How far out is a division, not a guess
The bottom falls away at some rate, and the depth you want is at whatever distance that rate delivers it. Nine inches per ten feet is 0.075 feet of depth for every foot out. Starting in six inches of water and wanting three and a half feet, you need three feet of extra depth, and three divided by 0.075 is forty feet. At eight feet a section that is five sections exactly.
Sound the bottom before trusting that. Two inches per ten feet on a shallow flat means the same target is a hundred and eighty feet out, which no permit is going to entertain and no sectional dock is going to like. Twenty inches per ten feet on a drop-off means eighteen feet, and the legs get long fast. The slope figure is the input that decides whether a sectional dock is the right answer at all.
Every leg is different, and three of the four terms are not
A leg is the water depth where it stands, plus the water level rise you want to be able to jack up for, plus the deck height above the water, plus whatever the foot sinks into the bottom. Only the first of those changes along the run.
On the defaults, the constant part is one foot of water level swing, sixteen inches of deck height and six inches of sinking allowance, which is 2.833 feet. Under section one the water is 1.1 feet deep, so the leg is 3.93 feet. Under section five it is 3.5 feet deep and the leg is 6.33. Two and a half feet of difference across a forty foot dock, which is exactly the amount that gets discovered after four legs have been cut the same length.
Sinking, and why it is a field
A foot on firm sand settles an inch and stops. A foot in silt keeps going, and it keeps going for weeks, which is why a dock that was level in June is a ramp in August. The allowance here is a field rather than a constant because it is entirely a property of your bottom, and because the honest answer to soft ground is usually a wider foot rather than a longer leg — spreading the load is what stops the settling, and length only chases it.
October, and where it all goes
Five sections at eight by four is 160 square feet of deck and, at 120 pounds a section, 600 pounds to move. Stacked four high that is two stacks and 64 square feet of yard, before any room to walk around them. Those numbers are small on paper and they are the reason people build a sectional dock one section longer than they need and then regret it every autumn.
The weight figure is also optimistic in the way weights usually are. A section is dry when you weigh it in the catalogue and wet when you carry it, treated timber holds a lot of water, and the section is always heavier than the two people lifting it agreed it would be. A section that lets go in the water goes toward whoever is standing beside it.
The plain hazards, named and not managed. Working alone over water is how people drown quietly; nobody hears a splash from the parking area. A dock section, a float, a pile or a lift cradle is far heavier than it looks and it moves the instant it lets go, usually toward whoever is standing in the water beside it. Cold water takes strength and grip out of a swimmer in minutes, and shoulder season water is cold even where the air is not. And electricity near water is its own category: electric shock drowning around docks and lifts is a real and specific way people die, it leaves no mark, and swimmers in the water have no warning. Anything electrical at a dock belongs to a licensed electrician who works on marine installations, not to a calculator and not to a weekend.
Questions people ask
How far out does my dock need to go?
It is the depth you want minus the depth at the shore end, divided by how fast the bottom falls away. If the bottom drops nine inches every ten feet, that is 0.075 feet of depth per foot out, so going from six inches of water to three and a half feet takes three feet of extra depth divided by 0.075, which is forty feet. The sensitivity is the thing worth noticing: on a flat of two inches per ten feet the same target is a hundred and eighty feet out, and on a drop-off of twenty inches per ten feet it is eighteen. Sound the bottom with a marked pole at several distances before doing any of this arithmetic.
How long should each dock leg be?
Water depth at that leg, plus however much water level rise you want to be able to jack up for, plus the deck height above the water, plus an allowance for the foot settling into the bottom. Only the first term changes from section to section, so on a typical run the legs climb steadily outward — on the defaults here, from 3 feet 11 inches under the first section to 6 feet 4 under the fifth. That two and a half foot spread is why cutting all the legs to one length is a mistake people only make once, and why sounding at every leg position matters more than the average slope does.
Why does my dock end up out of level by mid summer?
Usually because the feet are settling, and settling unevenly. A pad on firm sand takes up an inch and stops; the same pad in silt keeps going for weeks, and the legs in the deepest, softest water go furthest. The fix is generally a wider foot rather than a longer leg, because what stops settlement is spreading the load rather than chasing it downward. Wave and boat wake work the bottom out from under a foot as well, and a leg that is sitting in a scour pocket it dug for itself will keep needing adjustment until the foot area changes.
When should a sectional dock become a floating one?
A useful practical signal is when the outer legs stop coming out of one piece of stock. Once the water at the end is deep enough that a leg is longer than the material you can buy or handle, the section is fighting the bottom rather than standing on it, and a floating section on the end starts to make more sense — it does not care how deep the water is at all, only what is holding it up from above. The other signals are a soft bottom that will not hold a foot and a water level range large enough that jacking legs every few weeks becomes the summer job.
How much room do I need to store a dock over winter?
Multiply the section footprint by the number of stacks. Five sections eight feet by four is 160 square feet of deck; stacked four high that is two stacks and 64 square feet of ground, plus room to walk around them and get a hand truck in. The weight is the part people underestimate: five sections at 120 pounds each is 600 pounds, and that is the dry catalogue figure. Treated timber comes out of the lake holding water, so plan on the sections being heavier than the number, and on a section moving when it lets go.