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Do you have a terrace or flat roof on the top floor that goes to floor drains?  I would guess that you have a restriction in your main drain somewhere between where your house goes to the main sewer.  It's just enough that during normal operation you don't see an issue but in a heavy rain all of the water hitting the upper roof or terrace and going down that drain is more than the whole house drain can handle.  All of the floor drains are probably tied together whereas the black water stool and probably sink drains are probably tied together a separate line to the sewer.

They probably just need to snake the lowest floor drain or a cleanout (unlikely in VN construction) towards the street.  If you have a rooftop terrace leaves and stuff can go down it and start to create a blockage in a bend somewhere.

We had ours do a similar but opposite thing.  The rooftop terrace floor drains got plugged with leaves which created a swimming pool on the terrace and the resulting waterfall down four flights of stairs when it backed into the house made for a very wet mess.  Everyone should check upper floor terrace floor drains at the beginning and during the rainy season.  It's my biggest fear when we have a six month house and we are gone during the rainy season to have all that water come into a closed up house and come back to a black mold covered interior.

Thanks for the detailed reply. 

Yeah there is an outdoor terrace on the second floor.  We've also had leaks in the roof, some work was done up on top of the house earlier this year, resealing it, that area is flat.  Maybe there is a problem up there.  A new leak sprung from the roof in a different room recently as well.  We don't own the house, we rent, but the landlord is typically willing to pay for repairs if we can find the problem. 

We will get some plumbers to come look at it, and I'll check the drains I can find outside.

I read about something called a check valve, maybe we can have one of those put in the floor drain that's backing up so it doesn't flood the first floor.

mtgmike wrote:

The past two times it rained heavy, water came up the drain on the ground floor, a lot of it.  We've been renting a house for a few years with no problem. 

Nobody else seems to be having a problem around us, and our water goes down the drain no problem when it doesn't rain.

I had to scoop up the pooled water and dump it down the toilet to keep it from flooding the house.  The toilet drains fine while the rain water comes up the floor drain.

Our first floor is also raised by about two feet or so over the street level, while other houses near are closer to street level, but they aren't flooding.  I don't get it.

Anyone know how plumbing tends to work here, why this happens, and how to fix it?

In addition to the good advice already given, I'd suggest this:

If the problem hasn't been fixed by the next heavy rain, when you see the water starting to back up, go outside to the street or wherever the nearest flood drain is located (yes, in the middle of the heavy rain).

If you can do so safely, compare it to the nearest flood drain to the right and the left of your property.

See if it looks like any of those are draining properly and if any of them appear to be backing up.

When we had the great floods a couple of years ago in Danang, it was mostly caused by trash that somehow magically gets into the flood drain system during dry season.

Then of course, the flood drains can't work properly and the streets flood.

You might have a blockage at your nearest flood drain or perhaps at the next one "down stream".

As Stein has pointed out, a blockage somewhere between your main drain and where it empties into the flood drain system could have a blockage that's causing the backup.

Your neighbors might not be affected depending upon the location of the blockage.

The path of least resistance might be your drain system, without enough extra pressure to spread to a neighbor's drain system.

And In fairness to those who worship at the church of global warming, it is true that global warming is supposed to cause heavier rainfall, so it's possible you are simply suffering the effects of worldwide capitalist imperialistic pigs gone wild...

One of the biggest issues in Viet Nam is that in most urban areas the stormwater and sewerage system are the same.
As Stein has said it sounds like a blockage somewhere. If you can find someone with an get them to check your system.

We had some plumbers come today.  They found a bunch of black cement in the pipe under the bathroom floor drain, it's the same kind of cement from the repairs done on the roof earlier this year.  Looks like they came down the drain pipe from the roof.  There were also construction materials jammed in the pipe near the front of the house.  The Drain on the roof connects to the floor drains and then goes out the front of the house to the bigger storm drains on the street.  The toilet and sewage in the house is on a different system and wasn't backed up.  We'll see how it goes next time it rains, but it seems to be draining better now that they broke the cement out of the pipes.  Not much clogging the pipes in the way of leaves, just construction material.

Sounds like they washed everything down the drain, very common in VN. It just disappears when you wash it into the drain, thats the mindset of workers.

colinoscapee wrote:

Sounds like they washed everything down the drain, very common in VN. It just disappears when you wash it into the drain, thats the mindset of workers.

Like most things you see, on the surface good work but look further and every tradesman see to take short cuts.

In my building the floor tiles in various apartments get changed a lot. Mine was done last year when I woke up to loud cracking sounds where the floor tiles literally erupted.

I watched the maintenance team remove, replace and grout the new ones. No margin of error for expansion. So when it gets very hot.. pop and they go again.

Depends who manages the buildings, savils and Novaland aren't a great combination

Jlgarbutt wrote:

colinoscapee wrote:

Sounds like they washed everything down the drain, very common in VN. It just disappears when you wash it into the drain, thats the mindset of workers.

Like most things you see, on the surface good work but look further and every tradesman see to take short cuts.

In my building the floor tiles in various apartments get changed a lot. Mine was done last year when I woke up to loud cracking sounds where the floor tiles literally erupted.

I watched the maintenance team remove, replace and grout the new ones. No margin of error for expansion. So when it gets very hot.. pop and they go again.

Depends who manages the buildings, savils and Novaland aren't a great combination

Expansion, now there's an unknown concept here. Our tiles drummed after 6-months in a brand new house.

colinoscapee wrote:

Jlgarbutt wrote:

colinoscapee wrote:

Sounds like they washed everything down the drain, very common in VN. It just disappears when you wash it into the drain, thats the mindset of workers.

Like most things you see, on the surface good work but look further and every tradesman see to take short cuts.

In my building the floor tiles in various apartments get changed a lot. Mine was done last year when I woke up to loud cracking sounds where the floor tiles literally erupted.

I watched the maintenance team remove, replace and grout the new ones. No margin of error for expansion. So when it gets very hot.. pop and they go again.

Depends who manages the buildings, savils and Novaland aren't a great combination

Expansion, now there's an unknown concept here. Our tiles drummed after 6-months in a brand new house.

In my ignorance as a first time home buyer, I once used an unlicensed flooring contractor to put down some really nice hardwood in my living room.

I used him because he was such an excellent Finished Carpenter, and he had experience laying down hardwood for flooring contractors.

But he didn't know to check for the moisture content in the subfloor, and my crawl space was very often wet after rains.

Fast forward 6 months.

I'd left Tennessee to go on a road trip with my son.

When we returned after 6 weeks, the flooring had buckled up, forming a "tent" along the length of the room.

That was actually when I first decided that maybe I was going to live someplace else...

I'm just mentioning it here because this is a really smart kid in many ways and highly sought after for his finished carpentry work.

He did it as a favor for me because I didn't want to have to hire a licensed contractor when I figured he could do the job well.

I'm guessing that Vietnamese laborers find themselves in similar situations, where they are hired to do a job beyond their level of training, by people such as myself, who are looking to avoid paying professional rates.

I had this problem in US once. I was a able to buy a ball valve that would float up and prevent water from coming out of the drain. No idea if available in Vn. If you can find it perhaps think of something like a rubber ball you could force tight during heavy rains, or perhaps you could pipe the water from drain to outside of house, like sump pump. Good luck