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Building on piers

| Posted in Construction Techniques on May 17, 2003 07:02am

I recently purchased a small property with cottage on the eastern shore of the Cheasapeake Bay.  After a great deal of contimplation; I’ve decided to demo the cottage & rebuild from the ground up.

The existing structure – approx. 30′ x 30′ – was built on block piers, on concrete footings, spaced about 10′ apart (16 piers total). The piers vary in hieght from 18″ to 24″. No rat slab. This cottage was built in 1946, before P.T. lumber was avail. Get the picture? I’ve dug test pits to varify solid footings & everything seems to be dead level & solid.

If I am to build a new foundation, code requires a new well & septic system as well as new setback requirements. The well was installed in 1998.

I am a general contractor with a good deal of experience in new construction & renovations…  but I have never built on piers.

Anyone with any feedback on pier construction would be greatly appreciated.

Thanks in advance.

 

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  1. skids | May 17, 2003 08:39pm | #1

    what about pouring between existing piers on exterior for perimeter foundation? would that eliminate need for new setbacks etc. most of the load on the foundation is at perimeter and all the old barns etc. that i have worked on that had problems had them on the perimeter, usually the interior is fine. so basically what i am saying is if you can modify, or reinforce existing foundation to improve it without calling it "new" then you avoid the setback and septic stuff. its like the old remodel instead of new permit because the new construction permit is so much more expensive so you leave a closet wall standing while the rest of the structure is totally new. it should be possible in one way or another to retrofit existing without having to move footprint. does that help at all? if you have to build on just the piers my main concern would be to tie them all in with sheer walls to spread the load and then its just build as normal

    1. Kerns | May 18, 2003 01:38am | #3

      Thanks for the input Skids. My only concern with a perimeter foundation is that it might trap moisture below the structure. To avoid a damp crawlspace I'd have to pour a rat slab (a 2" thick concrete pad under the crawl space) & poly moister barrier wich would need a 36" min. clearance to the bottom of the floor joists. I'm sure at this point the inspector might start raising points about the definition of a "new" foundation.

      Why do they call it a "Rat slab" anyway? 

      1. skids | May 18, 2003 03:42am | #5

        andy out here in california it is rare to pour a rat slab to prevent moisture, we just have to install tons of foundation vents to allow air to circulate. the general code for perimiter foundation is 6" below native for depth of excavation but to limit water intrusion or for other circumstances you can certainly go deeper. and if you still have moisture intrusion french drain at drip edge. but either way this retrofit can be undertaken at any time as long as you have stable level piers to start from. IMO they call it a rat slab because it is what you pour first after excavation and then build up from that surface for the structural concrete. anyways before that first rough pour is dry there will be some tracks in it and it aint from no bird. the ones i saw in san francisco near the embarcadero must have been the size of a house cat. wharf rats get pretty big.

        1. Kerns | May 18, 2003 05:37am | #7

          Thanks again Skids

          I think I'll make an anonymous call to the building dept. & see if this retrofit can fly without being considered a new foundation. With a 36" frost line it's probably a little more intrusive than the west coast version. 

  2. TrimButcher | May 18, 2003 12:33am | #2

    I'm a Canuck, so can't speak to your building codes (although northeastern U.S. and Cdn. codes aren't all that different, from what I've read on this forum).

    However, literally hundreds of thousands of cottages up here on piers (block/grout with footing or poured sonotube).  We're talking a good amount of snow and nasty freeze-thaws.  It's only in the last 10 - 15 years, with escalating land prices, that cottages started getting basement foundations.

    Think of building on piers as the same as building a deck.  Decks got piers (if it helps, omit the 4x4 post), then built-up beams spanning the piers, then joists/rims on top (or Simpson strong-tied).  Cottage/house no different.  Our building code basically says decks have to have the same foundation support as a house, ie. I should be able to build a floor or two on top of any deck foundation I build.

    Check with your local building inspector whether PT wood has to be used for the beams, you might only need it contacting the concrete (since the underside of the cottage shouldn't see any moisture, and has plenty of air movement to dry it if it did - assuming you're at least a foot or two off the ground).

    The only other biggie is insulating the exposed underside of the cottage.  That's one place where that spray-on foam with the built-in vapour barrier really pays off.  I haven't heard yet whether it stops the porcupines from chewing the wood, though.

    If those piers are solid, no settling in the old cottage, seems like they're worth keeping. 

    Sounds like a beautiful property...enjoy!

    Regards,

    Tim Ruttan

    1. Kerns | May 18, 2003 02:16am | #4

      Thanks Tim.

      Do you think that I could expect a relatively long floor life with this type of construction.

      I'd probably use pressure treated 2x12" & 2x10" floor framing with 3/4" T&G plywood sub-floor (P.T.)

      I neglected to mention that this property was built by my grandfather & as you can imagine I'm having a hard time tearing down what his hands have crafted. I wouldn't want to put my grandchildren thru this same thing in another 60 years. It has been emotionally difficult. At least this way The new construction would be built on his foundation.

      1. Ronbaby | May 18, 2003 03:51am | #6

        Hi Andy, sounds like Im just south of you, In Va Bch. Down here all houses except for slab houses are built on exactly what you got, block piers. Pressure treated first floor system used only on the beach front. IMO, Id leave what you got and spend them somewhere else. 

        1. Kerns | May 18, 2003 05:41am | #8

          Ronald

          Are any of these pier homes of recent vintage?

          1. Ronbaby | May 18, 2003 05:20pm | #11

            All new homes. I did do a renovate last year, semi ocean front (other side of street). House was on block piers, no PT, and the floor system was stilll soild.

      2. TrimButcher | May 18, 2003 06:38am | #9

        I can only speak from personal experience of doing some cottage building, not professional engineering knowledge as to lifespan of floor, but that's never stopped me before... {idiot grin}  Now you know why my messages are so long.

        I don't know how much moisture/humidity you're going to get on Chesapeake, or how close you are to the water.  You said you were on the east side, so that's the sheltered (lee) side?  Or are the prevailing winds pounding you?  You mentioned setback, though, so I expect you're pretty close? 

        If you took an ordinary SPF 2x12, covered it over but let the air circulate around it, how long would it last?  There's definitely thousands of cottages up here, on piers, within 50' of (fresh) water, built 50, 60, 80 years ago and the unpainted, untreated  beams & joists are still sound.  The 24'x40' cottage my parents had built in early 70's (30 years...time flies!) is block piers (ranged from 1' to 6' above ground) with plain SPF 2x10 built-up beams, exterior ply subfloor, facing the prevailing wind, about 80 feet onshore on a big lake, fibreglass insul between joists, and the wood and subfloor are still tops.  Not a stick of PT, and it'll last as long as a city house.  And it sure wasn't old-growth SPF 2x10's.

        How have the wood beams and subfloor (pine T&G?) faired that your Grandad placed?  You didn't specifically say, but it doesn't sound good.  Any particular areas that are rotted, or is everything going bad?  Any fungus, mildew, or other enviromental issues?  I am surprised if it's all rotting, but I don't have experience with ocean humidity.   I'd be tempted to add a block or three to the existing piers to get more air circulation underneath (and raising would be helpful for when you need to get under the cottage), but I'm no engineer/designer, so I don't know whether my "commonsense" is actually sensible.  They put houses on stilts on tropical beaches, though (I know that's for storm surges, just joking).  Now some people don't like the look of all that much exposed pier, might seem hillbilly-ish, but I grew up cottaging...the sight of tall piers makes me nostalgic for childhood...except for lugging 16' drywall up the steep cottage hill when I was 10 years old. 

        Any ground water problems?  I suspect not, if your piers haven't budged, but I've seen rot caused by it underneath a cottage which you'd swear had enough ventilation.  What are the winters like, cold and damp/rainy?  Up here it's many feet of snow but the winter air is dry, so the wood gets as dry as a fossilized bone in a desert as long as the snow can't pile close enough to touch.  If Chesapeake is wet basically year-round, yeah, I could see using PT wood.  How well does the current cottage handle roof run-off?  Seen problems with that, too, rain water getting/splashing where it shouldn't and mould covering cedar walls that didn't have time to dry out between rainfalls.  Gutters and a decent-sized overhang are musts, I expect.

        Take a look at http://www.icynene.com   Insulating and vapour barrier application is never easy when working with piers.  Seems to me spray-foaming the entire underside would solve any/most/all rot problems under the cottage.  Perhaps you've used it in a garage ceiling that has living space above, or in the rim/joist/sill area of a basement?  More expensive, but seems like just about a perfect product to me: excellent insulation, built-in vapour barrier, sticks to anything and can get just about anywhere.

        Most importantly...talk to the local/closest building inspector, and local (piered) neighbors.  See how their buildings are surviving and why/why not.  Except for my modest amount of pier experience, I'm sure you know all this stuff better than me, so my questions are essentially rhetorical; sorry if the babble seems endless.

        A rat slab (that's a new term to me), adding walls between piers, complete tear down, etc., just doesn't make sense to me.  And yeah, I can imagine what you're feeling about destroying any of Grandad's work.  If I said they don't make'em like that anymore, I wouldn't be talking about the cottage.  Wars and the depression forged a unique generation.

        Regards,

        Tim

  3. jgjgjgnn | May 18, 2003 04:56pm | #10

    2 ideas:

    1//Consult a local Structural Engineer. Probably wouldn't cost a thing to see if they could help.

    2//Go right to the Building Insp. and ask for some of their time to discuss. My experience (Colo) is that they enjoy sharing their knowledge. The knowledge is sometimes good and provides alternative ways to get the job done. Sometimes the "knowledge" is just an acknowledgement that their suggested way of doing something would be accepted as correct.

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