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Flood Resistant Construction: 7 Minutes of BS

March 06, 2018

If you can't build to stay dry, build to tolerate wet

In this episode of 7 Minutes of BS, 'Dr. Joe' Lstiburek, PhD , P. Eng, ASHRAE Fellow, and founding principal of Building Science Corporation explains why flood water cleanup is critical, and how to rebuild with tolerance in mind.

What it is:

Flood resistant construction: fləd | rəˈzistənt | kənˈstrəkSH(ə)n (n)

Flood resistant construction is two things:

One approach is to build above the flood level, and it doesn't matter. That's an insanely logical way of doing it, you don't have to use special materials once you get above the flood level.

The second is that you assume the materials in the assembly are going to get wet, and saturated, so you build it in such a way that its easy to dry and clean the building and pout it back together after the event happens.

—Joe Lstiburek, PhD , P. Eng.

How construction resists floods

Many natural phenomena can cause floods, from heavy snows and warm weather in the mountains, to heavy rains over long periods, to massive hurricanes, as we saw at the end of 2017 in Houston, Florida, Puerto Rico, and all over the Caribbean.

The mechanism of property damage goes beyond the gallons of water soaking into the building materials, its—

...dirty water. Flood water is not clean. They call it category 3 water for a reason, there's sewage in it, there are animal parts, there are oils, it's just not good stuff. If the flooding and the wetting was clean water, this would be easy. What makes this more difficult is, this is dirty water—exceptionally dirty water.

And if all you do is dry, and the nasties are still there, the nasties are still there.

If all you do is dry the materials out after a flood with category 3 water, then you just got yourself a category-three nasty wall.

That’s not good.

One solution to construction in flood zones is to eliminate construction in flood zones

...surely we’re smart enough not to build where it floods.

But for a number of practical reasons, that’s not in the cards.

That’s not in the cards.

We’re going to build with flood resistant construction because we have no choice.

How to build new homes for flood resistance

In a perfect world, we could start from the beginning. Well, in a perfect world we wouldn’t be talking about category three walls, but setting that aside,

It’s always easier with a clean sheet of paper.

New construction is easier than remodeling.

...without having to rehabilitate or renovate an existing building.

The easy thing is to build up…

On piers, or sacrificial spaces,

Build it above where you expect it to flood.

People living in the coastal southeast know all about this. North and South Carolina, Florida,

Especially the panhandle

And along the gulf coast into Texas.

In places where it’s not practical to build up, we build out of stuff we allow to get wet.

Sacrificial spaces that are going to be perfectly dry 95% of the time. This method uses concrete and masonry on the first floor.

And we line the concrete and masonry with stuff that's in essence, disposable, so we build the first floor out of stuff we allow to get wet. When it gets wet, we dry it and throw out the stuff that we can't save, but we've designed it to make it easy to throw out the stuff we can't save.

Disinfect and clean, put it back together, and go on.

That’s the easy stuff.

How to retrofit for flood resistance

The more difficult stuff is what happens if you have a building that wasn't designed to be wash and wear, and cleaned and rehabilitated. And it wasn't expected to be flooded?

What if it was a slab on grade, somewhere in Houston? With brick veneer over stud framing with drywall or even lath and plaster? And what if the people living there cannot afford to knock the house down and build on top of piers.

Well, you take it apart. You take out the water damaged stuff that can't be saved, which would be the fluffy insulation in the cavities

Gypsum board, carpet, trim, cabinets, furniture, rugs, dog beds,

You toss it out. You're left with, bare studs and probably the backside of the brick veneer from the inside.

The first thing you want top do when you're reconstructing one of these, is you've got to kill the stuff that's already bad. You want to clean, and disinfect. You power wash, then you dry and ytou do it again with some bleach and some water, and you...

Lather, rinse, repeat.

All of this demo and cleanup gets pretty labor intensive. It can be expensive, too so when you put it back together, put it back together so that it will resist absorbing water: category three or otherwise.

We need to install from the inside, a water-control, and an air-control layer. the water-control is way more important than the air control.
When water passes through your cladding or your brick, you have to drain it out the bottom back to the outside.

There are a lot of mortar droppings on the back side of the brick and you’ll never get a chance like this again to clean them out. Take special care to clean away any mortar drops at the bottom which can prevent water from draining out.

When the water runs down the back side of the brick, and gets to the bottom of the wall, we can drain it out the bottom.

So you need to clean that step in the slab well

You’re going to have a chisel, and a hammer…

And you’re going to what the chisel with the hammer.

You’re going to have an airgun to blow things away, and...

...and a shop vac to suck things up and...

...and you’re good to go.

...and we’re good to go. Where we are going, is reconstruction.

Beginning at the bottom of the wall with paint-on membrane over the bottom plate and the...

... the seat in the slab into this beautiful flashing system. That is the key element—that flashing-drainage-weep assembly at the bottom of the wall.

So now, any water that gets in, can get out easily. Time to close up the wall.

Then you line the thing with a drainage mat from the inside. If you're lucky you can run the drainage mat past the studs. If you're not-so-lucky, you run it between the studs, no problem, they both work.

This drainage mat ought to have a facing on one side of it, and that side ought to be toward the interior of the house. Because you’re going to spray polyurethane foam

2-lb density foam, NOT the ½ lb stuff

The half-pound stuff is open cell, and can absorb water.

We don’t want a sponge in there.
Stand back and admire your workmanship.

Look at that workmanship

And then you coat everything with acrylic latex paint. Why? Well, so when we have the next bl;ack water event, we're not absorbing sewage, category three stuff into the wood and the two-pound density polyurethane foam.

That should be a robust water-resistant wall assembly that you can hang some drywall on—

If you’re smart paperless gypsum board

Then you can put it back together and call it a day.

You know, the next time this happens, and it will happen again, folks, just bstrip out the gypsium board and the trim, power wash, and put it back together. It's going to be a lot easier the next time.

An escape hatch from the attic to the roof could save lives

One last thing to mention is an emergency escape hatch.

One of the things that we've learned over time is that sometimes the water gets higher than anybody expects, in a very short period of time. And there are people rescued from boats and helicopters from roofs. It would be smart to be able to get to the roof without much difficulty.

I think if we are going to be building in a floor area we shopul;d put a shylight in, and the idea being that it is an escape hatch to allow us—younsters, old people, the cat and the dog to get up on the roof so that we can be rescued.

If you're going to rebuild it, do it right, make it impervious to water and easy to clean. And add a back door to the roof in case things get out of hand.

I'd like to thank Dr. Joe for joining me today in what clocked in at about nine minutes of BS.

You get paid for what you do and what you know. If you know how much it sucks to clean up after a good, you'll want to know how to make it easy next time. If you like this podcast, please give it a this up, passive review and share in iTunes, Google Play, SoundCloud, or however you like to listen in.

—Seven minutes of BS is a production of the SGC Horizon Media Network

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