Pier Foundation – Joist Underside Detail
I am in the planning phase of a barn/ADU on piers in CZ 6 (Coastal Maine)
Looking through the code, I believe I need 18″ of clear space below the building. The building will be about 20×30, and in my imagination will have 30′ beams on piers and joists spanning the 20′ gap. I want to fill the joist cavities with dense pack insulation to hit code+ for floor R-value. I have seen examples of pier buildings with Zip System on the bottom, but the building was ~8 feet off the ground.
Questions:
1. Does the bottom of the building need to be clad in marine plywood? or OSB? or will housewrap suffice? (I have a few ideas of how to construct this, but right now I want to figure out the requirements.)
2. Does the ground below need to be covered in a vapor barrier?
3. Should I just hire Michael Maines to figure this out for me?
Thanks!!
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Replies
Hi Tim- Fellow Mainer here. This is my non-expert $.02/idea:
I would make sure there is a decent grade to shed water away from the building footprint including underneath the building. I would install some thicker poly and stone under the building, extending out 6-8 feet.
I would use sheathing on the bottom of the framed floor. Maybe Advantech. Taped/ Air Sealed. I would also consider some sort of critter proofing- 316 stainless mesh?
Maybe someone already has a process, but this is what I might try: After the piers are complete and floor is framed, use several sheets of plywood as temporary/movable ground cover under the framed floor. Make a little wheeled cart out of 2x4s to load the underfloor sheathing on to, such that it would carry sheet goods a few inches below the bottom of the framed floor. Make/buy a creeper for work under the floor in a comfortable position. Wheel yourself and the sheathing under the building. Maybe a quick test run would tell you whether a 24" height or something else would work best. I actually think that this would provide a better working height/body position than some sort of "in-between" height. I have a barn that is ~3-5ft off the ground, murder on the back to work under it.
Let us know what you end up doing. Good luck!
EDIT- Perhaps a better way of managing the underfloor sheathing: Skip the sheet goods cart. Go under the building on your creeper, screw a few temporary "t-tracks" under the building. These tracks would basically be wood strips that are maybe a 1/4" thicker than your sheathing with a furring strip over the top to form a track that you could load sheathing into from the exterior of the floor frame. Slide several sheets in and the roll yourself under the floor on the creeper and the sheets would almost be fully in position to nail off. Nail one row then adjust the tracks.
The big question is whether the assembly can dry downward, 18" is kind of marginal, if it's too close to the ground the bottom of the assembly is going to be damp all of the time and keep it from drying. If it can dry down you want it to dry down, which means a vapor block on the warm side and vapor open construction on the cold side. If it can't dry down it has to dry up, which means vapor open on the warm side and some sort of impermeable insulation on the cold side to prevent condensation and block the flow of vapor upward.
In any case you want the assembly to be thoroughly air tight. You also want the bottom to be rodent proof. Whether the bottom has to be weatherproof depends on how protected from the weather it is and how moist the ground below is.
In terms of constructability, it's much easier to do a floor like this after the roof is on, because then you don't have to worry about keeping it dry. So is there a way to build this building where you build the walls first, then the roof, then the floor? So the exterior walls rest on the beams, and the rim joists hang off of the wall?
Are you running water through the floor system?
If you want a simple, easy to build well insulated floor system that can be air and vapor sealed easily, put in your beams, put Advantech on top of the beams, then put a 12 1/4" thick Sip panel on top of the Advantech. Screw down through the SIP panels to attach them to the beams.
Since SIP panels are both structure and insulation all in one you shouldn't need joists between the beams- the panels will come 8' wide and up to whatever length you want and will have 2x material around the outer edges for strength.
Tape your SIP seams on top and then build your exterior and interior walls on top of the SIP panels.
You can then build a raised floor system out of 2x6.s and OSB floor sheathing within the walls if you're planning on running plumbing lines and waste pipes.
rockies63,
What supports the Advantech?
You tack 2x4's in place between the beams to hold the sheathing level and then after the SIP panels are set in place, go underneath and screw the Advantech to the underside of the panels.
One trick is to put a short piece of sheathing tape (have it overhang the sides of the beams by about 6 inches) sticky side up on top of the beam wherever two Advantech panel edges meet and set the panels down on top of it. Then when all the SIP panels are in place and secured go underneath and tape seal the rest of the Advantech panel edges.
After that you can apply a finish material to the underside (which in the event of wildfires could be something as simple as corrugated metal).
rockies63,
Ah, got it. I suppose y0u could also just clad the SIPs before you laid them on the beams.
OOOOOOOH, where were you last year? DRAT DRAT DRAT! That would have saved some time lol!
rockies63,
I vaguely remember someone here saying there is a SIPs supplier that uses either Zip or Advantech as their skins?
tim_william,
I'd suggest three rows of piers, and each row consisting of at least three piers. Otherwise your beams and joists will be very deep, meaning the floor will be even further above grade.
Working from the underside is awful to do, and yields a poor finished product. I've done quite a few of these floors, and tried various methods to make it easier - building the floor in sections upside down and flipping them works, but is finicky.
The one I used on a 14'x28' outbuilding this summer has my vote from now on. All the work is done from above, your lower level of plywood protects the foam, and the lower framing only adds 3" to the height. The link shows the floor construction, and also includes a clever way to bring your services up while protecting them from freezing. https://www.southmountain.com/wp-content/uploads/2019/02/19-Small-Home-Harmony-Article-PDF.pdf
As DC said: the big problem, with insulated floors on piers is they rely on good weather to install, or the insulation gets ruined. Definitely grade the area underneath towards the perimeter, put down poly, and cover it with 3/4'" clear-crushed gravel.
Agree that more, closer, piers work better than fewer, wider.
I like that floor detail. It looks like you could build the walls first and then put the floor in after the roof is on if you didn't do the double-stud walls. Or maybe do the inner wall after the floor?
DC,
I've tried sketching details of how you could frame walls and then put down the sub-floor. It's fairly easy in one direction, but the problem I bump up against is what to do with the exterior walls parallel to the joists. Someone brighter than me needs to set their minds to it.
Yeah, there's also the problem that the framers need a flat platform to build the walls on. Although I think if you had to you could use the beams, especially if you keep the joist span less than the wall height.
I'm thinking on the gable ends you'd put the rim joists in and build them like headers, two 2x with 1/2" plywood sandwiched in between so they're wide enough to hold the wall. But it's not a well-formed idea.
I tried playing around with sketches. You know how normally you do the eave end walls first, and then the gable ends go between them? I think you have to do it in the other order. If the gable ends run the full width of the building you can build up their rim joists as headers and have them rest fully on the beams at both ends.
You'll probably end up with the framers, plumbers, electricians and drywallers all hating you.
Malcolm- thanks for the link. Very clever process
steve41,
I felt like going to church and lighting a candle in thanks when I first read it. Anything that eliminates working from the bottom is a huge improvement.
Haha, I just read your last question. I am currently booked out about 18 months if you'd like to get on my wait list ;-)
The relevant code section is R317.1.1: https://codes.iccsafe.org/content/IRC2015/chapter-3-building-planning#IRC2015_Pt03_Ch03_SecR317.1. Having crawled through several spaces less than 18" high, it's also just a good idea for accessibility.
Coincidentally, just last week we finally got the underside of the addition in the lead image here covered: https://www.greenbuildingadvisor.com/article/all-about-helical-piles. It's more than 18" and we used Zip sheathing. We used 1/2" because I wanted the reddish brown color, but the 7/16" green version would have been fine. For two years it only had Typar covering dense-packed cellulose, which was not pest-resistant to be a permanent solution, and it's very vapor-open, which is not what you want close to grade. Zip sheathing is less than 1 perm which allows some downward drying but should work well at keeping moisture from migrating up from the ground.
The first new house I helped build, 30 years ago, was on concrete piers and we used Celotex, an asphalt-impregnated wood fiberboard. It was easy to work with and pest-resistant but not an air barrier.
I have worked on other houses on piers that had conventional OSB on the underside. Exposure to the elements swelled the OSB, and where it was very close to grade there was some fungal growth, so if you're close to grade I would place a heavy-duty vapor retarder on the ground.
Michael,
Clearance underneath these open pier foundations is something I think is perhaps situation dependant. If the lower structure is built from pt materials, the ground has a robust protected VB, and unlike a crawlspace there are no services, then is providing adequate height for access really necessary?
For the outbuildings I've used piers on I've provided 8". That is enough to visually inspect the space to make sure no pests have set up shop, and get a rake under to clear debris. It also provides good ventilation. Of course that demands on the underside being made of a material that can be relied on to protect the floor system, and that won't need any maintenance over the life of the building.
My head is presently sporting four large lumps from working in a crawlspace which had a beam at 12".
Malcolm, that's what the IRC says--if the framing is closer than 18" it just needs to be rot-resistant. As far as I know there are no rules for what the height must be for access reasons.
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Most of the houses on the Atlantic coast south of Connecticut are built on pilings or piers. Most are much higher up due to flood elevation. You didn't say from your post whether that was the issue in your case but if so, I would suggest you add a bit of additional "free board" above base flood elevation if all else allows it. Base flood elevations are based on previous storm surges, but future storm surges could be worse. No matter how tight you make the underside, if you get a storm surge that engulfs it, you will have to rip it out, so adding an extra foot or two of free board above base flood elevation makes sense if other considerations (and your knees walking up additional steps) allow it.
Others on this thread have given good ideas on how to install sheet goods so close to the ground. As for what type of sheet good to use, builders around here often use sheetrock or hardieboard sheets to seal the underside and carefully tape them, then paint with a high gloss exterior paint.......but again, they are higher above ground. For your situation, I would not use regular sheetrock due to the paper face. You might consider the hardieboard sheets or fiberglass faced exterior sheetrock (e.g., Dense Armor Plus) of the type commonly used on the exterior of commercial buildings. Then finish with fiberglass exterior tape and a true waterproofing coating. Put the first coats on before install, then tape and do a final coat. Make sure it is a waterproofing compound or coating that can be applied up side down (wont run).
Thanks for all the thoughtful replies. All the ideas about cladding the bottom are great. Malcolm that article is solid gold! I've been kicking around this idea for a while and we finally are in a place to execute.
Some more detail: I plan to build this myself. Partly out of necessity since builders are booked out, and partly out of mid life crisis. I am planning a simple rectangle with a gable roof. The grade is sloped so it drains well towards what would be the gable end of the building. I am not worried about floor height, we'll build stairs (and plan for a ramp). For plumbing, I'll have to build an insulated chase or mini basement to accommodate the supply and drain.
I want to use as much local material as possible and as much natural material as possible. Here in Maine there is no shortage of pine, Advantech is produced here, and now we have TimberHP making wood fiber insulation.
My fantasy assembly is to use 20' I Joists to span the 2 beams. I-joists are light and solo-friendly, and have a flange I can take advantage of.
From above, I would lay a 1" rigid board on the bottom flange of the I-joist, then have the loose fill blown on top of that. No need for ply on the bottom. My concerns are the weather resistance of the wood used on the I-joist and whether a rigid wood fiber insulation board is an appropriate layer to leave exposed on the bottom. Also this is obviously not air tight, but I hope to lean on an interior barrier to to the heavy lifting for air sealing.
If I decide to commit to local lumber, I would have to use 3 beams and lap the joists on the center beam and add my own flanges to the bottom. I need R-30 for code minimum, which is 9" of loose fill, so a 2x10 would work.
One concern I have about building a structure with a solid bottom is what happens when a pipe bursts? Will the floor fill like a bathtub?
Thanks!
Tim, this sounds like a fun project.
Some back-of-the envelope calculations: I assume in Maine you have a 50 pound per square foot snow load. With a 10 pound dead load for the floor, 15 pounds for the roof, 40 pound live load for the floor and the snow load you need to support 115 pound per square foot. That's 1150 pounds per linear foot for the beams along the long walls, and 69,000 pounds for the whole building.
According to the beam calculator at https://learnframing.com/wood-beam-calculator/ , three 2x12's can span 7' 7" under that load so you'd need four segments/five piers for each side. (Four 2x12 can only span 8' 5" which still has you at four segments so it doesn't buy you anything.)
With ten piers, each has to hold 6900 pounds. Assuming your soil can hold 1500 pounds per square foot, that's 4.6 square feet per pier. That's either a square 26" on a side or a circle 33" in diameter. You probably have to go down 48" in Maine to get under the frostline, so that's a big hole. Ten big holes, actually.
One thing to consider instead would be a more conventional crawlspace with a poured or block foundation. To support 1150 pounds per linear foot your footer only needs to be 9-1/4" wide. You basically end up moving the same amount of dirt, but a straight narrow line is easier to dig with an excavator.
Another thing to consider would be a slab on grade, or even one of the slab-less on-grade foundations currently featured on the GBA home page.
Thanks! I shpuld clarify that I plan to use helical piers. I had them installed for a deck and I was impressed how quickly they were installed. I really want to avoid a concrete foundation. I know a slab would simplify this project immensely, but the disruption involved (days vs hours) and the carbon footprint really turns me off.
Once I have initial drawings I plan to involve an engineer to get specs for my pier plan. Fortunately there’s a top notch firm around the corner from me. If it ends up requiring an absurd number of piers I may reconsider.
Helicals should be able to hold 6900 pounds each, you'll still need the same beam spacing so ten helicals total. I did some a couple of years ago and paid a little over $1000 each.
Your engineer can advise further but they may need some sort of lateral support. Mine were embedded in a 18x18 block of concrete that ran the width of the building. It was reinforced with rebar that was tied to the heads of the piers so it served the purpose of the beam. My piers could hold up to 20,000 pounds which allows for greater spacing and fewer piers.
Dang, when I had piers installed 3 years ago they were less than half that. I love the simplicity of piers vs. a slab or wood foundation that requires so much disruptive site work. I never thought of embedding them in concrete to increase capacity, thanks for mentioning that.
I received an estimate for helical piers a few years ago, but did not go that route. At that time they were $350-400 per pier.
Out of curiosity I just looked at a few helical pier installer sites. One thing that struck me is the short design life that was stated by one of them: 50 years. Soil conditions would be a factor of course. Just thought I would mention it as it could be concern depending on how coastal your site is.
What if you tried precast concrete piers like these, or similar?
https://www.ez-crete.com/products/solid-precast-footings/