PWF Permanent Wood Foundation Question

[MountainDon]

I got the following PM from new member willardP. (We've been away). .... I can't see any reason he can not post... that's a puzzle.

Mountaindon,  For some reason it won't let me post something that I basically just wanted some feedback on so I'm going to try and send it to you and maybe you can help.....

I've been wanting to put an addition on my house and just purchased the land to make it feasable.  I had a contractor come and give me a estimate and asked about a basement under the addition,  which is going to be 22' X 26'.  He's trying to convince me that a PWF would save alot of money and give me even more space.  My house is currently 680 sq ft.  My Question to anyone out there that knows about this subject is, "Can you build a PWF and butt it up against a brick foundation?"  It is kind of hard to explain.  The PWF is going to butt up against 2 different style foundations.  The north end of my new PWF basement is 22' wide.  What it is butting up against is 11' of crawl space and 11' of a brick basement. And I do want a basement.

[John Raabe]

Any type of new basement foundation will be a little tricky, especially where you will be undermining the support for the crawlspace foundation with the new basement wall. Perhaps the new PWF foundation wall can replace the crawlspace and brick foundation walls on the 22' side?

A PWF could work that is carefully done with:
• foundation grade studs and plywood
• hardware and nails designed to work with the wood treatment
• a 6 mil moisture barrier to the gravel footing
• a way to drain or pump away any site water coming against the foundation
• it is important that the right blocking and hardware tie the top of the wall into the floor diaphragm. This replaces the mass of of the heavier masonry walls.

The PlanHelp membership site has a scaled drawing that can be added to your plans with the details on such a wall.
(See the download info near the bottom of this list: http://goo.gl/XbPWp.)



Here is a more general informational diagram

[paul s]

I built one in 1996 and it is just fine today.  But beware this is not your ptw at lowes or home depot.  Mine had to be 0.6 lbs per cubic ft and has to be redried after it is treated and stamped kiln dried. yes it was a special order.   mine was put together with ss nails and screws. can save money but  the local inspector is much more used to brick and mortar in most parts of the country.

[AdironDoc]

My project is underway and also uses a PWF. To bring the PWF diagram here to life, I've posted the link to photos contained in that thread.
http://countryplans.com/smf/index.php?topic=10791.0

[new land owner]

I ordered my Footer material (2 x and top and bottom plate material today (2 x 6).  I am also going to go with a 6' crawl space and am looking forward to getting started.

[Don_P]

This month's Journal of Light Construction has a good article on PWF's.
As a side not there does need to be a floor or something at the base of the walls to resist the pressure of the fill on the outside. The floor system attached to the top of the walls provides lateral resistance at the top

[Rob_O]

This month's Journal of Light Construction has a good article on PWF's.
As a side not there does need to be a floor or something at the base of the walls to resist the pressure of the fill on the outside. The floor system attached to the top of the walls provides lateral resistance at the top

Looking at Docs picture below, I see 2* going from side to side on top of the gravel with the jacks on top. I'm guessing these function as "a floor or something" keeping the fill from collapsing the bottom of the wall. Is this correct?

[MountainDon]

This is not a prescribed method for building a PWF when it is back filled on one side only. See section R403 of the IRC 2009. 

[Squirl]

It depends.  I don't know what Doc's unbalanced fill height is.  
Per IRC, floor bracing is only required if greater than 4 ft of unbalanced fill.
Since most of the US has a frost depth 4 ft are under, if you are building just at the frost depth, floor bracing is not required.  It is simply extra protection.

http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_4_sec004_par039.htm

Technically, you would be placing the gravel at frost depth and the wood part of the footing just a few inches above to get the proper slope of fill from the foundation.

[Don_P]

I don't believe that is a correct or safe interpretation... go carefully.

[MountainDon]

I made an earlier post here, but didn't like it so I removed it... Now this...

All the illustrations I seem to recall for PWF show at least some backfill on the inside of the PWF, or a wood or concrete floor. There is another reference from the Southern Pine Council. The download link is on this page.... http://www.southernpine.com/applications_permanent-wood-foundations.asp  We have listed it on the forum before.

IRC Section R404.2.3, Height of backfill, states "For wood foundations that are not designed and installed in accordance with AF&PA PWF, the height of backfill against a foundation wall shall not exceed 4 feet (1219 mm). When the height of fill is more than 12 inches (305 mm) above the interior grade of a crawl space or floor of a basement, the thickness of the plywood sheathing shall meet the requirements of Table R404.2.3."

First I can not access the referenced AF&PA PWF document. I only find links to pay for view copies, so I have no idea what it espouses.

That section of the IRC simply goes on to state the backfill height shall not exceed 4 feet and goes on to caution on plywood thickness under certain conditions.


The next section reads, R404.2.4 Backfilling. Wood foundation walls shall not be backfilled until the basement floor and first floor have been constructed or the walls have been braced. For crawl space construction, backfill or bracing shall be installed on the interior of the walls prior to placing backfill on the exterior.

So, okay bracing may do the job. However, there is no explanation as to what type of bracing, the spacing of the bracing or how it should be secured, how many nails of what size, etc.


One of the reasons the IRC becomes longer with every new version is the need or the attempt to flesh out the regulations to cover all conceivable situations. When interpretations are made, different people may reach different conclusions.


In cases where the method may be subject to interpretations it would always be advisable  to check with the local authority to see what interpretation the builder would be facing.


One thing that bothers me about a PWF crawlspace without infill and without a floor of any type is what is preventing the lateral inward movement of the lower wall plate? That plate is simply sitting there on gravel. Even the use of "earth pins" raises questions to me. Note the IRC doesn't reference pinning; I believe that came from a design book where it came under alternatives that could be engineered to be used. A thought I had about the pins is how similar the load bearing ability is to the issues involved when trying to support a load by nailing joists to the sides of piers; the loads are sideways instead of vertical and the pins larger in dia. than nails. I don't know how that difference affects the relationship between the forces and fasteners here.


All the above is of course an opinion from a non technically trained person.

[AdironDoc]

I have now seen 3 projects now in the upstate NY area. All were built by different builders and used PWFs. All used gravel flooring. An additional 2 seen on this site used gravel but were not braced. All had wood 2x6s under adjustable posts. Seems that, as was the case with my builder, the engineers and inspectors are instructing builders to do it this way.

[Don_P]

If these foundations are actually engineered then I have no problem, I suspect they were not.
The 4' maximum unbalanced fill limit in section 404 I believe is in reference to the first drawing fig 403.1(2) rather than fig(3). For a full basement with a basement floor the maximum unbalanced fill height is 4' without design according to the PWF standard. Going from there to no bracing and 4' of fill is not the intent, nor is it safe. Notice in fig(3) they do show fill inboard of the crawlspace wall bracing it.
http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_4_sec003.htm

From the code referenced PWF Standard;
"Note that a partially excavated crawspace foundation does not have a basement floor system at the bottom of the wall to provide resistance to net inward soil forces at that location. For those cases where the difference in backfill height is relatively insignificant, it can be assumed that the net lateral inward force can be resisted by friction between the footing plate and the granular fill below. Otherwise additional methods of restraint may be required, such as by use of a concrete slab as illustrated."

There is room to discuss "insignificant" and "other methods of restraint". There are formulas given to determine the net inward forces. Reading human law creatively does not change natural law, this is what we are really trying to wrap our heads around. Don't ignore these forces, they are real.

Thus far we are only talking about lateral forces, uplift and overturning are further considerations and are discussed in the standard. Some of those details would also adress lateral resistance.

I have no experience with these foundations, I have seen block foundations fail from lateral soil pressure. The failure is slow, taking years. The soil pushes from either shrink/swell or frost then recedes when those conditions change. More soil filters into the recesses and the process repeats, over and over, gradually shoving the wall inward.

[Rob_O]

There are formulas given to determine the net inward forces. Reading human law creatively does not change natural law, this is what we are really trying to wrap our heads around. Don't ignore these forces, they are real.

If you have a link to these formulas they would be real handy. I understand that as long as gravity keeps working, there will be forces conspiring to wreck my foundation. If gravity quits I will have other problems to deal with...

[Don_P]

Here is the formula for crawlspace lateral force;
R_b=^w/₂[h²_o-h²_i-(h³₀-h³_i/3H)]

R_b= Reaction at base of crawlspace wall
w= design lateral soil load, lbs/sq ft/ft of depth
h=backfill height from top of bottom plate to ground level, ft... o&i= outside& inside
H= height of foundation wall from top of bottom plate to bottom of top plate

w is the tough variable to pin down. If you know your soil type or can access it from the web soil survey I'd use the worst soil type for the area to be conservative, still backfill with gravel and drain it well. If you are in an area of expansive soils, or a high water table I'd reconsider. This table is not in the codebook but dovetails with table 401.4.1 and the 2 letter soil classification.

Class1	Class 2	Class 3	Class 4	Class 5
30 pcf/ft	45 pcf/ft	60 pcf/ft	75 pcf/ft	90 pcf/ft
GP,GW,SW,SP,GM.	GC,SK,SC,SM-SC.	ML,CL,ML-CL.	OL.	OH,CH,MH.

I'm going on the fly here so we are learning together... not a good scenario.

Here we go, the PWF standard is still online, I'd suggest downloading and saving it;
http://mshi.org/Reports/Wood%20Foundation%20Design%20Specs..pdf

[Rob_O]

I'm going on the fly here so we are learning together... not a good scenario.

Soil is type ML, beasly silt loam. 60 pcf/ft

Walls are 4', deepest section of the wall will be maybe 3' in ground

Edit: Found the AF&PA guide here http://mshi.org/Reports/Wood%20Foundation%20Design%20Specs..pdf

Edit 2: the answer is... 202.5 lbs per linear foot.

[Don_P]

Looks like I'd better put that quote back in there  This should be reviewed by a qualified engineer.
I see we both found the online version of the PWF standard at about the same time.

I haven't had time to check the math but that sounds better. Most structural problems are a matter of identifying and quantifying the loads and designing adequate resistance. We now have a load to be resisted. If the bottom plate is continuously supported then it's easy. If there are several crossbeams across the foundation then the load needs to be collected and transferred to those points. The bottom plate would then be acting as a beam and checked for bending, the crossbeams would be acting as columns and need to be checked for buckling. The bearing between those members would need to be checked. If the force is being resisted by angled braces the same basic method applies, the connections at each end of the braces and the floor joist it is attached to would be checked for bending. If you pin the bottom plate to the soil to resist the load this is getting into a geotech's playground.

This is from an old JLC article I was reading;
"Most structural wall failures occur during construction from careless backfilling without proper wall bracing. In service, most wall failures happen because of the gradual consolidation od saturated fine soil, resulting in excessive soil loading after many years of increasing pressure." The PE that wrote the article also cautions that without exterior footing drains the wall design should use the soil pressure of the next higher class. Keeping the soil well drained is very important.

[Rob_O]

I have now seen 3 projects now in the upstate NY area. All were built by different builders and used PWFs. All used gravel flooring. An additional 2 seen on this site used gravel but were not braced. All had wood 2x6s under adjustable posts. Seems that, as was the case with my builder, the engineers and inspectors are instructing builders to do it this way.

You have links for any of the projects on this site? I've seen the pics new land owner posted and his basement looks a lot like yours

Looks like I'd better put that quote back in there 

Thanks for the info you provided. It's always helpful to have an extra brain in the middle of a brainstorming session

[AdironDoc]

You have links for any of the projects on this site? I've seen the pics new land owner posted and his basement looks a lot like yours

My bad. New Land Owner's build shares features with mine. I though I had seen another online, but i can't recall if it was this site. The others I've seen this past month in the area around my own build. Two were braced diagonally at what appeared to be 8ft intervals, presumably until the first floor was completed. I don't know if their diagonals will remain. My builder said the town engineer highly recommended I leave them as an added lateral support against frost heave. All had gravel and a 2-by laying across to brace the floor plates. I've never gotten a gander at the "soil anchors".

[Squirl]

Sorry I ducked out of the conversation for a while. I was backfilling my foundation this weekend.  I hope I didn't hit a nerve for anyone with my interpretation of the law.  I spend part of my days reading and interpreting the language of laws.

"For wood foundations that are not designed and installed in accordance with AF&PA PWF"
So if all AF&PA PWF foundations are to be braced, then "not in accordance" would be unbraced.  If unbraced is within AF&PA then there is no debate and the argument would be moot.  I'm not trying to say that installing a tall unbraced PWF is a good idea.  What I was trying to do was state the plain language of the law.  If this was not the intent of the code, it should be rewritten and the language is seriously deficient.  I always had a lot of faith in the accuracy of the building code.

I always took the approach that building to code is not the worst way to build.  I was looking at it as good, better, best.  Good in this case building to the exact standard set by the language of the code.  Better building beyond that, such as in the bracing styles in Doc and Newlandowners. Best would be a full concrete slab as designed in the southerpinecouncil guide.  I can't say for sure if Doc's design is not covered in the AF&PA PWF foundation guide because I did not get a chance to read through all of it.

The original chart I referred to also states that it is for "30 pcf equivalent-fluid weight soil pressure" which is pretty light.  That is part of the reason for the "it depends" answer.  There are a lot of variables to account for.

[MountainDon]

Hope your backfill went well.

I always took the approach that building to code is not the worst way to build.

Not the worst way to build but the worst, or least good, you can build and meet building code. Big difference.

I mean that in a friendly, stop and think about what is being planned/done before forging ahead and cutting lumber or hammering nails.

For example, Don_P has found that a better roof results when he uses 5/8" OSB for roof sheathing. IRC states 7/16" is fine, but 5/8 is better according to experienced builders and engineers. However, the IRC doesn't list options as good, better, best. They list minimums; minimum thickness, minimum nail size, minimum number of nails. It has been shown that better roof sheathing uses nails every 4 inches along edges, not 6" as per code.

So sheathing a roof with 7/16" OSB with 6" edge spacing along edges is the worst roof you can build and get away with it, get approved, under code.

Just a blunt way of saying, think.


There are lots of examples of worse than code, not up to code, outside of code, that can be found on the internet.


And yes, maybe that section needs a rewrite, expansion, to be more clear. Unfortunately laws of all sorts frequently need rewriting for clarity.

[Rob_O]

Sorry I ducked out of the conversation for a while. I was backfilling my foundation this weekend.  I hope I didn't hit a nerve for anyone with my interpretation of the law.  I spend part of my days reading and interpreting the language of laws.

No maybe about it, code is exceptionally vague on this subject if you're not using a concrete basement floor. I'm still trying to wrap my brain around it all

[Squirl]

I'll tell you one thing I am really happy about is that the link to the AF&PA PWF guide was posted.  Talk about a wealth of information.  I am going to enjoy running through those equations.  Also I sparking a good debate can flesh out the parameters and limitations of the "it depends" variables in a problem.

IIRC in my first glance at the document there was a reference to diagonal bracing the studs to the joists as was done in doc's design.  Much more learning to do.

[Rob_O]

I'll tell you one thing I am really happy about is that the link to the AF&PA PWF guide was posted.  Talk about a wealth of information.  I am going to enjoy running through those equations. 

You got that stuff figured out yet? I have some numbers for you to run through whenever you're ready

[Megaminya]

My project is underway and also uses a PWF. To bring the PWF diagram here to life, I've posted the link to photos contained in that thread.
http://countryplans.com/smf/index.php?topic=10791.0

Lord builders I live in Russia where to get information about PWF the construction and preferably alone or built houses. Sorry for my English. Writing through an interpreter