pier foundation for attached garage
Yet another hitch in my addition plans…help me out here guys! Ideas would be very helpful before I go see my engineer friend and his hourly rate clock starts ticking…
The city has a new tree protection by-law, and my planned attached garage’s foundation wall excavation would definitely sever too many deep anchor roots of a nearby tree to be acceptable to them. The tree is beautiful (though it would be two beautiful walnut logs if it were to die…) and it would be costly to remove (i.e. a boom truck and a professional crew- this is no “chainsaw and a case of beer” job… Removal would also require either an act of God, an act of city council or an act of real bastardliness to make it possible – the neighbours like the tree too and they’re very good neighbours…
The planned garage is L shaped. It adjoins the sidewall of the planned addition for 11′, and is 11′ wide with a 9′ garage door in the front gable-end wall. It then widens out to 17′ once it gets behind the house and goes back another 20′- the back part will be my shop. It’s the front 11’x11′ section that causes the trouble for the tree. The front section will have a shed roof between the addition and the exterior wall.
The arborist says we can get away with installing a pier at the front corner if we hydrovac excavate and prune any roots needed to get the pier in place. Ten feet back from there we’re outside the danger zone for the tree so we could install more piers or continue on with conventional footings and concrete block foundation walls. Footings are required to be 4′ or more below grade here due to frost heave.
For some bizarre reason, the Ontario Building Code requires a foundation under the gable wall that the garage door will basically occupy almost all of- but again, a conventional foundation wall would probably sever key anchor roots for the tree and the city won’t allow it. So- what the heck can I put under this doorway to make the building code inspector/plan reviewer types happy, without digging more than 6″ below the existing grade? Hard to design a supporting structure for something which supports NOTHING except the weight of the door…
The OBC also requires full foundations under an attached garage- a thickened perimeter slab is no good as there could be differential movement between the house and the attached garage.
If I wanted to found this garage on piers instead of on conventional foundation walls, how the heck would I do it?
What would I use to span the distance between the tops of the piers (i.e. at grade, or a couple of inches below)? Whatever it is, it would need to withstand continuous soil/sub-slab drainage layer contact. A concrete/rebar “grade beam” would seem to work, except how would you keep it from being affected by the freezing/thawing of the ground? Normally a grade beam has to go below the frost level, which I can’t do here. A structural slab founded on piers is another option but that would cost big bucks…and if I just go with a built-up beam of PT 2x12s and build the walls on top of that, I’d be asking for the thing to get saggy in 20-30 years, wouldn’t I? And ordinary steel beams would rust, even if I epoxy coated them, wouldn’t they?
I’m hoping there’s an easy solution that I’m missing here. If there is, I really would rather install this thing on piers than on a concrete block foundation wall anyway. There’s no future possibility of building over top of this garage so the block walls and strip footings are overkill. I can set forms and rebar and pour piers but would be paying somebody to lay block for me, so maybe there’s a few bucks to be saved here too.
Your help is greatly appreciated!
Replies
What a condumbdrum!
(Yes, I intended to spell it that way)
are you sure you are reading the code right in the requirement for full foundation at that wall. The obvious ( to me) solution is a reinforced grade beam over the piers to support the whole thing.
My long term concern would be what are those roots going to do to the building as they grow and enlarge?
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Piffin:
Condumbdrum indeed- this tree thing wasn't even on my radar map. We knew we'd have to be careful around the tree, but we figured it would just have to lose the roots near our garage and that would be the end of it. Walnuts are apparently fairly tolerant of root damage. The forms that I signed when I applied for the Committee of Adjustment and for my building permit didn't say anything about trees on private property in my zone. The tree by-law for the old City of Toronto was only extended to the rest of the greater city at the end of September, and I began the application process mid September...Imagine- four months with the city and still no building permit- and thousands of bucks in permit fees before I've so much as stuck a shovel in the ground!
OBC 9.35.3.1 (1) Except as permitted in this Subsection, foundations conforming to Sections 9.12 and 9.15 shall be provided for the support of carport and garage superstructures, including that portion beneath garage doors. (so I'd say that the building code section in this area is pretty clear...they want a foundation, even though there's nothing there to load it...)
The next section states that foundations for attached garages shall be approximately the same depth as the foundations of the house. So no thickened edge slab for me I'm afraid...
So- grade beams and piers sound great, but in this case I can't bury the bottom section of the beam below frost penetration since doing so would potentially sever big tree roots. The deepest I can go is about 4-6" below existing grade, and that much I have to do anyway because I have to remove the sod and topmost organic layer of soil before I construct my slab.
Here's what I'm thinking: I'll remove the 4-6" of topsoil, install the piers, then install the drainage layer (4" of crushed stone- not required by code for a garage floor, but I think it's needed to do a good job- am I wrong?), then my 2" of XPS foam- at that point I'd be back at the existing grade.
I'd extend the subslab foam under the grade beams. Then I'd set the forms and install the rebar and cast the grade beams. Since the basement of the house addition is being done with ICFs, maybe I'll just use a row of ICFs for the grade beams too, founded on the underslab foam- the foam under them would act as a buffer against frost heave, and the beams themselves would be more or less inside the conditioned space of the building.
The "grade beam" under the garage door/gable wall is basically just a doorframe, so it would be formed to end up flush with my finished slab. The grade beams for the rest would end up ~12" above the existing grade or ~6-8" above the top of my finished slab. Then I'd put in my wire mesh and PEX tubing and have somebody pour and finish the slab for me. The exterior of the grade beams and sub-slab foam would be protected by a layer of lath and parging or cement board.
My paving-stone driveway has to end up constructed above the existing grade at the garage (again to protect the tree) and I want it that way anyway for drainage. My slab would work out to be about 4-6" above existing grade with my plan, so that should be just enough for paving stones.
Will this work? If so, I can run it by my overworked engineer friend for his say so and so he can design the grade beams and the piers/footings.
As to the effect of the growing tree roots on the building, I'm not too worried. The tree is pretty big already and hardwoods grow slow up here. I doubt it'll move my piers, and I'm far enough from the tree that I doubt any surface roots will heave my grade beams- the frost has a much greater risk of doing that.
I can't see the rest of your code where it is refered to - "except as provided for in subsection..."I don't think the ICF blocks for grade beam will do you any good - just waste money. You would gain far more by having 2" XPS run horizontally at 6-8" for four feet out from the beam. That is standard in shallow frost protected foundations and it works, by keeping the earth's heat radiant in the soil just under the grde beam. Principle is that by eliminating water that can freeze with good drainage as you adressed, and by keeping temps above freezing, you make it impossible for the soil to experience frost heave.
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The rest of the subsection doesn't give me any outs.
Your shallow frost-protected foundation sounds like what you need to do around a basement walk-out. I've never seen this done, nor do I have any drawings showing this, but I have a dusty idea of what you're talking about. Is it as simple as putting in an extended layer of crushed stone sloping away from the grade beam, putting 2" XPS sheets widthwise against the grade beam, and burying them with sloped topsoil? Not an option in my case because this would cover even more root area.
I'll have to insulate the grade beam anyway as the top of them will be in the weather- they'll be buried in snow all winter. Maybe rather than buying ICF blocks for the grade beams, I'll just rip 2" XPS, reinforce it with lumber and make my own forms for the grade beams.
http://www.google.com/search?hl=en&q=FPSF&btnG=Google+Searchespecially
http://www.nahb.org/page.aspx/category/sectionID=235
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Thanks again Piffin. An interesting system which would have been attractive to me if I'd heard about it sooner- but unfortunately I don't think I'll get away with it now because of the "root" cause of my problem (i.e. the tree). I'd have to either excavate or raise the grade to a greater extent than I'm permitted to install the footings deep enough (i.e. 12-16" below current grade as they recommend). I think I'm stuck with piers because these I can sink to below frost level without damaging the root system too much- especially if the grade beams are designed to give me some flexibility about the spacing between the piers to avoid major roots if I come across them. The only question is how to keep the earth from heaving the grade beams themselves...drainage will help, but will it be enough?
In this shallow frost-protected footing design, would you put insulation sub-slab if you were using a radiant heating system like what I plan? The section given on the site you referenced showed insulation along the vertical outside of the slab, but nothing under it. Is the intent to use some of the building heat to heat the earth enough to keep the earth under the footings from freezing? Or will the earth's heat alone be sufficient if you put foam under the slab to minimize heatloss to the earth?
Depends how efficient you want the heat system to be. I use foam under the slab. The perimeter insulation wil contain earth heat so the ambient temp under slab here is about 55°F or so. From that to 70° is a fifteen defgree temp gradient that I would not want to have to constantly overcome with energy losses. I suppose you could say it is not all lost energy 'cause it is a heat sink that would let you coast for a week or so in bad weather and loss of the system. But where ground water exists, it has been shown to suck heat away by conduction and increase the design heat load required, so I always use the insulation under slab too.
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Just for the record, here's what my engineer and I came up with: 24" piers 10' on centre, supporting a 16" deep x 8" wide reinforced concrete grade beam sitting just below finished soil depth, with 2" of foam under it to cushion against frost heave. Good drainage of a well-compacted drainage layer under the slab and grade beam. The grade beam is doweled to the house foundation and thins to 8" deep under the garage door to end up flush with the top of slab. The grade beam has 4x 1/2" rebar throughout with bends at corners and stirrups 6" o.c. for three feet from each pier.
It's overkill, but it's my house and I don't mind so much. Whoever has to demolish this (after I'm in the ground myself!) will be cursing us, that's for sure...But if we do it right, the tree might outlast me...
Just hope the plan reviewers at the city building department and "urban forestry" department accept it!
Thanks again for the brainstorming help, guys.
Came across a similar condition as far as a full footer under the garage door in Buffalo, N.Y.....which we then had to core through when the HO decided to install an area drain & assosciated oil trap in the garage....I was perplexed at the code requirement....anyone have a reasonable explanation?....on second thought, we don't leave off the footers at the gable end of a house foundation so why would we on a garage.....maybe if we were trying to do for footings what was proposed for framing (I don't remember the acronym) a few years ago to reduce the amount of lumber used...anyone remember what that was called?
Depending on what seismic zone you're in, there may be a requirement for tying piers in two directions (fortunately not in mine). I suppose that the lateral tying effect produced by making the foundation a box with four walls rather than merely being two strips is beneficial, even if there's no basement and no backfill pressure to resist. But we did have to put a grade beam under the garage door and under the gable wall at the other end of the garage too.
Molten,
11x11 hunh?
So, how about you move the garage door to the shop wall and consider the 11x11 part as a driveway.
Would the city ok this?
Now, how about a pole-barn type structure over the 11x11 'driveway'? Unattached, but very close to the house and shop? 4 poles, 5 beams (1 as garage door header,) skin and roof it.
Use that brown closed cell pipe insulation as a weather gasket between the P&B and the other structures and a sacrificial board as the bottom piece of exterior sheathing. Return the interior DW to the Sacrificial with another piece of pipe insul, leaving room for the 'driveway' to heave.
Use slip flashing between the two buildings so, technically, they're not attached.
SamT
SamT- I'd considered just sacrificing the front section of the garage and turning it into a carport, but then our bicycles and such would be exposed to wind and snow in the winter (that's what the front of the garage will be for- to keep stuff like that out of my shop!). I can't add a space for that at the back because I'd have to go through the damned commitee again- and that's time and money I don't really have. The only time a car will be in there is when I'm repairing it- I'll need to start doing my own mechanic work again after the city's done taking my money in permit fees...
If I can do the whole garage with piers and grade beams and make it work, that would suit me better- the carport option is my last resort in case they reject my plan and don't give a permit for it! Thanks for the advice on how to isolate the two structures, but since I'm permitted to install a pier to found the main outside post of the carport, the whole assembly would be properly founded below frost level and I wouldn't need those slippage planes.
I wonder if this post foundation carport idea would pass muster if you used top plate beams that would carry the roof loads and suspended curtain walls with a gravel floor.
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