Frost protected shallow foundations

[Bill Houghton]

I did a search and didn't come up with too much here.  Has anyone in cold climate with clay type soil built with a frost protected shallow foundation?  There are some excellenet resourses on line about FPSF. http://www.cs.arizona.edu/people/jcropper/desguide.pdf

I am planning on using a 2' treated wood foundation, about 16" buried with a slab on grade floor for my project.  I will overbuild a bit with the insulation, like go thicker and out farther, just in case.  I am not sure about out local building codes but I am also not sure I am going to ask.   

Bill in the U.P.

[tc-vt]

Bill,

I built my 20x30 1-1/2 story in northeast Vermont on a frost protected shallow foundation (aka FPSF)in 2004-2005.  The perimeter concrete footing is two feet tall and 18 inches wide.  Then, inside the footing wall was filled with compacted gravel and then a slab was poured.  No interior footings were needed for weight brearing because I used I-joists and roof trusses.  I have not seen any indication of frost movement of any kind.  If I remember correctly I used the design specs for a semi-heated building in my climate.

Tom

[JRR]

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http://countryplans.com/smf/index.php?topic=79.0

[Bill Houghton]

Tom,

Thanks for the reply.  Do you keep your place heated all winter? Our plan is to build a cottage for weekend use.  It may not be heated 24/7.

JRR,

Thanks for the link.  Read those before I posted.  Lots of great info.  I even printed up the manual in case I apply for a build permit I'd have a copy for the officio's.

Bill in the U.P.

[tc-vt]

Bill,

I found the thread with some pictures of my project:

http://countryplans.com/smf/index.php?topic=2095.0

I started building in August 2004 and by November got as far as framing with sheathing and a roof with no insulation.  It sat that winter with no heat without any problems.  I live in the house full now time but heat only when I am home so the average temperature is 50-60 degrees.  The inside has never been below freezing since I moved into the house in 2005. 

The other part of my design was to omit insulation under the floor slab.  I did this for two reasons: first, the supposed benefit of more thermal mass for the radiant heat which I put in the slab and second, for increased heat loss from the building which is part of the FPSF system of protecting the footings from frost.  In hindsight, I would rather have insulation so the radiant heat would react faster.  I don't often use the radiant, but I think insulation under the slab would have been better - but would it compromise the frost protection??? 

In a few locations around the footing/stem wall are placed lengths of PEX tubing with one end under the horizontal insulation and the other coming up to grade.  I was thinking I might check the temperature down there.  I haven't done it yet but since it is now winter, maybe I'll try it.

I placed a perimeter drain around the bottom of my excavation (which produces a good amount of water in the spring season) and then placed two feet of compacted stone before I formed for the footings.  The FPSF design guide allows for the use of non-frost-susceptible fill to be used to achieve the necessary depth of the foundation below grade.  So, I incorporated a vertical piece of two inch insulation in the fill that is under the footings so what I have is a total of almost four feet of vertical insulation, the upper two feet of which are actually affixed to the perimeter footing/stem wall.  The pictures are not in order in the link above.  The sixth picture shows the horizontal insulation and the vertical piece which is going to be buried in fill.  The fifth picture shows the top edge of that insulation sticking out above the fill.

Tom

[Bill Houghton]

Great looking place Tom.  Thank you for the excellent information.  I am probably overthinking this  but because this is a weekend cottage I might not be able to keep it heated 24/7 and am wondering if I shouldn't just bite the bullet and build the 40 or 42" depth foundation to our local code.  I would like go go with treated wood so that I can do the work myself.  The main thing that concerns me is that if I go with code (40 -42") I have no way to drain my footings to daylight.  I am only 2'8" higher at my build site than the adjoining ground level.  I'd still like to incorporate the insulation and water avoidance of the FPSF system, just in case.  My concern though is my two neighbors have 2 - 4" of heave in their unheated buildings every year with the code depth foundations but they are pole structures.  If I go with the protected shallow foundation, then I have enough grade to put in plenty of drain tiles and drain to daylight, so there are good points and bad points with each choice.   

Bill in the U.P.

[tc-vt]

Bill,

I remember reading somewhere that a depth of two and a half feet of non-frost-susceptible fill will remove you from the 'zone of influence' (I think is what it was called) of troublesome soils whether they be frost prone soils or expansive clays when wet. 

A load of 10-12 yds of gravel or stone I think was around 180-200 dollars, delivered.  Each load would give me another 4 inches of lift under the house.  That is, I was filling an area roughly 1000 square feet, the entire footprint of the house and slightly beyond.  I came up around two and a half feet from grade, 7 or 8 loads, before I formed for the foundation.  There are many benefits to this - more altitude for plumbing (septic); a house sitting up high looks better than a house at grade or sitting in a hole slightly below the surrounding landscape; much better drainage around the house and foundation.  For about $600-700 of fill and the rental of a compactor you can put your house up a foot higher.  If you have good soil on site, it may be usable instead of importing it.

There was a commercial building being constructed near me for which the footing excavations began as trenches dug at grade, just deep enough for an 8 inch tall form.  It looked odd at first.  Then they poured the four foot frost walls and imported fill.  The building is elevated four feet above grade.  It looks good and also has added benefit since it is an area that can flood.

Here are some tips if you use any kind of fill.  The coarser fill materials require less compacting than finer materials and the moisture content when compacting coarser fill is not an issue as it is with finer materials.  Three-quarter inch crushed stone will compact faster than finer material but it is difficult to shovel by hand.  To move it you should have a tractor with a loader bucket.  I started with 3/4 inch crushed stone, then 3/8 crushed stone which can be shoveled fairly easily, then the upper layer is crushed gravel.  The crushed stone settles after one or two passes with the compactor.  The gravel shovels very easily but needs to be moistened the proper amount when being compacted and requires several more passes with the compactor than do the crushed stone to achieve compaction.

If you need to do any clearing and are bringing in a dozer, consider clearing more than you think you need.  You'll need access for delivery of materials, propane delivery in the future?, and material from excavation.  The increased cleared area will also give you a head start on making the area around the house clean and livable.  Get rid of any trees that you wouldn't want near your house for whatever reasons like blocking the sun or fear of them falling over.  I had one that I regretted never cutting before I built but wouldn't dare cutting once the house was in place because it leaned toward the house.  Fortunately I had removed last year when an excavator was here.

Tom

[Bill Houghton]

Tom, thanks again for the excellent information.  It is a good idea about the trees.  BTDT.  Had a leaning tree that I kept saying we should get rid of and had to work like heck to get it down AFTER the place was built.   

I am thinking that it wouldn't be that much more difficult to just drop my footings to frost depth and use the FPSF techniques for extra projection.  Especially with a slab on grade set up.  I will try and keep the excavation as close to the size of the foundation as possible and use good grading, plastic and insulation to try and keep H2O from getting near the foundation.  Overkill?  Yep.  That is a style we are very comfortable with.   

Seeing as I have 6 more months before we can realistically put anything in the ground, I am going to start purchasing materials and putting things together here in my garage.  Treated wood foundation, exterior walls, trusses, etc....  Try to buy on sale whenever possible.  I have to check around on how to span the 24' for the upstairs yet, but that will come.  I might put a beam across the middle of the 24' and use floor joists to span the 14 - 16' on either side (may be 28 long or 32 long).  The next challenge is to find the supplies I will need to put in-floor heat in the slab.  I had a new neighbor start whinning about my outside boiler so our city officials are forcing me to take it out.  They were kind enough to let me use it the rest of this heating season, though.  Lets see, they force me to make two very hefty payments to them each year and they can tell me what I can and can't do on what is supposedly my land.  Hey, they must be my landlords?      Oh, I better not get started on that....

Bill in the U.P.

[tc-vt]

Bill,

I did a quick lookup and 16 inch deep I-joist would span that at 16 oc.  The floor would be stiffer floor at 12oc.  Here is the link to their specifier guide.

http://www.bc.com/dms/wood/ewp/3_Guide_Resources/east_south_us/1_alljoist_Spec/alljoist_spec_complete.pdf

I have Boise AllJoist AJS20 12 inch deep joists spanning 20ft (technically, 19ft 1inch, allowing for the wall thickness).  The floor has some bounce in that section that spans the full 20ft with no downstairs partition wall beneath it.  The floor has not sagged at all but I am going to put in a 3-ply 2x8 or 2x10 beam across the center to stiffen it.  I liked using the I-joists.  They are lightweight and easy to handle.

Tom

[Bill Houghton]

Wow, thanks again Tom.  I sure appreciate all of the help.  My confidence is getting a great boost with all or your help.  What a great forum.   

I had time today to work on my foundation plan.  Here is what I came up with.  I will have the excavator take 6" of grade off my whole building site.  It is an elevated area, kind of a ridge.  Then, I will have trenches dug for the size of my cabin (24 X 28 possible 24 X 32).  The trench depth will be 20"   I will have the excavator trench outward from the 4 corners, away from the building location, with a 6" drop for drain tile (to daylight).  Then, after I install the drain tile in the bottom of my trenches, and to daylight on the corners, I will fill the trenches with 20" of gravel.  (I wonder if I just dump it in, or does it need to be compacted?)  Then, I will build a 2' high treated foundation resting on the gravel. I will add rigid insulation out from the foundation base the required distance and thickness per the FPSF instructions (probably a little extra, overkill remember).  Then I will add backfill 16" to the outside of my foundation and 18" inside the foundation.  This will give me 8" of exposed foundation above final grade on the outside and 6" from the top of the foundation on the inside to leave room for 2" of insulation under the slab and a 4" slab.   

This design won't give me the 42" frost wall depth our local code requires, but I believe it will be more than adequate.  This design will give me 20" of non-frost-susceptible fill below my frost wall as you suggested above.  I am limited to the 20" because I want to make sure I can drain my trenches to daylight. As it is, I will be paying for enough fill to raise the final grade 16" plus gravel, excavating, etc...  I might be able to reuse some of the original 6" of fill we took off the site to start, but there is a lot of clay there so I will be cautious in reusing it.

Now that I have this plan worked out, I will spend the next couple of days questioning it.  If it passes all of the "what ifs" I can throw at it, then this will be the plan that I will use once spring comes.  Plus, like I said in a previous post, I would like to build the frost walls here in my garage this winter.

Bill in the U.P.

[tc-vt]

Bill,

My foundation drain drains to daylight at only one corner.  If you have one corner that has more of a downslope grade away from the house than the other corners, you could probably be fine with just the one exit point for drainage.  More exits are okay in case of a clog, I guess. 

I wrapped my foundation drain pipes in landscape fabric before back filling with 3/4 stone - see the picture.  The vertical pipe supported by the stakes is the conduit I made for the incoming black poly spring water supply line which can be seen in the background.  The other vertical pipes are the overkill accesses to flush the drain out if ever necessary. 



I read many different opinions about the best way to install drain tile and to prevent clogging of the holes in the pipe.  One method I think was to cover the drain pipe with sand.  I can't remember if the method using sand omitted the fabric.  I think I heard of stories of the fabric clogging over time.  If I remember correctly, the sand is supposed to act as a filter barrier between native soils and the drain pipe but will not be prone to clogging.  It seems to make sense.  I think this is how my curtain drain for the septic leachfield was done. 

Tom

[tc-vt]

Bill,

I posted some chronological pictures of the foundation construction in the owner-builder projects forum.  I hope they help you.

http://countryplans.com/smf/index.php?topic=5873.0

Tom

[Bill Houghton]

Thanks Tom, I have a file called Tom's Project with all of your photo's in there that I have been studying.  That was a lot of work but a brilliantly executed plan.  Did you do all the work yourself?  Any design help? If you have any more photos you want to share, I'd be glad to have them.

Most of the drain tile I have seen here is the black flexible rolls.  I don't want to put that in the bottom of my trenches because of the possible load from above, so I will seek out the solid stuff like you used.  I don't mind the flexible stuff off to the sides where there is only dirt on top, but not under the gravel supporting the cabin.  The uprights for future flushing is an excellent idea   

Tom, could you offer suggestions for help with my in floor system?  I searched and found a few opinions here where folks could have their system done by their local contractor complete for the same money it would cost to get just the materials from Radiantec and do it themselves.  I will be moving my Central Boiler outside system down there.  I am thinking of using glycol and then having a back up heat (electric and/or propane) for when I can't get there for a week or two to fire the stove.  I have a propane wall heater from the old cabin and I had electric brought into the site this past summer.  I will also have a wood stove inside the cabin for when I am out there and want to have the view of the fire, etc... (There is the overkill thing again. )  Did you staple up any tubing for heating your second floor? 

I have to tell you, this whole process is a blast!     At least once things start to take shape.  I really appreciate the help you gave me with my foundation.  The design of that has had me hung up for weeks. 

Bill in the U.P.

[tc-vt]

The design I did on my own.  I had a huge amount of help from two friends up here.  One helped has construction experience and helped put up the concrete forms and helped with shoveling, wheel barrowing and spreading the fill when I didn't have the rental tractor.  The other was a help mostly with the fill work.

There wasn't much planning on paper at all.  Matter of fact, the final size was made 30 feet instead of 28 feet the day before I excavated for the foundation.

Another thought popped into my mind - when you are clearing with the dozer, keep in mind where you will be putting your septic tank and leachfield and have that area cleared of trees and brush.  I had two house site options right next to each other.  Both were cleared and one of them became the leachfield.  Make a good wide driveway, too.

I got my radiant supplies from Radiantec.  They are 20 minutes away from me.  I think radiant heating and PEX became more popular and widely available (ie. at Home Depot) after I had built.  Maybe it seems that way because I just wasn't looking anywhere else other than Radiantec for the stuff.  Now it seems you can get all sorts of fasteners and tools and everyone is making pre-made manifolds.  Here's a caution about Radiantec and the other local (to me) spinoff company, Radiant Floor company  http://www.radiantcompany.com/
They both sell some odd, proprietary it seems, sizes of tubing.  I used Radiantec's 7/8 inch.  I just looked on their site and now they even have 5/8 inch.  I usually find 1/2 and 3/4 inch when I looked elsewhere like Home Depot and a quick check at homedepot.com confirms this.  Back when Radiantec was helping me with mine they were using, and had been for years, doubled hose clamps to join tubes to barb fittings.  This is considered verboten now and there are several different crimping tools available for PEX.  I don't think I have seen one of these tools with crimps for 7/8 tubing.  So, go to your local supplier and get tubing in standard sizes for which fittings are widely available.  For a small footprint like what I did and you are going to do, I would use the 1/2 inch.  It will bend in a tighter radius and make it easier for you to get good spacing between the runs of tube in the floor.  I used the 7/8 and you might be able to see in some of the pictures how the loops touch in many areas.  It makes for a few places in the floor that are favorites on which to stand when the radiant is running.
I could see it being not much more money to have a contractor do it.  They might get the supplies cheaper than we can.  I have heard this same cost savings about drywall, too, since they get the materials cheaper and they are fast at what they do.  I also put the 7/8 tubing in the upper floor, one run per 16oc joist space, using those metal pans you see on the radiantec site.  I was broke when I was building and they gave me a bunch of 'seconds'.  I placed the double foil faced bubble wrap insulation beneath the tubing, for what it's worth.  I don't know whether or not it will make any difference in the performance of the radiant, but the only thing it hurt was my wallet.

I did a few things to make the house able to go unattended in the cold.  All of my plumbing is mounted to the interior face of the walls, ie. not drilled through the studs.  This will reduce the chance of a frozen pipe, and was lots easier to install, especially when there are no plans on paper.  Most, if not all, will be hidden by cabinets and such.  I pitched all of the hot and cold water lines toward a drain valve.  The spring line has a curb stop just outside the house so I can turn that off.  I made an access where I can pour antifreeze into the short standing pipe in the house so it doesn't freeze. 


The spring line comes up just behind the drain line which is coming from the kitchen.  I still have to run a vent up from that drain where the cap is.

You can't have enough different sources of heat.  I have the radiant which runs off the water heater, a wood stove, and a no-vent wall propane unit which I am soon going to replace with a direct vent propane wood stove-style unit (a Hearthstone Sterling) which I bought used.  I use metalbestos double wall stainless for my chimney for the wood stove.  The water heater is a direct vent unit.  You can see it in the above picture.  The direct vent units were around $750 and the B-vent units were around $250 when I bought mine.  Considering the ease of installation compared to running B-vent through the roof, it was worth the extra expense.

Tom

[MountainDon]

.... I am thinking of using glycol ....

Just be sure to use propylene glycol, not ethylene glycol if you want to stay within the law.

[Bill Houghton]

.... I am thinking of using glycol ....

Just be sure to use propylene glycol, not ethylene glycol if you want to stay within the law.

Thanks MountainDon,

It has been a few years since I had to freeze protect my system.  Have just water (plus mfgr recommended additives) in it now.  I will contact the local CB rep for the "right" stuff for the install.  I know it is the stuff for potable water systems that we used before NOT car antifreeze.

Thank you again Tom.  If I send you a plane ticket, do you have any spare time?     I told my wife I have a partner on my cabin build named Tom.  You sure are making my decision making easier.  Information is great.  Sometimes it is hard to come by.  I may take a trip out to our local truss shop today, just to talk with him.  I found some do it yourself truss options, but I am worried about insurance coverage with that.  They are gambrel (barn, because I am not sure if I spelled that right) style.

As far as the in floor heat, I might just have it contracted or at least a bid put on it so I know what the damage is to compare to DIY.

Bill in the U.P.

Bill in the U.P.

[tc-vt]

Bill,  I unfortunately have lots of spare time right now, as work for me has come to a standstill.

As far as the trusses - mine were about $100-110 each and I think I estimated about $40-50 of materials in each one if I built them myself.

Tom

[Bill Houghton]

It is hard to justify building my own trusses with that small of a mark up, plus they deliver.  The shop I visited today tends to be higher priced than other nearby truss shops, however, they are very personable, very professional, take a ton of time and really do build a very nice truss.  I am amazed at the spread in turss design up here.  I can go to the big box store and get trusses that they claim will handle our snow load and they are all 2 x 3 except maybe the top cord.  The shop I visited today designs their trusses very heavy.  I will see what the quote is next week.

Bill in the U.P.

[tc-vt]

Bill,

My trusses I think are rated for a 40 psf snow load.  I think design for my area would call for 60-70, but with the metal roof they were able to go lighter.  He said he was having trouble getting a design to work.  I don't know why he didn't suggest going to 16 oc instead of 24 oc.  I don't think there is anything bigger than a 2x4 in mine.

Tom

[MountainDon]

Trusses transfer loads manly by using compression loads on the components. Wood compressed along it's length is much stronger than compressed across the grain or in a bending moment. So it's amazing the loads that a 2x3 or 2x4 can actually carry.

[glenn kangiser]

Trusses also use tension mostly on the bottom chord.



http://www.rltruss.com/trusseducation.html

[Bill Houghton]

The trusses will solve my span problem on the second floor.  The DIY trusses I was looking at just sat on the outside edge of the second floor.  I was still challenged with how to span my 24', either center beam and posts on the main floor    or 14" or 16" deep I joists or possible gluelam beam across the 24' span with floor joists running perpindicular.  I am big and hate bouncy floors, another thing the trusses solve.  The trusses are designed for 24" OC and I will buy some extras and put them 16" OC (overkill again  ) 

Bill in the U.P.

[tc-vt]

Bill,

My trusses are tray trusses.  I'll dig up  pictures but it might be after the holidays.  They have a short vertical 2x6 piece that sits on the knee wall, then the truss extends up the slope of the roof with the appearance of a floor truss until it joins the bottom chord.

Tom

[Bill Houghton]

Good morning Tom,

I Googled Tray Trusses, didn't find anything.  I look forward to your pics. 

In my style of never leaving anything alone, I am now wondering if I can do something besides a cement floor in this design.  My 2' frost walls with FPSF insulation, sitting on top of 20" gravel filled trenches and backfilled on both sides should be held in place pretty firmly by the fill on both sides.  I am looking at the possibility of filling the inside with gravel, compacting, then 4" of blue board with treated 2 x 4's  flat on top of the insulation with my plywood/osb (or other) floor screwed to the 2 x 4's.  I could place my heat pipe in the spaces bwtween the 2 x 4s, maybe even using some of those aluminum heat transfer plates.  I could dry in the cabin then put the floor in.  Plus, it would create a lot of flexibility in getting my plumbing in, etc...

Bill in the U.P.

[jr1318]

Bill,

I used sfpf in northern Wisconsin on a off and on heated weekend cabin, the inspector had no problems and did not balk at this method at all, I myself was worried but this method is adopted in the UDC code. It has been 3 years now and no heaving at all. Make sure you use the high density foam.