Crawl space foundation - Wood or CMU?

[bearzod]

My wife and I are in the process of building a new custom home in the Braes Heights neighborhood. We have retained the services of an architect and an engineer, and we have come to a crossroads.

Due to the fact that our house is in the floodplain, we will have a crawl-space foundation with flood vents. The question is, what is the best material to use between the pier & beam foundation and the bottom of the first floor. Apparently this "pony wall" can be constructed out of CMUs (concrete blocks) or pressure-treated lumber.

The argument goes like this: if the neighborhood floods again, and water floods underneath the crawlspace, you don't want any wood in the "wet" area, therefore CMU would seem to be the best material. However CMU is significantly more expensive than wood, and the frequency with which the crawlspace floods (in my opinion) should be rare, so is it worth the extra money?

Of course the engineer recommends the CMU style, but says that using PT wood is also a "viable" option. I get the sense that the architect and the engineer don't want to come out and say "wood is fine" in case there is a problem down the road, even though the risk seems low.

Does anyone out there have experience with these types of foundations and/or recommendations of what we should do?

Thanks!

[RedScare]

Houston, Floodplain, Rare. One of these words does not belong with the others.

Unless the added expense of CMU construction is so exorbitant as to make the entire project financially impossible, why take the chance? At a minimum, you can use CMU construction as a selling point, should you sell the house later. If you intend to live there for a long time, the odds of flooding are not if, but when. This is really just a variation of the argument about insurance...if the chances of needing it are rare, why get it?

BTW, PT .40 psf lumber is rated to come into contact with wet ground, and will stand up to it for years, but all wood eventually rots.

Edited August 7, 2007 by RedScare

[bearzod]

Good points all. I guess to clarify, my feeling is that when (not if) the crawlspace floods, it would drain out rather quickly through the flood vents. After all, that is what this type of foundation is designed to do.

I guess my question is: if such a system is designed to function this way, and is permitted by the building code, is there anything wrong with it? Specifically, what are the pros and cons of each method (PT wood vs. CMU)?

[musicman]

My wife and I are in the process of building a new custom home in the Braes Heights neighborhood. We have retained the services of an architect and an engineer, and we have come to a crossroads.

Due to the fact that our house is in the floodplain, we will have a crawl-space foundation with flood vents. The question is, what is the best material to use between the pier & beam foundation and the bottom of the first floor. Apparently this "pony wall" can be constructed out of CMUs (concrete blocks) or pressure-treated lumber.

so are they building an exterior foundation "wall" with vents and piers on the interior? and the beams/joists will sit on those?

when you say "between the pier and beam foundation and the bottom of the first floor" i guess that is throwing me.

[bearzod]

My understanding is that we are talking about the short wall (sometimes called the "skirt" I think) that surrounds the perimeter of the house, above the ground level but up to where the 1st floor starts. In our case, we have a 36" high crawl space, which is designed to allow water to flow in and out in case of flooding. As long ast the flood water is under that 36" mark, technically your house doesn't flood (with the exception of the garage, which is built at grade-level).

From the exterior, you can't tell if that pony wall is made up of wood or CMU, nor can you tell where it starts and stops, because typically they bring the exterior finish material all the way down to grade level. So, if it is a brick house, you end up looking at a brick wall which looks continuous, but really there is a point behind the brick where you go from crawlspace to living space. The framing for the living space is all wood, obviously, but apparently you can use PT wood our CMU for the crawl space structure.

I hope I'm explaining this right - maybe someone out there has a better way to explain it.

[musicman]

My understanding is that we are talking about the short wall (sometimes called the "skirt" I think) that surrounds the perimeter of the house, above the ground level but up to where the 1st floor starts. In our case, we have a 36" high crawl space, which is designed to allow water to flow in and out in case of flooding. As long ast the flood water is under that 36" mark, technically your house doesn't flood (with the exception of the garage, which is built at grade-level).

From the exterior, you can't tell if that pony wall is made up of wood or CMU, nor can you tell where it starts and stops, because typically they bring the exterior finish material all the way down to grade level. So, if it is a brick house, you end up looking at a brick wall which looks continuous, but really there is a point behind the brick where you go from crawlspace to living space. The framing for the living space is all wood, obviously, but apparently you can use PT wood our CMU for the crawl space structure.

I hope I'm explaining this right - maybe someone out there has a better way to explain it.

that clarifies it for me. go for the block. having any wood sit on the ground isn't a good thing. i have a concrete wall at the front of my house for the foundation. it has lasted since 1938. i just don't think wood would last near that long.

if you do use wood, it can be replaced down the line but now you're risking cracked sheetrock.

are they going to put a moisture barrier on the ground? the city will force you to insulate under the house. do you know what type of insulation they will use? the fiberglass tends to hold moisture. but i know that if under the house is fairly enclosed they recommend a moisture barrier on the ground to keep moisture out of the area, particularly if ventilation is poor.

Edited August 7, 2007 by musicman

[bearzod]

Yep, sounds like CMU is the way to go. I did a little surfing and found this website: http://www.southernpine.com/spvideo.shtml

What I've been calling a pony wall is also called a stem wall. The website has video of the construction of a house with a concrete stem wall and a wood-framed stem wall. It seems that the only advantage of the wood-framed stem wall is lower cost... at least up front. Long term costs are harder to measure!

[musicman]

sounds like an ambitious "project" if you can post some occasional pics as it progresses.

[lwood]

My wife and I are in the process of building a new custom home in the Braes Heights neighborhood. We have retained the services of an architect and an engineer, and we have come to a crossroads.

Due to the fact that our house is in the floodplain, we will have a crawl-space foundation with flood vents. The question is, what is the best material to use between the pier & beam foundation and the bottom of the first floor. Apparently this "pony wall" can be constructed out of CMUs (concrete blocks) or pressure-treated lumber.

The argument goes like this: if the neighborhood floods again, and water floods underneath the crawlspace, you don't want any wood in the "wet" area, therefore CMU would seem to be the best material. However CMU is significantly more expensive than wood, and the frequency with which the crawlspace floods (in my opinion) should be rare, so is it worth the extra money?

Of course the engineer recommends the CMU style, but says that using PT wood is also a "viable" option. I get the sense that the architect and the engineer don't want to come out and say "wood is fine" in case there is a problem down the road, even though the risk seems low.

Does anyone out there have experience with these types of foundations and/or recommendations of what we should do?

Thanks!

I was facing the same question two years ago in the Heights. A wall around the permimeter (on piers) was used going about 8-inches above grade. The treated wood was then used on top of the perimeter wall. This design was totally above my pay grade so I had a meeting with the engineer and architect and this is what they decided on, based on the soil reports.

[gwilson]

As a builder, I've built in Bellaire and had to deal with this same problem. I noticed many builders using wood and in spite of the engineers willingness to sign off, we simply weren't willing to risk the homeowners investment. When you've got a few hundred thousand pounds of home pressing down on waterlogged wood, it just didn't seem secure.

So we opted for the CMU. Granted we went a bit overboard thanks to an overzealous engineer who possibly just didn't pay attention (we built much higher than necessary because of contradictory engineering plans).

Here are a few pictures to illustrate what was done (this is the start of the build). The foundation guys took too long getting there to knockout their forms, they had a fun time doing it after the walls were built. Additionally, the vents have not been knocked out yet.

The long and short of it is, when the trusses were installed, they were much more secure and there was far more even loading. This house will be able to endure many floods without any issues. The only wall with wood even touching the ground is that of the far garage wall, and it was built out double studded with pressure treated wood. It was also framed in such a way that if a really bad flood caused any damage, the studs could be replaced, one at a time without risking any structural problems.

It is more expensive, but worth it.

[gwilson]

I should also note that you should install drains under the house before you get too far along (to go UNDER the footings) otherwise, you will end up with a sloppy mess that is impossible to do anythign else.

In addition to the drains, you will want to grade the material under the house and possibly lay over some aggregate stone so there is a non-muddy surface to get around on under the house when you need to do any repairs.

[bearzod]

Wow, great info and pictures. Thanks for the advice. In that particular house, do you know what the cost differential was between doing wood vs. CMU?

[musicman]

I should also note that you should install drains under the house before you get too far along (to go UNDER the footings) otherwise, you will end up with a sloppy mess that is impossible to do anythign else.

In addition to the drains, you will want to grade the material under the house and possibly lay over some aggregate stone so there is a non-muddy surface to get around on under the house when you need to do any repairs.

unless it floods, why would it ever be wet?

[Original Timmy Chan's]

unless it floods, why would it ever be wet?

Capillary action?

[gwilson]

unless it floods, why would it ever be wet?

These crawlspace foundations aren't meant to simply raise the house above a likely water level. Their primary purpose is to not displace water, so they are meant for water to run under the homes. For that reason, it is likely that you will end up with a lot of moisture under the house (which is a reason you should use a VERY good decking material for the first floor that can resist moisture and even lay a moisture barrier.

[gwilson]

Wow, great info and pictures. Thanks for the advice. In that particular house, do you know what the cost differential was between doing wood vs. CMU?

I was actually looking for the invoices so I could tell you exactly because I didn't remember.

The number that keeps popping in my head was around $15,000 including materials, but I could be wrong. I don't have a copy of that invoice, so that's the closest I can get for you unless I were to have someone bid it out again on a home that doesn't exist, lol.

[musicman]

These crawlspace foundations aren't meant to simply raise the house above a likely water level. Their primary purpose is to not displace water, so they are meant for water to run under the homes. For that reason, it is likely that you will end up with a lot of moisture under the house (which is a reason you should use a VERY good decking material for the first floor that can resist moisture and even lay a moisture barrier.

humidity from the ground in an enclosed space is understandable. this is where moisture barriers come into play as may a forced ventilation system.

IMO it isn't a good thing for water to run under any home. that leads to other problems like excessive settling and the dreaded termites.

Edited August 9, 2007 by musicman

[gwilson]

humidity from the ground in an enclosed space is understandable. this is where moisture barriers come into play as may a forced ventilation system.

IMO it isn't a good thing for water to run under any home. that leads to other problems like excessive settling and the dreaded termites.

Settling isn't really an issue. These are pier and beam and the house in the pictures above has beams that are 13' deep with 16" of beam on top of it. The crawlspace isn't enclosed. It has vent holes every few feet to allow water to pass through (as that is the purpose).

The city of Bellaire began requiring these types of foundations for two reasons. The primary reason is so that as the new homes take over the old ones, they displace less water when it rains/floods. This is an attempt to keep from flooding as badly as it did during Allison. Basically, to give the water somewhere else to go rather than UP. Second is to give the homes added protection from rising water.

So you see, the water HAS to flow through. Granted, that doesn't mean it will flow through during EVERY rain, but heavy storms, you can expect the ground underneath to quite wet.

As far as termites: The foundations are treated for termites and guaranteed for the lifetime of the home.

Edited August 9, 2007 by gwilson

[musicman]

Settling isn't really an issue. These are pier and beam and the house in the pictures above has beams that are 13' deep with 16" of beam on top of it. The crawlspace isn't enclosed. It has vent holes every few feet to allow water to pass through (as that is the purpose).

if you remember, take a shot of a vent hole. i'd like to see it.

[gwilson]

if you remember, take a shot of a vent hole. i'd like to see it.

the pictures I posted above were BEFORE the vent holds were knocked out. They are every fourth block.

If I ever get back by that house, I'll try to remember to snap a picture.

[musicman]

the pictures I posted above were BEFORE the vent holds were knocked out. They are every fourth block.

gotcha. i guess the confusion on my part is that the only blocks i see are all above ground level sitting on the concrete beam.

[gwilson]

gotcha. i guess the confusion on my part is that the only blocks i see are all above ground level sitting on the concrete beam.

That is correct. There is no ventilation THROUGH or UNDER the grade beam. The ventilation is a knocked out block every few feet. The water doesn't get in from under ground, just when it rains heavily would it ever make it over the grade beam. There would be some water getting in during rains anyway, simply because the vents are open as is the crawl space opening (which is on the back left of the house) and the openings in the CMU wall under the house.

When I first started building this house, I really thought it was just to elevate the house above any potential flood waters. My friends at the Bellaire planning office told me what else it was for, as well as the engineer confirming it for me. Makes sense.

[musicman]

That is correct. There is no ventilation THROUGH or UNDER the grade beam. The ventilation is a knocked out block every few feet. The water doesn't get in from under ground, just when it rains heavily would it ever make it over the grade beam.

concur with the last sentence! it would take a flooding event for water to flow thru.

[gwilson]

concur with the last sentence! it would take a flooding event for water to flow thru.

To generate any significant flow, yes. For the ground under to be moist? No.

Long and short is that it makes sense to prepare by laying down something under the house so it isn't a rotten mess if some repairs need to be made after a storm or flood.

[jm1fd]

These are pier and beam and the house in the pictures above has beams that are 13' deep with 16" of beam on top of it.

13' deep beam?!!? Is that right? Holy shat. That's a lot of digging, concrete and rebar.....must've cost a small fortune!

[gwilson]

13' deep beam?!!? Is that right? Holy shat. That's a lot of digging, concrete and rebar.....must've cost a small fortune!

I was actually wrong, it is a 36" deep by 12" wide beam. I just double checked the plans and it was on 10' piers. It didn't really take any more concrete because there is no slab. I did hear plenty of complaining because of all the excavation that was required.

DOH! I just noticed my typo.13' piers. rofl

Edited August 9, 2007 by gwilson

[jm1fd]

I was actually wrong, it is a 36" deep by 12" wide beam. I just double checked the plans and it was on 10' piers. It didn't really take any more concrete because there is no slab. I did hear plenty of complaining because of all the excavation that was required.

DOH! I just noticed my typo.13' piers. rofl

Ooooh. Okay. Much more plausible.

[gwilson]

Ooooh. Okay. Much more plausible.

Definitely.

Good catch btw.