Forget the common answer that it allows water to collect at the lowest point in the house. The question REALLY is why can’t a house be 100% water tight therby eliminating the need for a sump? I never understood this…
What would happen IF a house were 100% water tight (including the basement) and there were no sump???
Could this house remain watertight through the life of the house?
Concrete is semi-porous. Houses settle. Foundations crack.
Then any leaks, condensation, drips, etc., would accumulate in the low point of the basement. Foundations are normally sealed so as to be effectively watertight, though with the course of time they will spring potential leaks – miniscule breaks that will allow water to leak in from saturated soil. But notice that water does not always come from outside. A broken pipe, or hot humid air cooling and condensation occurring on exposed surfaces, are ways in which purely interior water accumulation can occur.
Here in New England, there are companies that can reseal basements and prevent future leaks but they cost thousands of dollars.
Not to mention that pipes break and water heaters sometimes have catastrophic failures.
I’ve lived in four houses with basements here in New England. Our current house was built last year. None of them had (or have) sumps.
I don’t believe sumps are typically used in new homes unless you have a serious infiltration problem due to high groundwater.
You don’t have to make the basement completely watertight, either.* What you do is surround the outside of the basement with a footing drain. It’s a french drain consisting of a perforated pipe surrounded by crushed stone. The pipe is connected to daylight at a lower grade, or to a catch basin in the road.
The footing drain acts to lower the groundwater level in the vicinity of the house, thereby preventing infiltration into the house.
Knock on wood, if I ever do have a pipe leak or water heater failure, though, I’m screwed. I’ll end up with an inground swimming pool. :eek:
*The exterior concrete of the basement is coated with a bituminous dampproofing coating, for what it’s worth. (Notice that it’s not called “waterproofing.”)
If you had a watertight basement sitting in a saturated soil environment right up to the basement windows, I’d expect to see your house float upwards.
I don’t understand this. If you are using a sump to pump out water seeping in wouldn’t there be the same amount of buoyant force?
Actually it would.
Very slightly. Very very slightly. As a matter of fact, the weight of the house’s materials will far outweigh any bouyancy effect, so it won’t really move.
But in essence, it’s correct that a sump is not truly necessary if the house’s basement floor is above the groundwater level. However, if you have a rain event that exceeds the ground and sewer system’s ability to remove, the water will look for otehr places to go, which is likely to include that basement.
Here’s the thing: you’ve got a whole bunch of water systems in your house; as we all know, water flows down. So the waste systems all collect at the lowest point of the house. This includes any basment floor drains or toilets you may have, which are at the basement floor level. Thus, they have to flow downhill a bit yet to drain properly.
If the water table is higher than the basement floor, water will come into the basmement through several means: the walls, any cracks, the joint between wall and floor, even around the floor drain. This can be dangerous because the water will then go into the waste system and can mingle with your waste stream, bringing the Bad Stuff (known in the business as black water) back into your basement. This happened on a huge scale in New Orleans.
A sump pump takes this water and pumps it out into the sewer system or septic. A sump pit or crock allows enough water to accumulate to activate the pump (they get damaged if they run with no water) without flooding the entire basement, but in sever storms, the water may come in faster than the pump can push it out.
Sometimes a pump is also necessary if the sewer system is at a higher level than the lowest point of the waste system. This is called an ejector pump, and it has to handle the waste stream as well as the water.
You can, actually, build a house waterproof. But it won’t stay that way due to things like workmanship, movement, wear, and the other penetrations in the waterproofing you have to make. It is not a system that you want to rely on. If the water table is high, a sump pump is simple and cheap.
Well, houses definately don’t need sumps - here in the South most houses don’t have basements, and I don’t know anybody with a sump pump. The water table is high, but that’s why we don’t have basements.
Then it would be a submarine.
Water always wins. Essentially what one does when dealing with water is to just slow down the rate at which one loses the battle.
Whetever last layer of protection you have resting directly against the water would lose. When it loses, the water breach would have to be dealt with (time for sumps/pumps/french drains). So, no absolute waterproof homes because you must prepare for some type of breach. And if you reduce the breach chance down to a very low risk number, you have pushed costs through the roof (costs that could have been used on better above-ground structures).
The best way to deal with water is not to create an absolute resistance against it (it will win, especially as it goes through its solid and liquid states over and over)…ahem, anyway, what you do is you use water’s properties to itself manage the water: You make it flow to low points, allow it to collect, guide it, steer, etc…and pump it out. The french drain system faciliates this, along with the sump/pump/etc.
A house situated on well drained terrain, a proper foundation and foundation drains does not need nor have a sump.
Many houses would not have a need for a sump IF the builder had done a proper job when he first excavated and installed an external foundation drain, gravel fill and poured the concrete foundation higher than the drain.
Poor construction practices keep the retrofit sump and drain companies in business.
Also, overdevelopment in greenfields and near waterways creates situations where the builders have no choice but to build in unsuitable areas. Poor development planning is the culprit in this case.
And to make matters worse, the new impermeable surfaces and earthwork decrease the ability of the watershed to absorb water, increasing the forces acting against the basement walls and making the matter worse.
It’s not always poor construction practices. Sometimes, no matter how the builder handles the foundations and construction, water can overload the system. If located anywhere near a flood plain, waterway, or even a low point, it can be an intelligent decision to include a sump system.
Thanks, all, for the SD!
I guess we were fortunate, and we took it for granted. Our basement was always dry, and the sump collected water from a drain at the bottom at the exterior basement steps. Without that drain, the bottom of the steps would flood. The drain emptied into the sump.
In my present house, the walls have dampness issues of which a sump pump will not solve. Again, I guess I’ve taken it all for granted.
Thanks, all, for answering a seemingly obvious question with patience and tact. 
Misread as: Why Do Houses Need Sumos?
A stupid Q if ever there was 1.
Yes, now Sumos we need in the house! …Instead of a guard dog, of course! 