The previous owners left us with a 9x12' slab they had poured for a
dog kennel; it happens to be in the same place I want to build a shed.
The slab is 7" deep and though it's an obvious DIY job it has held up
for 10+ Minnesota winters. Assuming I want to build an 8x12 shed on
PT sills atop this slab, what's the best way to anchor the sills?
With a new pour I'd just use bolts but I'm not sure what the best
method is in this case. FYI, we're in tornado country and do see
gusts in the 50-60 MPH range during thunderstorms or blizzards a few
times a year, so I want to make sure this shed stays put.
Check some anchor bolt catalogs such as Hilti or the like. They will
explain the loads and how to install the different type bolts. I
think you will have to drill into the concrete and use a bonding agent
in the hole with the bolts set in.
You might also want to check the stability of the entire structure in
The last poster has it spot on,
rotary hammer drill and set in epoxy threaded rods where you think
they are required. Expanding masonry anchors are great but close to the
edge of a slab they could split the concrete. Manufactures have tables
specifying edge distances for this reason.
A length of threaded rod is cheap so I'd go for 12mm. (half inch,) set
in a minimum of 4 inches.
Put your threaded rod, or hold down bolt, near a stud at each corner, and
say maybe a couple spaced equally to each wall, but close to a stud. Say
10-12 bolts in all.
Fix a strap or timber connector from the bottom plate to the stud
(vertical) near each HD bolt and also tie in the top plate to the same
studs. Get the picture, a link from the concrete to the plate to the stud
to the top plate. Then strap down your roof, same way.
All this is a fair bit more work than maybe you want to do Kiwanda,
but the material costs are nothing compared to the rest of the shed.
Do you want feel responsible if your shed blows through you neighbours
living room window?
What with all the global warming and extreme weather stuff going on, you
never know what the next storm will bring. Build it strong and sleep easy.
Thanks again to all for the advice-- it's very helpful. I'll take it
all into account as I plan the project this summer, but it seems quite
reasonable to build this thing as solidly as possible since I plan to
finish it to match the garage and it will be a permanent part of the
property. Really high winds are rare here (nothing's gonna stop a
tornado but we do get big downdraft gusts from t-storms on occasion)
and if the cost/design issues aren't that great it makes sense to do
everything I can to make sure it will stay put when my neighbor's shed
(which is just sitting on railroad ties) blows away.
I would use Tapcon screws. They aren't cheap but you wouldn't need many for
your size shed. I believe that they come with a drill. They also have a
large washer to prevent the screw head from sinking into the wood. I've
used them with good results, not hard to use and I'm no carpenter.
Glad I asked-- I've never seen these expansion anchors before, only
being familiar with TapCons that wouldn't be up to the job. 10"
expansion bolts are about $1 apiece and should make this a simple
Nobody else said it, so I will. Putting sill right at slab level, even with
treated wood, is a bad idea. Rain will pond, and that sill will be wet most
of the time, and even treated wood rots at some point. Much better to lay a
ring of 4" block, and anchor through that to the slab. My shed that previous
owner left behind is also in an old dog pen, but it is made of metal. If you
don't want to mess with block, at least put down a ring of plastic deck wood
or something, so the real sill is raised a little.
This slab is raised about 4" above the ground level, which is well
drained (sand). Given that the eaves will shelter the sills from
direct rain, I doubt there will be much exposure...snow is a different
issue of course, but my 50+ year old garage has untreated sills right
atop the slab, on grade, and after a half century the only damage to
the sills was in one corner where the snow often piled 3-4' deep from
Thanks for the advice though. I may consider using a composite for the
sills, and typically will use treated plywood for the lower band of
sheathing (with the edge protected by aluminum j-channel) if it's
going to be exposed to snow for 6 months of the year.
Anchoring the sole plate to the concrete will keep it there but the
rest of the shed can still go off to Dorothy land.
In Florida they either want the whole shed built to wind code and tied
to a real bell footer with steel in it or you strap over the roof. Now
days they won't even accept straps on a new shed. It needs to be wind
code construction based on your zone with engineerred drawings.
Forget about those metal or plastic sheds you get at Home Depot.
BTW the straps are a lot stronger if you anchor them to the side of
the slab so the bolt is in shear than to put them in vertically. That
is how screen cage anchors are set. A 3/8" x 3" tapcon drilled in in
the center of the slab will work for that.
On May 18, 9:13 pm, email@example.com wrote:
Thanks for the tips. No code issues here for sheds at all, so my
primary concern is simply to keep it from becoming a projectile in a
typical wind; most of the sheds in my neighborhood aren't anchored at
all but typically sit on treated timbers or pier blocks. My site is
sheltered by an existing garage, 6' fences, and a huge silver maple,
so I'm just planning to anchor the sills and design in reasonable wind
First, if the slab is 9x12, it would be best to build a 9x12 shed so the
siding can overlap the edge of the slab. Otherwise, rain water is going to
run down the sides of the wall, hit the slab, and go under the wall into
As for anchoring, the only real option you have is to drill through the
sill and into the concrete slab with a masonry bit. Then install expanding
anchor bolts and a large washer. The more bolts you install, the more
secure the shed will be to the slab. You can usually find these "retrofit"
anchor bolts in the same area as regular anchor bolts and framing brackets
in the home centers. But regular expanding anchor bolts are essentially the
same thing. I'd select 3/8" or 1/2" bolts, as long as you can fit in the
slab (don't drill all the way through the slab).
Remember the sill is only part of the building. You'll need to nail the
sheathing every 6" to tie the sill, studs, and plates together. You should
also use "hurricane" straps on the rafters.
It's probably overkill for a shed, but this is standard building practice,
and the costs are minimal so why not...
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