmly. use whatever you've got to find the highest corner and mark and cut the tops of all of the corner posts so that they are all level with one another ( a water level will work!) mark the bottom plate with your stud layout. stretch a chalk line between the corner posts. one by one, take a stud and putting one end on your layout marks, rub the upper end against the chalkline - transferring the correct length to the stud. toenail it down. complete the wall and install the top plate....sheathing is harder but use the trick with 2 16d nails between the slab and the plate to hold the plywood up - hang the sheathing plumb!...it all sounds so complicated! isn't coffee time ?
Or.....you can do it right by breaking up the unstable slab and make a new stable one.
If the slab is settling then whatever you do now will be out of plumb later on.
Putting money over bad is always bad.
Gabe
hey paul , you might try levelling the slab at the start . Problems have have a way of coming back at diff stages during the build if they"re ignored .
How bout if you cast a beam of concrete and steel at the perimeter of the slab , the formwork will give you a straight reference point throughout . If the slab is unstable maybe you could cast the ringbeam outside it . This will allow you to have a level strong bearing point for your new shed .
When I grow up I wanna be a carpenter.......
Or use SLC to level out the pad.
Boris
"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1927
What is your total material cost going to be?
Jon,
That is really the issue. The structure will be somewhat expensive (have not yet figured out how expensive), and, more important, take some time to build. I've designed it to go with the house, provide storage for the garden tools and have an area for my wife to do her potting, and have an upstairs play area for the kids that can later be turned into a small office or studio. If it were just a shed I would not be too concerned, but this will have some investment in it. I would do the un-leveled sill if I thought the slab would not move any more, but I have no way of knowing that. The Idea of spacer blocks between a double sill apeals to me because if the slab moves more, I can loosen the bolts attaching the framing to the slab, jack the structure and change out the blocks...although a slight lean could give it character.
Paul
Ah Hah! So it's not "just" a shed. Coulda said that in post #1.<G>
Now you really do need to spend some extra bucks and do it right, which includes a better foundation than just a slab of unknown quality. The original slab was probably just a 2x4 form on the ground, following the lay of the land, with no thought for level or even thickened edges.
Maybe you live in an area that has enforceable building codes and nosey neighbors who will report building activity. But even if you don't have to look over your shoulder you should determine the need for a footing to proper depth along with a new slab formed and poured level. If you are going to spend the money for a structure that fits in with your main house while also being configurable for various needs it should also enhance the value of the property by not being cobbled together on a questionable slab.
A new slab also gives you an opportunity to grow beyond the current fixed dimensions. If you were to pile all the garden tools, potting table(s) and materials and the lawn mower, edger, line trimmer, leaf blower, wheelbarrow and whatever on the original existing slab you will quickly find that you don't have enough floor space for what you want to do.
How bout that Ralph! Take the guy at face value..."I am (a novice) building a shed/playfort"...... so Paul says...only to find out later that this little shed is to be an actual 2 story castle! ( or so it seems)!
Hey Paul, Disregard my previous advice. Thought you were constructing a shed type (single story) playhouse for your kids. Now that you've let the cat out of the bag.....the best thing to do is to demolish your existing slab ( 11 X 11 is not very big and should not be much trouble to demolish...call someone you know with a backhoe...job will be over by the time you finish your 2nd beer watching your backhoe buddy doing all the work.)
Layout and install a new foundation. Build it as big as you like and then errect whatever you truly want to build. No sense building a 2 story structure on a slab that is marginal at best.
Davo
Paul,
Did you do the slab? If so, then what were the conditions previous to the pour?
Gabe's right. If you are planning on a significant investment, than do it right. No sense on hurting yourself at every stage down the road when you could get it right now. If the slab is sinking, you have a problem. Either leave as is and deal with a structure that is getting worse or do over.
It all comes down on your costs vs. benefits. Only you can do this. Is it worth it to do it right now?
Jon Blakemore
Thanks, everyone, for the advice.
The slab is 4" thick poured by the previous owner who tended to do things right. There is a large tree right next to the slab that would be damaged if I do the foundation correctly below the frost line. I will do the scribed PT 4x6 sill to make the top of the sill level. I'll put plenty of nails into the sheathing so that if I have to eventually jack the structure back to level it won't rack.
Thanks again,
Paul
WHOA! Back the truck up!
What are the chances that the tree roots have lifted the corner of the slab? or that they will continue to do so? You suggested that it might still be settling but we really need an idea why that might be so. If this guy tended to do things right and it was poured on undisturbed soils then the tree is the culprit and it won't get any better before it gets worse..
Excellence is its own reward!
Paul,
IMHO a 4 inch slab is not really suitable for a 2 story structure. You say the guy who owned the property before you had poured this slab and normally did things right.
Well, first off, if things are done right, how come the slab is 3-1/2 inches low from one corner to the next over just an 11 foot span? That's a pretty big error.
Secondly, since no structure was put on this slab before, do you know the original intended purpose of this slab? Was it just to be a patio slab to set lawn furniture and the barbeque grill on? If something along these lines was the intended purpose, chances are good the slab is not even reinforced. Do you know if slab was reinforced? Chances are very good that slab is not even a full 4inches thick. Lots of sidewalks and patios are framed with 2X4 lumber; which is yields a true slab height of only 3-1/2 inches.
Perhaps my questions and comments seem trivial, but 4 inches is sidewalk grade material...not really suitable for foundation work. For a small, single story structure, yeah, you might get away with it. But I would strongly reconsider if planning to errect a 2 story structure.
Paul, I've poured many a slab for many a small outbuilding, and even in those instances, I never poured a slab that was less than 6 inch minimum; and they were always reinforced with a 1/2 inch rebar mat. This might be overkill, but we were never allowed to pour a slab less than 6 inches thick unless used for pedestrian use (sidewalks) only. And our foundation slabs were always reinforced...sidewalks hardly ever.
If the nearby tree is causing problems with your slab ( which may be the case) then the slab is already subject to point loads that it would not be subject to if simply lying undisturbed on compacted ground. If the roots are lifting one end slightly, chances are good that your slab is gonna start cracking once the new structure is errected on top of it; which will create more unequal stresses at various spots throughout.
My advice....RETHINK and RE-EVALUATE before proceeding.
The costs associated with tearing out the old pad and repouring new (11 X 11)should amount to approx $600 or so if professionally contracted out, and way less if you do most with sweat equity.
LOL.
Davo