Advice for replacing post/pier system on 100 year old shed on a hill
Just bought a new home and it came with an old 12×16 shed that has some historic value and character. Sits on a good slope, so the downhill side is about 3′ above ground level while the uphill side, which butts up to an alley, has a foot or two of dirt that has built up above the bottom of the walls, which I will also have to address.
The shed was built with 2×8 joists, looks like 2′ on center – running the long way. Supported by a bit of a hodgepodge of posts of varying sizes, most of which appear to be feeling their age. No diagonal bracing, definitely some off-plumb posts. There are 20 of them total. The floor is also sloping along both axes.
So, the first thing that came to my mind (besides safety working under a stable but less than confidence inspiring structure) was to replace the posts one at time, while at the same time installing concrete piers (what diameter would be proper?) to frost depth. Plan to use PT 6x6s, rabbeted to fit the joists, with diagonal braces too. While I’m under there I would probably sister another joist next to the existing ones to beef up the floor. But maybe that’s not necessary? I would start with the highest point, which is also conveniently one of the easiest to access, and gently level out the floor to that height as I move around.
I’m also not a builder, so maybe I’m looking at this the wrong way and there’s a better way to tackle it. Also, as you move to the back of the building (alley side) space gets tight and fitting a proper pier in would be nigh impossible. Ideas on how to secure/support that side of the building? And also do a better job of protecting it from snowplow berms/gravel from the alley that has apparently filled in whatever gap was there when they built it? Could that side be framed in and a concrete footer and stem wall poured? If it’s important the soil is kind of a broken gravelly shale mix.
Appreciate any thoughts from you folks.
Replies
From what you are describing it sounds like it will be very hard to get access to dig footings at frost depth. In my head I’m picturing that the shed is very close to the ground and not a lot of room to work and as you say maybe not so safe to be under anyways. Do you have the ability to move the shed? At 12x16 a backhoe should be able to drag it out of the way. Will need to brace the structure some when you do it. Can dig proper footings etc and move structure back in place after. Also, you can possibly slide it to a new location that is less complicated and cheaper to deal with. If this is not feasible, you may want to consider removing the floor to get access to joist etc and reinstall after the repair. You can install a basic retaining wall w/ landscaping blocks or wood timber if it’s only a 3’ drop in grade, not worth the expense of forming and installing a concrete stem wall. Also, if you can best to have the structure away from the retaining wall 3’ish, so the structure can breathe a bit.
Here are some photos to make it easier to visualize. It might be four feet of space below it but it doesn't really matter, it's still diggin on your knees type of work. Second one is the best access side, you can see the plywood piece that must have been someone's previous access. Third, fourth and fifth are what's underneath. The fifth photo was taken with the phone held at a bad angle, the posts aren't that crooked. But you can see it looks like they just grabbed whatever was laying around to support the floor when they built it.
So, as might be evident, moving it with a backhoe is probably not feasible, and the floor looks to be old T&G hardwood over planks, so not easy to remove... Do you think a few well anchored and braced piers on the tall end would reduce the risk of lateral movement enough to where working under is a reasonable idea? The lower siding could be removed and those posts dug and installed from outside.
From the comments and descriptions of what you are intending to accomplish,
I would strongly urge you to hire a professional !! You're lack of experience,combined w/the structural/engineering knowledge needed to achieve what you want to do,
is WELL beyond your capabilities, you could easily be injured or worse! You're lack of experience is a significant drawback!! Hire someone, ask if you can work as the laborer
and do some OJT ( on the job training )!!!
Good Luck,
Geoff
That slope really complicates things. Are you on bedrock or is it just a steep hill? To do it all properly it will take a lot of work and in truth probably more cost than the value of the shed. I can appreciate if you are keen on keeping the structure, but would be cheaper and easier to start over. If you are looking for more of a bandaid repair and not wanting to go all in on the repair I would suggest placing footings on the outside of the footprint of the shed, so you do not have to work under. Land posts on top of the footings which will in turn carry girders under the floor structure. Make sure to attach posts to footing w/ proper anchoring and install lateral bracing from posts to girder and double up outside joist (if not done so already) and install a lateral brace from posts to the outside joist as well. I would make the footings at least 12” wide (refer to your local code authority for specifics on footing size). With 12” wide you have enough space to place a jack and jacking posts while placing permanent posts next to that. You should really try to tackle the slope a bit as well, but that will require heavy machinery (unless you want to hand dig for months). It is really a job for more than one person and you should have experience doing this type of work. If it where on flat grade and easier to tackle it might be a good option for if DIY person, but even than a bit complicated. Have you considered saving the siding and boards of value to make a new shed have a similar look to the original? I’m assuming you like the look of the existing and is why you do not want to take down.
Thanks for the feedback guys. In restrospect, Finehomebuilding.com is probably an odd place to ask for advice on this old thing :-) Yeah, nothing is really ideal about the whole setup. I suspected it was more than I really needed to be getting into, which is why I was asking for opinions. The building was used as part of a Korean War-era aircraft defense system and is the only remaining structure from that operation. From that standpoint it would be nice to be able to keep the existing structure. (I have a troublesome appreciation for history I guess). I will reach out to some local pros after the COVID settles down and stay out from underneath it in the mean time. Take care.
A piece of cool history there. If it is considered a historic property you may need to confirm before any work is done with your local historic preservation authority. Also, you may want to seek out funding as it sounds like a structure that the city may value and not want to collapse. The historic authority would be a good start on that as well. It is possible that you might be able to receive enough funding that you will not have to pay much or anything for the repair. You may also want to see if any large construction companies are willing to do the work pro bono. I know that as a contractor I would be willing to work at a reduced rate or work the project pro bono. Your shed sounds like a great piece of Americana that should stay standing. Good luck.
I like the look of the shed and I think you have the right idea to save it. Nothing in the pictures looks terrible and I think you can do a lot to help the structure.
I worked on a bunch of bungalows at a Hudson valley camp that rested on nothing more than 3/4 inch bluestone. A hole was dug as deep as possible with a backhoe the hole got filled with stone and then we placed 6x6 for the bungalow to rest on. The structures were 60 plus years old.
This was far from ideal but it goes to show how little is really needed.
If it were me I would jack the structure up and rest it on cribbing which will give you the headroom to sink sonotubes as support for your posts. 4 ton jacks are not expensive- the cribbing will cost more than the jack.