PSP as Foundation for existing frame addition

I've been contemplating digging out the crawl space under a 70 year old addition on my 93 year old house for a root cellar. The existing foundation is fairly shallow concrete which is wandering away from the house in places. The main house has a full basement. The addition is on the west wall at the north corner of the main house. Frost line is generally considered to be 24" but building dept calls it 36".

I've been read Oehler's book and thinking that my all wood addition can't weigh more than 2' of dirt.

Does anyone have suggestions or cautions for installing PSP basement/foundation walls under an existing frame structure? I'm quite handy and understand the scope of this extra-legal project. I'd also like to keep the existing foundation in place to avoid attracting the attention of inspectors. If I need to just slide the old foundation back under the wall after PSP is complete I think I could manage it in only a few pieces.

Hi Tim,

Welcome to Permies!!!

For clarity what do you mean by PSP?

Also I am not following on what you want to do, add an addition, dig a root cellar?

Photos would be really helpful for us to see, so we can perhaps help you.

Regards,

jay

> PSP...



My usual approach to habitat mods is to do the work to code if at all possible. It removes a serious complication to resale; if you do have an "inspector problem" remediation is way easier; it can give you some understanding of the building elements involved and possibly throw more light on your own plans. Note, I did not say get a permit, just understand the code and build accordingly as much as possible.

How old is your inspector? If he's been on the job more than 5 years he may well know your property - most good inspectors know their territory. Which should not really be a problem - just don't bet on official ignorance if you don't have to.

My only other thought would be that you want to maintain a weather barrier (wind and water) on the exterior of whatever foundation elements are exposed above grade and a good water barrier below grade. In freeze/thaw territory this is as much for the longevity of the structure as it is for your comfort.

It sounds like a challenging little project that you can probably pull off if your ground isn't all sand. But do install hefty temporary supports before you dig down close to the existing foundation wall. Safety/Safety/SAFETY! Especially if you're working alone.


Rufus

Sorry for the confusion about PSP. I'm stuck in bed recovering from surgery and only have my new (first) smart phone to post with. I was trying to type as little as possible and apparently cut too many corners.

I am referring to Mike Oehler's underground house method of construction: Post Shoring Polyethylene. I live in an old suburb in a small bungalow. A small extention of the back porch (from the '40s) is just the right size for an ample root cellar. The lot is 1/7 acre and flat so a separate root cellar would be quite the logistical feat. Our soil is mostly clay and would be quite stable during excavation. The structure itself is made entirely of solid wood products (no ply or pressed stuff and no rock) making it relatively light weight. Total load on the proposed PSP walls can't be more than the 2' of back fill that Oehler puts on his roofs. Biggest problem is the fact that current foundation is no longer viable and needs to remain in place solely for it's visual effect (both aesthetic and concealment). The structure needs a new means of support, I need a cellar, and the city doesn't need to know. Does that clear up my previous question?

I should point out that, while the main foundation is fine, the foundation wall supporting this addition extends only a few inches below grade; well above frost line. This is why an entire wall has broken off and is creeping out from under the house. There's only an inch or so of foundation still supporting the wall across that end. Therefore I'm bumping up this project in priority since I may only have a couple more years before I've got bigger problems that can't be fixed without costly permits and contractors.

Hi Tim et al,

Well Tim folks that know me here on Permies know that I am always game to help someone, and 90% of the time I make a good effort to just that. This one I am going to leave to others.

In 35 years of working in, and around the building trades I have seen two similar attempts like this with porches and attempts to undermine them without proper preparation and materials. Both ended in fatalities. If the foundation needs to be reset than you already have an issue that is alarming, the other is what could happen digging and undermining in a confined space, and lastly violating could in this manner is not acceptable. I work outside of code most of the time, but I have PE support, insurance and 35 years of experience doing it. I with Rufus most of the time on this one...do it to Code, or have your plane approved by a structural PE (professional engineer) with experience overseeing alternative-traditional architecture. To do otherwise may risk not only you and your family, but others that may someday live in the home.

So I don't leave on such negative note. Yes this can be done, but you will need a PE, also you must with blueprints of the project with both cross sectional elevations, plan views with by sections at different strata levels, and the foundation to the porch will have to be replace first for this project to even come close to being safe. 3D CAD renderings would be helpful to you as well in guiding and planing the root cellar dig. You will also need to present pictures of the proposed project area. I will be following along, and if you do all that I will lend guidance after the porch foundation is repaired, and the photos are presented. The drawings you can develop over time before "root cellar" dig.

Regards,

jay

Yea its already been said. Get an engineer to approve whatever you're doing for sure.

Here is how I look at these types of problems/projects:

1. You want X and you have Y but Y has Z issues. Most people want someone to tell them that they can just have X and this is how or this is how much. Everybody is ignoring Z because Z is likely costly or inconvenient. I have worked on customers homes where they want to fix their leaky roof but don't want to address the structural issues that are contributing to the problem and I totally understand cause new shingles cost 1x but fixing the structural type issues and then the roof cost 2x or 3x. Its simple math.

2. Its good that you identified the slab shifting from the house as an issue. Problem is that back in the 40s people just built whatever however and with whatever they had easily available. Where I live we have all kinds of older homes with multiple types of foundations competing, usually the original house foundation is good and the addition is junk. This sounds a little like what you're facing. So you're going to have to come to terms with the fact that you have a bad foundation that needs to be addressed. This unfortunately is primary above your root cellar goals which are commendable. First things first I would find an engineer who will help you address the foundation issue and give you options, generally you want whatever new foundation to be of the same type and scale as the existing house foundation. You may need to replace the addition foundation, only your engineer can answer this.

3. Now that you are addressing your foundation issues either yourself or through a contractor it should be really easy to incorporate the root cellar into your plans at minimal additional cost.

Often when I explain things this way to people as a contractor they keep looking till they find a contractor that will tell them what they want to hear which is they can give you X for some marginal cost. Then they call someone like me a couple years later when their house is leaking etc to fix and I address Z then again address Y, or they just live with it. Like was said above DO NOT mine under any foundation especially one that is 70 years old as though it is a weekend project. Good veggies are hard to enjoy when your in a hospital recovering from a bad accident.

Thanks, Jay, for the well deserved cautionary advice. I am not really interested in jeopardizing anyone's life for this project. Unfortunately, our financial situation changed drastically at the beginning of this year and will not support the conventional and legal repairs needed to make the structure safe into the future. Since our financial situation isn't going to return to the level necessary to fund those projects in the foreseeable future I am forced to find a "freaky cheap" and SAFE alternative since life and limb are already approaching imminent danger.

Btw, I realized this morning that my thread should have been posted in the "Wofati" forum rather than here. If anyone with the ability to move it happens across here, feel free to move it.

A recon mission through the house has brought the laptop to my recovery station here. With a proper keyboard I will attempt to at least alleviate any concerns that I (a new guy who hasn't posted before now) am not a complete idiot. (c:

First of all I am reasonable experience in the building trades as I grew up working with my dad in construction/remodeling. I have lived in my house for 9 years now and have done ALL the repair and restoration work on it in that time. I have replaced all electrical and plumbing in the house. Much of it permitted and all to code. I've replaced bearing walls (knowing they were bearing) and both built and removed non-bearing walls (knowing they were non-bearing). I am excruciatingly familiar with my house and with the soil & dirt around it. I've had the foundation inspected by a PE and discussed the various ways and means of conventional repair. My wife is constantly complaining about how long I think about projects before beginning work and how long it takes me to get to the "exciting" part of the job. I think things through thoroughly from all angles before formulating a plan.

About this project: The proposed excavation will actually be under the original back porch AND an addition which created a roughly 11'x13' space incorporating a portion of the original porch, original cellar doors/stairs, and about 6 additional feet beyond the edge of the original porch. The total space comprises two rooms above. The south half is a long ago enclosed porch approximately 11'x7' which serves as a laundry/family closet. The north half (which includes the addition) serves as an art/textile room and back mudroom (great combo) and inlcudes the "new" stairs to the basement which follow the house foundation wall and turns a precarious corner to enter the basement at the original cellar access. (We use the almost-tall-enough-to-be-legal basement as living space and the climate is not suitable for food storage.)

Access to the crawl space under the porch/addition is through an access panel in the stairwell wall next to the original foundation wall which originally formed the outdoor cellar stairwell. The access panel opens to a portion of the crawl space which was part of that original stairwell. The footing of this portion of the foundation gets progressively shallower till it reaches the top of the original cellar steps. Beyond that is the "new" foundation wall which has no footing and barely extends below grade. This wall wraps around the addition and meets the original porch foundation at the midline of the house (which isn't buried any deeper). This original porch foundation originally wrapped around the entire original porch but much of the portion that extends under the addition has been knocked out (presumably for access to plumbing and electrical). The portion still supporting the original porch extends along the west face of the laundry to wrap around the south end and meet the southwest corner of the main house. This south end of the laundry room is the section of foundation that has crept nearly the full width of the foundation out from under the porch. Being that the floor joints all run north to south, this could be exciting when the foundation finally clears the joists. That said there is visible light along the entire section which leads me to believe that it isn't structurally necessary (at least for now).

My preliminary thoughts run along the lines of digging out the center of the north art room (the addition) to no less than 3' of the existing north foundation wall, installing post and beam bracing to support all joists while I excavate another foot closer to the foundation and install another post & beam support. My thought is to work this way until I can install diagonal support against the foundation wall to prevent it falling inward. Even though my access is directly against this north wall, the cellar stairwell affords deeper excavation at that point allowing me to dig my way toward the center of the room while maintaining decent headroom at the entrance point. The final post and beam support will be installed as close to the inside of the foundation wall as possible while still allowing shoring to be installed above the base of the concrete foundation. Posts will be placed 3' apart in 24" deep holes with a flagstone and gravel footing. The 2x shoring will be installed from the inside with staggered joints with plastic and backfill placed and packed as I go, all per Oehler's designs. The new PSP foundation wall should be within 6" (preferably less but that would require excavating under the existing concrete foundation which seems reckless at best) of the edge of the floor above and 2.5" from the inside edge of the bearing wall of the room. I'm figuring that this is an acceptable cantilever distance that should last at least as long as the structure it supports. A beam will rest on top of the posts that extends to support all joists and joists will be attached to the beam using hurricane ties. I'll then excavate south in the same manner as before, installing joist support after each lateral foot of excavation. I'll leave an ample ledge inside the west foundation wall except at the corner where the final supporting PSP wall will be installed under the south wall of the room (which is the midline of the house). Once the north and south walls of the north room are supported I can install lateral bracing between the two walls so that the floor joists attached with little hurricane ties aren't the only thing resisting the lateral pressure of the earth on the PSP walls. Then I can excavate toward the west wall using diagonal bracing to keep the concrete foundation from caving inward but joists do not need bracing. Once the west wall of the north room is excavated and the PSP is in place and tied in to the adjacent walls, I can begin excavating toward the south wall of the laundry room. The north wall being already supported at the midline of the house. A ledge must be left the length of the west wall of the laundry to prevent the concrete foundation from falling inward but the south wall of that room will be installed first as it supported the joists that actually bear the load above. The west PSP wall of the laundry room will bedirectly tied to the original beam that supports the original porch at the midline of the house. This beam in continuous to the front of the house and directing westward lateral forces from the PSP walls under the laundry and the art room will be transferred all the way across the house agaisnt the foundation and soil at the east end of the house elliminating focused inward pressure the the center of the west basement wall (which would eventually result in collapse. All work will be slow and meticulous.

If anyone who has experience with PSP or excavation has constructive criticism (other than don't do it) I would greatly appreciate it. If you are certain that PSP absolutely won't hold please explain why and what DIY solution you would recommend instead. If someone lives in a place where PSP is legal I would love to know what would pass code where you live!

Sean,

If Y=PSP what, in your experience is the value of Z? I'm quite familiar with the value of X.

Tim I wish I was closer to see what you're up against face to face. I can tell you that I personally have never built a root cellar although I do think its a pretty awesome idea considering how much we threw out in bad veggies this year :$. Anyhow I really can't tell you what to do with your foundation repairs. I can tell you this if you have a section of your house foundation that is currently distressed then mining around it is a really bad idea if you don't want to aggravate the problem and make it worse in the short term. This will mean that the value of Z can start taking away from the value of X (X being your current home and Z your apparent foundation problems) which is a big step backwards.

I have no idea what the best and or most cost effective solution is to your foundation issues. This I do know, right now the foundation isn't completely ruined and its still holding up your house. If you mine underneath it that could change, it could cause you or someone close to you some serious injury and/or cause your home some serious injury, neither of which you want to risk. So the real question is does the value of having a root cellar in your basement exceed the risk? I can tell you this houses, specifically older houses weigh a LOT more then we generally expect, the pressures on the earth under a house are amazing, its why we need sure foundations. Don't underestimate the weight of a section of wall.

Given you can't afford to address the foundation right now is fine, people live with issues in their homes all the time. My advice to you is not to mine under any of your foundation walls without first having a PE look it over and give you some advice if not specific direction. If you can't afford a PE then you probably can't afford to do anything anywhere near your foundation, pretty simple math. Wofati is a building system all its own, and what you have in your home is also a separate theory, combining them is definitely cool but everything in life has a cost. I know this isn't what you want to hear and I'm really sorry.

Now for the positive advice cause thats whats really important. Why don't you build a separate underground earthen outbuilding? Something far enough away from the house that you won't have to worry about stressing your current structure? To me its a way more engaging project that is much safer then what you currently have in mind. Plus having done something like this will allow you to gain some knowledge that could help you address the foundation problem, who knows. Often it is much easier to build from scratch then to fix broken structures. Put the entrance under a light shed and you'd avoid the prying eyes of those around you. My $.02

I'm having a hard time envisioning your predicament, as well as your vision. It is for stuff like this that the adage "a picture (or drawing in this case) is worth a thousand words". Without being able to fully grasp what you are trying to do I, and perhaps the others on here, advise extreme caution.

In my mind the PSP model is a stand alone design. The posts in it are meant to hold up its own roof, not a whole other story. Unless you know of structures Oehler has used his design as a basement for an upper story, I can't advise using it in such a capacity as you desire. For someone on such a tight budget, experimenting with so many unknowns with no worst case scenario capital just doesn't add up to me. I'm sure a more "conventional" approach would be easier, faster and cheaper, such as incorporating the additional beams as permanent, and using larger posts, lally columns or concrete piers. I think you could do a concrete perimeter and still back fill it so you have an earth berming effect on your root cellar.

I understand the desire to use exciting alternative and natural building practices, but most of them don't incorporate well into remodeling, as they are usually whole house systems. There is something to be said about a seamlessly updated classic house using standard techniques. Being a builder, only you know your capabilities and vision for your structure, so best of luck however you proceed.

Tim

Wood-Framed Shear Wall Construction; An Illustrated Guide by Thor Matteson

This is not a direct fit, but the guy writes in English (mostly) and doesn't waste words. It contains a lot of building weight information about wood frame construction. Search around for forums and articles by this guy and you'll probably find a lot of the info in the book available online. But it's worth the trouble to get an inter-library loan if you can. It also discussed in great detail the forces present in wood frame buildings and how these forces can/are transferred to the ground. Although not directly addressing your design, the thought processes and examples may help in figuring out how to think about the problem and what to look for.

There is, of course, a lot of other engineering info out there.

The picture/diagram comment was spot on. It's the difference between confusing and absolutely opaque. <g> Sketchup has a nasty learning curve but it's free and does the job as well as anything else you get for less than $5-10k.

Nobody can recommend anything specific with the info you provide, although it's clear you've spent a lot of thought on it and clearly have investigated at some length. Even w/your detailed diagrams, nobody can say anything because we can't see the actual site and be sure it's like we think it is and there isn't anything just outside the picture or around the corner that would stop the show. I mean no offense and I suspect if you've been in the trades you've experienced the same problem: "Lady, I have to SEE it before I can give your a price!"

That said, here are a few thoughts:

1) Sean's point about combining two different systems hits the spot. It might not be immediately intuitive but there is a LOT of detail in any even slightly complex system. The reason a system is considered reliable is because a lot of people over a long period of time have messed things up over and over again until everybody has figured out what details are Bad and what details are Good and how to make them play nice. They have got in the habit of recognizing the bad and doing the good. Ie. Long track record. Right. You already knew that. But here's the thing. The good habits and automatic insight people have gained about the one system or another - they don't apply when two different (reliable, tested, well understood) systems meet. All a sudden you have a fault line where two systems meet and that line is clotted with hundreds, even thousands, of details which nobody has had experience with before. No "real world" tests, no hundreds of smart guys making it work, failing and fixing it and getting smarter. You're effectively the pioneer here. And I think you know that figuring it out ahead of time.... Isn't even a close 2nd to thousands of successful examples well tested with the bugs worked out. Can or do you want to afford to be the pioneer here, if you don't have to be?

2) The PSP system is effectively untested compared with standard construction, so add that factor into the fault line between two different systems. You don't have two well tested fully understood systems. One of them is, relatively speaking, almost completely unknown. That is not good for your odds. Note I'm not saying that a lot of thought hasn't gone in PSP; not saying there are not outstanding examples of success. There just aren't _millions_ of examples with 100 millions of hours of engineering and testing behind them. You don't sound like you want to bankroll any testing - I sure wouldn't.

3) If you do a quick and dirty estimate of a standard construction solution, then double it, then factor in time, code compliance, resale value etc... The standard solution might start to look better and better price-wise.

OK, thought specific to your project:

1) First thought I had reading your description: Find out for SURE what's holding up that house! Especially the back part that you're playing with, but all of it really. Light under the length of the rim joist? Or the sill? Bugger. Find out what supports what! Then (if you haven't done it already) poke around _carefully_ at the bottom of the supporting member(s) and see how much it scares you. What's under there and how deep does it go? Basic stuff, like I said - just my first thought. But it tells you what you must support for absolutely sure before you start if you want things above your head to stay there in the same position as always. If there is one 2x rim joist floating above the sill and holding up 8 floor joists toe nailed into it with a wall above... You might, depending on the size of that rim joist, want to give it a little... no, a LOT of help pretty quick. But maybe there's other things that need more help first... Check.

2) Along the same lines: Is the existing structure (above the foundation) sound? My sister's 100 year old 3 story house has 30% or more of the main floor joists splitting and some have sagged at the split more than 1". I spent several weeks last summer dealing with it as part of other work in progress. What has lasted until now undisturbed may not last once you start to shake it's world. If the existing structure doesn't look peachy, that's probably the 2nd thing to fix (after making sure you floor isn't walking on air). Put in sisters, use some steel ties (in the way they're designed for), add blocking to transfer some load... Ah. Transfer load. Say that again. Transfer... Where is that load going? Make sure that _where_ you transfer that load to is sound - don't have something failing at the other end of the house bring your end to ruin.

3) Clay... Water... Water is not your friend here. A good long rain could, like, reshape your world. Probably want to plan for that.

4a) Privacy. Depending on your site it might make sense to do some tasteful, natural landscaping as a first step.

4b) Neighbors. Long thoughtful moment here. Anything you can do or not do to ensure they absolutely are not involved and that their not-involved feeling is all good will - is probably worth it. Truly. You have no idea... Or maybe you do. Anyway. Neighbors.

If I had to bet. I'd say structural cinder block with rebar in it or just plain old ferro cement might be your winning system. However. It might be interesting to study your local permit records at your building department or, better if you're lucky, at the library or somewhere. Or go visit the next county where the ground is the same but the people are not. Access to the documents varies by district and bureaucrat. You're looking for jobs that involved foundation work like you're thinking of and which have plans and possibly engineering documents and inspector's comments. When reviewing foundation work for a friend of mine in San Francisco we were lucky enough to find a nearby neighbor that had done similar work. There were plans and PE documents on file the described a type of clay that likes to swell and shrink with moisture changes. Standard foundations w/footing were not acceptable in that area - it required concrete posts sunk a certain depth connected by grade beams carrying the load. We got a heads up that saved us a lot of time and angst. That was a special case. The idea is that taking a quick look around the local info sources might bring in some good nuggets.

Last but not least. The more informed (friendly) eyes looking at your site the better. I'm not dumb or careless but I've had my bacon saved several times by others who reviewed the job and immediately saw stuff that I'd just plain missed. Get somebody on site to critique your ideas and your job as you go along, if at all possible. An anal retentive hyper active sour puss would be perfect. <g>

Sean said it - houses are real heavy. Oversights, miscalculations and mistakes happen. Count on it. Have the next layer of safety always there. Redundancy is good. Overbuilding is good insurance. Before you get going underneath it, see if you can arrange for that house to _have to_ standup straight even if you vaporize that foundation all the way around the porch! This is not your day job. You have no reflexes, no moves, no instincts about foundation work. You're making it up as you go; that's as bleeding edge as it gets. Paranoia and old lady fussiness and deep and abiding suspicion are you best friends.

Rufus

Thanks Ruffus. All excellent advice. Just the sort of practical (and understandably cautious) advice I was hoping for. (2nd, of course to someone who's already done the same thing.)

I'm looking up that book now and it never occurred to me that I could search for other people's past permits.

I am working on a SketchUp model to post but since it'll be for my own use too I'm taking the time to make accurate drawings. In the meantime, I'll let my convoluted description stand.

Btw, concrete pier foundations are very stable here when the piers are completely surrounded by soil but I'm skeptical of it's stability under a one-sided lateral load. That's why I am looking into wood piers.

Tim

"Full basement" technology is well understood and you're approaching that type of structure here. Lots of info out there. Permit documents are not generally available online to my knowledge - only in person.

IIRC one important factor w/basement walls is the water content of the soil. More water means more pressure. Draining water using foundation or curtain drains, lowering the water table at the wall, doesn't just keep the basement dry - it reduces the inward force on the wall span; but you still have to build for the worst case, saturated soil. Part of the strength of basement walls is the structure spanning their tops which acts to prevent any bowing in at the top; the slab at the bottom does similar duty. More than a few new basements have been destroyed by backfilling before the first floor structure and diaphragm were complete.

Might want to take a few hours and a shovel and see what you find down 5' or so (like water?). Have a care, though. Deeper than that would be interesting but when you're in a hole over 4' you're getting close to betting the farm (that you won't die) if it collapses. In fact the SF plumbing community was sobered around 1986 when a 23 year old apprentice died in a 4' hole when the sides caved in on him. There were people standing right there and they dived right in but he got knocked to the bottom and they couldn't expose his head quick enough - he suffocated.

Best luck.

Rufus

Well aware of cave in risk. Oehler 's recommendation to cut vertical cliff faces rather than tapered raised all sorts of flags for me.

We average 18" of precipitation annually and it's spread over a long "wet" season. People still talk about the 3" of rain we got in one night a few years ago. Soil saturation is very rare here. That said, anything is possible and I want to include safeguards for that possibility. I may dig the entire thing out with a spoon but I'm not planning to do it recklessly. (c: