slab or post & pier

[Hotzcatz]

I wonder if some of the "tanglefoot" sticky stuff put on the posts would stop centipedes from getting inside. I'm not even sure where to get it anymore, though, my grandpa used to use it on his trees out at the farm to keep some sort of bugs out of them.

[HADave]

absolutely, positively, without a doubt. This is the greatest stuff, depending on what else gets stuck, a bug bridge could be made for the ole wise centipede.

[mella l]

Bump.

Anyone who has built in the last year or so perhaps might give an update.

mella l

Paris London New York PUNA

[MarkP]

Interesting I never saw this topic before but I have posted on other similar topics.

I have the opposite reaction from leilaniguy regarding the crack across the property and what would happen to a slab. A slab would be much better equipped to deal with something like that than individual piers. I have stated in the past that the best option to protect against such ground movement would be to build on a slab, just make it a very good slab. The slab might cost twice as much as a "regular" slab. So be it. Do what it takes. Spend what it takes. After you do your entire house, if it settles, will settle as a unit and can be jacked back up as a unit. Pretend you are building a bridge. It baffles me how little effort and resources are typically put into the most important part of the house, the foundation.

All slabs crack to some degree. Unless the steel reinforcement is pre-stressed, the steel is just there to hold the pieces together. It can not be otherwise. Concrete has a very small strain at failure. Steel by comparison has a very large strain at failure and is hardly carrying any load at the amount of deflection that will break the concrete, after which the steel will carry all the load.

Hotcatz's story gets to the point. People are used to doing things a certain way and even builders who have been building houses for years may not really know the theory of something like how different slab designs affect the outcome unless they have personally tried different designs AND gone back to review the results over time. This is not something that many builders have done. They just build the same thin slab over and over.

I am not saying you can't get good results with pier and beam but the old plantation style houses were built that way because it was CHEAP. There are so many of them around because the plantations ran everything and it was the cheapest way they could provide housing, not because it was the most durable system.

[MarkP]

If longevity and quality of construction were your defining concerns you would have a very robust slab to tie everything together. After that, if you wanted the benefits of pier and beam with an elevated floor, you would build it on top of this slab. What I am hearing is a lot of people talking like these are mutually exclusive or maybe there is some special reason why people don't do both. There is - cost. People are presenting technical reasons in favor of pier and beam and perhaps without realizing it are implying that these are reasons against a slab. In fact they are independent issues.

[MarkP]

With regard to centipedes I spoke with someone who as a child lived in a house that had an extensive dry-stacked stone wall. He said thay had a lot of centipedes. The father then grouted between the rocks, not as a structural measure but to deny the centipedes access. After that there were noticeably fewer centipedes. They must like the following things:
1. Rock. Don't know why but maybe it holds heat better.
2. Cracks to hide in.
3. Cracks close to the ground. I figure this because otherwise why would elevated houses have fewer centipedes? It seems logical that they would not like to travel either horizontally or vertically where they are exposed. I have also heard that a CMU stemwall a few feet high with wood construction above allows you to detect termites before they do damage because they don't like to travel exposed and so they build mud tunnels that give them away.

The benefit of building with individual piers where the ground is not flat enough for a slab is very real, can't argue against that. I only mean to say that there are benefits to a slab that are independent of whether the rest of the house is raised and which in my opinion you can't reproduce without the slab.

[StillHope]

Mark,is the concrete under a house with P&P the same as "slab"?
(Learning the terminology).

[MarkP]

StillHope:

By slab I mean a continuous slab of concrete large enough to tie the whole house together and support it and with enough rebar and also thick enough to not crack unacceptably. If you are looking under your house and seeing concrete but don't know how thick it is or how much rebar there is then I don't think you can claim the full benefits of having built on a slab.

There are some parts of the country that have really expansive soil where pre-stressed steel is installed in the slabs. In these cases the soil expands and contracts several inches over the course of the seasons. The foundation must literally be able to be supported at odd random points without cracking. These slabs are highly engineered to hold up while the ground drops away beneath them. The pre-stressing allows the slabs to do this without cracking and with a minimum thickness. There are big hydraulic rams that are used to stress the steel and expertise is required so usually this is too expensive to do on a single house. Developers are able to take advantage of prestressing when they are doing several houses. Otherwise you can make the slab thicker and put in lots of closely spaced rebar. There will be cracks but they will be many small cracks that are too small for bugs to get through.

I think that when people think about slabs they are picturing older thinner slabs where the effort was not made to get everything you could out of the slab. A badly cracked slab certainly sticks in your mind. Decent slabs that are holding together OK are out of sight and out of mind. I have also heard repeatedly that an amateur built slab is easy to screw up so there will be a certain percentage of crummy slabs out there.

I have been reading about traditional techniques of building stone houses out of rubble stone and timber in seismically active areas of India and Pakistan and there are some remarkable houses that have stood through hundreds of years of earthquakes. They are usually on built-up stone foundations a few feet high. I think this forms a sort of base-isolation, but at any rate a tremendous amount of effort is put into the foundation before they even get to the rest of the house. I'm no expert of course but looking at it that way I can see that if you choose building on piers because then you can perch the house on a rough slope where you can't put an extensive foundation, well that skips step #1 in the traditional earthquake resistant house construction book.

[StillHope]

Thank you,Mark!
I learned a lot.

[Dennis]

Just how big are these guys? We have centipedes in Ohio but they are really small.