Is there a reason to use forming tubes for piers?
I’d think that an augured hole would, itself, be a fine form and space between the concrete and the dirt would be tight. The tube looks like it’d create an air gap that’d encourage wiggling. It’d be difficult to pack the dirt tightly back into that air gap.
(My daughter-in-law needs a new deck and I’m watching lots of DIY network).
First let me say: I don’t know the answer, but I’ve been wondering this myself, and look forward to someone who is a professional contractor answering the question. But here’s a few non-professional observations…
I always thought Sonotubes were to minimize the amount of concrete required, and this does indeed seem to be a feature. But they apparently serve two other purposes: in colder climates the irregular lumps of concrete that would otherwise be poured directly into the ground would be subject to potential ground shifts above the frost line, and in certain geographic areas it’s desirable and sometimes mandatory due to code to have the concrete six inches above grade, which the tubes facilitate. My deck, which was built by the builders as part of the house, has such Sonotube-contained concrete above grade. Apparently this is much more important for load-bearing structures like decks than for fence posts. Though I personally have built both fences and decks without the tubes and not had problems, ensuring that the holes were well below the frost line.
Depends on soil conditions. If your ground is solid, and the local building inspector O.K’s such, it might work. Of course some places you might not need to tell the gov’t what they don’t need to know. In some cases it may be necessary to pour a footing in the bottom of the hole, which would mean a bigger space than just for the tube. Refilling the space around the post is not difficult. Just use a bar or piece of 2by4 to tamp the fill as you shovel it in. Or, water from a garden hose as you fill the space will settle every thing down. As a side issue, make sure you understand how to attach the deck to the house, specially if it is high.
The piers will be stronger and last longer if poured in a form. The rough interior surface of a hole will greatly increase the surface area exposed to water. A smooth finish prevents cracks from forming also. Local codes may require their use. Another aspect is the inspection. If you fill a hole in the ground with concrete a building inspector can’t see how far down the hole is or if it had sufficient diameter all the way down. It does save money compared to using a square form, but not much, concrete is cheap, and the tubes are expensive for what they are, and they aren’t resusable. but the result is a structurally strong pier with less concrete than a square form would require.
I wouldn’t be surprised if there was lobbying by the tube manufactures to make the tubes a building code requirement. But since it makes an inspector’s job easier it wouldn’t have taken a lot of lobbying.
All the building codes I’ve found so far just refer to the national standard codes.
What I’ve yet to find is whether I need the flared bottom on the pier. Is there a standard for this?
(Denver, BTW, official frost line is 36" down).
I assume that a flared bottom in essense is a footing. I haven’t seen flared piers around these parts, or footings for deck piers. How big is the deck and how many piers are there.
Also, 36"? Around here it’s 48" and we get down to 0F occasionally through the winter.
ETA: Why did I think you were in Denver?
Yes the use of Sonotube will save on the waste factor of concrete and that can be substantial in certain situations.
In other situations it is more cost effective to drill, or auger, a post hole and fill that with concrete. This is done a lot and in many situations.
The decision between the two can reside on a lot of factors. Perhaps the most important one is the soil conditions. Another is what equipment is available you can work with.
There are some rigs, and the name of them escapes me at the moment, that will pump in the concrete as the auger is pulled out. More used in heavy construction that residential.
Many times the decision is made for you by the specifications of the job.
There is a specific flared bottom for sonotubes. One brand is called bigfoot. Link here
Yeah, it’s a wide base you put on the bottom of the tube. Dig a 2-foot dia hole, drop the wide base in the bottom with a sonotube coming up out of the center. Like an upside-down funnel.
I just found this, “Heavy clay soils don’t drain well, so they tend to have more frost heave problems than sandy, well-drained ones. But even if footings are deep enough, ice lenses can latch onto the rough surfaces of wood and concrete and lift footings and posts from the side. That’s why concrete piers poured in waxed cardboard tubes and smooth wooden posts work well for below-grade support.”
Dek Block piers are an alternative. Check with your local Lowes to see if they are allowed by code in your area. Home Depot may have them too.
here is how they are installed. the board slips into the slot.
or your corner post 4x4 goes into the center.
http://www.rblank.com/wp-content/uploads/2011/10/IMG_1965.jpg
I’m sure there is a height restriction for using the pier blocks. IIRC the deck shouldn’t be higher than the pier.
you shouldn’t put a 3ft 4x4 corner post into the pier block. It wouldn’t be stable or safe build three ft off the ground with these.
Most of the time that I have used sono tubes it has been in an open excavation for a new home, and it is specified on the building plan. This is usually to support concrete steps or for a structural post to support an overhang. You are pouring footings and foundation wall anyway, and it probably has to be inspected, so its a no brainer. It is very important to brace the living hell out of it and have a spotter directing during backfilling.
For a deck or porch the advantage is that you can use a sono tube to extend the pier out of ground which is good (if somewhat over rated) practice.
If this is for a typical deck I would not worry about it too much. Auger your holes and pour the concrete. You can bell out the bottom with a spade. Build simple box forms to get some ground clearance. Even better just use some 6x6 PWF timbers and pack them in with gravel and don’t bother with the concrete at all. You size them rough and trim them to level when you do your layout.
If it is a small you can even build a floating deck and put it on concrete deck blocks on gravel base (just make it independent of the house). Its just a deck, big deal if it heaves a bit for a few weeks every spring, no one worries about their stone patio heaving.
Edit: I see someone mentioned the blocks while I was typing (slowly). It is best practice to remove all top soil down to clay and build up your base with gravel rather than just placing the blocks directly on lawn as in the picture. Its actually good practice to remove and replace the top soil under any deck.