shop drainage


Staff member
most problems youll have with a shop can be AVOIDED with careful planing,and doing some research BEFORE starting construction, the foundation pad size, height and surrounding slope has a huge effect on your shop drainage and how solid the slab will be supported a few things that should be mentioned is that the shop floor should be at least A MINIMUM OF 24"-48" higher than any surrounding area, and 48"-60" is a far more reasonable minimum pad height if you want the shop floor to remain dry under all conditions where water from a hard rain might collect and the area around the shop should be sloped away to speed drainage, 48" is really the safe minimum in most areas, but some building departments don,t require more than 24"-36", now you might not have hurricanes in your area but a hard rain and wind can do a good deal of damage if the drainage for the shops marginal. french drains can be installed in problem areas to speed drainage, but having the shop floor higher than the surrounding area is the best plan as water tends to run down hill. and its best to have the main shop doors not face the prevailing wind direction, or up hill
IF you intend to install a bathroom check building code requirements, in many cases youll need to have 50-200 feet from a property line
when they pour most shop floors you have the option of having it dead level or slightly sloped to the doors to make clean-up and drainage easy, or to install floor drains in the shop floor with pipes to drain water out of the shop, just take the time to plan carefully because those pipes could be allowing water from a hard rain storm,to drain into your shop is placed incorrectly
a great reminder for anyone planing a garage to have a pad built thats at least 24" taller than the surrounding area before pouring a garage floor pad, yes that usually requires a good deal of fill and having it firmly compacted to build the pad but its also going to mean a dry shop during hard rain storms, and in Florida where we get hurricanes thats MANDATORY if you want to keep the cars and tools dry as is selecting dade county approved doors designed to withstand 150 mph winds and sloping the area around the shop for decent drainage.
water runs to the lowest local area and pools after a hard rain, its your job to plan the shop site so its not located in the area water pools and to have decent drainage and a floor height thats high enough to avoid water intrusion under normal or even some rare conditions
, obviously you'll need to check building codes and your site plan but in most areas thats the MINIMUM mandatory pad height.
I had my shops pad built on a 48" tall pad with a 5:1 slope for drainage, again your deed restrictions and building codes will effect what you can do, but planing ahead always beats making corrections later
your going to be way ahead if the pad height the floors poured on is significantly higher than the surrounding area.



read the linked info

viewtopic.php?f=27&t=116 ... ch-drains/
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Staff member
just some info, one of my friends built a really nice shop and he built it up on a 4 foot high pad so that should have provided excellent drainage and no water intrusion, its a really nice shop and he had zero problems until he got a little spare cash and decided to have a 30 foot wide and 40 foot long concrete pad built in front of his shop so he could park cars and do welding out side in the open air,the problem started with the design, and its as built in that application.
the result was that his dead level outside pad with no drain allows wind and rain to flood the shop floor, hes now having to cut the pad to install a covered. grated drain channel, which has added a good deal of UN-necessary extra expense
most problems youll have with a shop can be AVOIDED with careful planing,and doing some research BEFORE starting construction,
,(1) he failed to build in a covered drain gutter along the front of the pad/shop to allow storm driven rain water to drain off the pad before it could flow into the shop

a drain slot like this ALWAYS needs to be placed and located between the shop floor and edge of the slab so water draining into it drains reliably when a slab is poured in front of a shop, to prevent water intrusion

(2) he failed to slope the pad about 2.5" of slope per 10 feet of slab surface minimum, sloping the slab very slightly to allow storm driven rain water to flow away from the shop, (its currently a dead level pad, and wind and rain cause problems)

(3) he failed to have the level of the upper edge of the sloped slab, thats sloped away from the shop,to be at about 1.25"-to-1.5" lower than the shop floor


Determine specifications. Driveways for one-car garages should be at least 10 feet wide. A two-car driveway should be 16 to 24 feet. The slab's thickness depends on the vehicles that will roll on it. A good measure is at least four inches for cars and at least six inches for heavy trucks. If the soil is soft or drainage is poor, add another one or two inches.

Design the slabs. For six-inch slabs, use a shovel to dig at least eight inches deep to accommodate for a four-inch gravel bed and raising the slab two inches above the grade. To prevent water from flowing into a garage, the slabs butting against the garage floor should be about one inch below that of the garage floor. For a six-inch slab, use 2 x 6 wood borders to create the forms.

Slope away the rain. The land should slope gently from garage to street, at least one-quarter inch per foot. A rise greater than one and three-quarter inches per foot will cause most cars to scrape the rear bumper on the driveway. If the slope is reversed, from street to garage, install a drain and drainage channel where the driveway meets the garage to divert water.

Build the slabs. Pour the concrete, smooth it with a wood float and cut the isolation joint with a trowel. Set isolation joints where the drive meets the garage, walkways and the street. Control joints should be no more than 10 feet apart. A longitudinal control joint should be included down the middle of the driveway. Cut control joints at least one-quarter of slab depth.

Curve entryways. To create a curved apron from sidewalk to street, set stakes at the edge of the sidewalk, about 15 feet from either side of the driveway. Hammer a nail into the top of each stake and attach a 15-foot string to the nail. Use the string as a radius to place additional stakes in the desired curved pattern. Dig 10 inches deep between the curves and extend the excavation nine inches beyond the stakes. Build slab forms with 2 x 2 boards, lay the gravel base, pour the concrete, smooth it and install isolation joints.

remember the footers surrounding the concrete floor slab will be about 2 ft deep x 2 feet wide
and the slab will need to be about 8 inches thick where the lift is mounted
youll need a certain amount of rebar in the footers and slab



(BTW the number is the number of 1/8" in diameter the rebar measures)
so a number 5 rebar is 5/8 diameter
if a building code calls for lets say #5 size rebar you can and probably should tell your builder to step up the size at least one or possibly two larger steps
(thats what I did, it only added a couple hundred dollars to the cost of the shops construction but significantly increased the slab rigidity potential.
and concrete quality varys if you don't specify the strength you get the minimum, which is generally the 3000 psi version, you can upgrade to the 4500 psi version for only about $15 a cubic yard (again what I did)


verify local building codes
and remember the slab and footers will need to be elevated above the surrounding areas height,
for decent drainage (usually 3-4 ft.) minimum
and the area sloped out at a minimum 5:1 ratio,
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Staff member
were always glad to help out where we can!


just a reminder to plan your shop and think things thru carefully.
you can and should learn from the mistakes made by others

we had a rather long wet rain storm recently and while it was not anything like a hurricane ,one of my neighbors, who refuses to take good advise is only now starting to think I might of been correct when I suggested he build the dirt pad he built his garage in the rear of the property on to more than the minimal required 24" height, as last nights rain storm had a large puddle in front of that shop form that was more than 12" deep, what he ignored was the fact that the front of the property and the rear of the property are slightly higher in elevation than the rear middle area, once the rain water started too accumulate he realized that the shop floor height was not nearly as high in relation to the water level as he had assumed.
I built my house and shop floor supporting pad area well above the surrounding areas , for that reason, it cost more to do, but it has potential benefits.
my builder asked me why I had the house pad built to 6 ft above the surrounding area, and that extra dirt in the pad cost me about $8K extra, and I told him that during a good long hurricane, here in florida where its not uncommon to get 20 plus inches of rain over a day or so, I wanted to be an island not a reef!
most of the property in this area has a drainage canal along one edge for a reason, here in this area we are only about 30 feet above sea level, so water drains slowly,people that are not familiar with florida hurricanes wrongly assume those drainage canals never get full and over flow, simply because they were not here during a long hurricane to see the results, and it can take 7-10 days for the water to be pumped from the canals.
theres really nothing he can do now but I'm darn sure he realized that building the pad the garage has been built on 2-3 feet higher than it is currently
(like I suggested at the time the pad was being built)
would have been money very well spent well before the shop/garage floor was poured, yes it might have cost a couple thousand dollars more but that's a drop in the financial bucket towards the total expense.

BTW heres two calculators (Id add 10% )

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Staff member
I keep seeing news casts about excessive rain fall in places like TEXAS lately and Im wondering how many have had the fore sight to build the homes and shops up a bit higher than the surrounding elevation levels for those admittedly rare times when you do get excess flooding, and how many people gave a minutes thought to WHERE they bought property and if that property location was at risk of flooding?
I know here in florida we get a good hurricane every few decades so its a given that the older and more experienced, and SMARTER residents do usually consider the area they build in


A link to pictures of florida flooding

yeah! my first home had a 10ft wide 30 ft long single car garage , so when I built my own home I built a shop out back, because I was not happy working in a place that was always far to cluttered with no room, I had the good sense to build a pad under the shop that puts the floor height 4-5 ft above the surrounding land surface and my home floor level 6 plus feet higher simply because hurricanes and water puddling around a shop after hard rains are not all that rare in florida

but I quickly found you tend to fill floor space you have with tools cars, parts etc even with significantly more floor area




yeah! hard to make out the shop floor height above the surrounding lot level,but the shop doors a bit over 8 ft tall as a reference and the pond water levels easily twice the height of the corvettes roof to shop floor lower
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The Grumpy Grease Monkey mechanical engineer.
Staff member
the predicted path of hurricane dorian does not look to be something that will bye-pass the area I live in
florida generally has decent weather but every 5-7 years you can depend on these storms being a P.I.T.A.
thats why you build any home or shop on a raised pad, to avoid problems.
my home and shop are both built on raised foundation pads , 5-6 feet above the surrounding ground levels, even building codes require a 3 ft raised foundation pad


we have hurricane shutters and I have a portable generator and
we purchased what I feel will be the required fuel, and I have tested the generator
naturally I have enough extension cords and food & water
,yeah, the weather hurricane path prediction service accuracy is less than accurate,
and thats always a big issue

keep in mind Ive lived in fla for 60 plus years and we get hit every few years,
in most cases its a P.I.T.A. and we lose electrical power for a week or two,
but I,m 26 miles inland,
so as long as its a category 1-2 storm theres generally no real problem.
yes a category 4-5 is potentially a major problem,
I hope thats not the case here.
if we get hit directly
hopefully, that will not be the case,
its like tornados in oklahoma, you have zero control,
its something you hope youll never personally deal with,
obviously, if we lose local power the internets going down,
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solid fixture here in the forum
Dorian L getting revenge? Stay safe as you are already prepared for the worst and hoping for the best. Let me know if you need any help afterwards.