Cottage Addition - Foundation Questions

jregier · March 28, 2020, 07:58:00 PM

Hey all,

Newbie here, I have spent countless hours over the past few weeks reading on the forum, everything from new construction to lakewater supply. I have learned a great deal and very much appreciate this resource!

I just purchased a hunt cabin in the Whitestone (Ontario) area. The current building is 16x24 and does not look to have much of a foundation. IT is supported around the perimeter with blocks/chunks of wood directly on the ground.

I would like to build a single story addition attached to the gable end... extend the 16' width another 24'. The finished cabin would be 16'x48'. The cabin is a 5 km ATV ride through the bush... with some mud holes... so access and ease of bringing materials isn't great. I am really struggling to figure out what my best bet is for the foundation... keeping in mind the access. I am thinking of doing 20"x20" x 8" thick footings 4' down, use angle brackets to fasten 6x6 PT posts that extend 18" or so above grade. Backfill with dirt or sand if I can find it on the property. I would love to do full concrete piers but I think that will be too much concrete to haul back there... need to run the numbers on how many bags. The piers would be 8' OC.. 4 piers long by 3 piers wide. I would notch the posts and set (3) laminated 2x10's along the long sides and center for beams, 2x10 joists with hangers 16" OC. I will work on a sketch to clarify the above.

So.. my questions are:

1) Given the existing foundation looks to be NOT down below frost depth, If I put the addition foundation below frost depth and attach to the existing cabin, is there cause for concern with differential movement? If so what would you recommend?
2) Do you think the footing with PT post is an adequate foundation? If not, what would you suggest?
3) Can someone point me in the right direction on how to correct the foundation on the existing cabin? Without having to tear it down...

I am going to try and attach pictures of the existing cabin. Thanks everyone for your input!

jregier · March 28, 2020, 08:01:27 PM

I can't seem to post pictures.. maybe because I am a new member?

March 28, 2020, 08:11:45 PM

Photo help..... https://countryplans.com/smf/index.php?topic=15096.0

jregier · March 28, 2020, 08:25:14 PM

Thanks! I have the pictures posted on another forum - https://www.small-cabin.com/forum/2_9658_0.html#msg133254

MountainDon · March 29, 2020, 05:03:07 PM

1. What rules or regulations, building codes do you have to satisfy? Understand those post foundations seldom, if ever, meet building code requirements. You do see them; some were built w/o a permit, other places/times the local enforcement is lax and nobody seems to care. And, in some very remote areas, there may be no codes. However, piers or posts have potential problems with movement.

2. What is under the soil? Canadian Shield bedrock? If that is solid rock and not too far below grade then probably clearing the soil away, drilling for rebar to be inserted and epoxied in place with a poured concrete pier would make a solid base to build on.

3. If the existing structure is more or less just sitting on an assortment of whatever you should likely build a separate new structure with a passageway connecting. That can be as short as a wall thickness. If you build a new solidly anchored building and hard-connect it to the older structure there will likely be a different movement. In any event, whatever is done will likely have problems with movement. It sounds like a difficult situation. Are there other cabins in the area? Can you talk with the owners? I would definitely ask the local government department.

March 29, 2020, 07:35:05 PM

MountainDon,

1) I am being told I shouldn't reach out to the building department as they will likely make me tear the existing cabin down because it doesn't meet code and it may be too close to the shore. I'm on the fence whether to discuss with the local authority or not... I am concerned if I reach out to them then they know about the cabin and may open a can of worms. The cabin is quite far back into a bush across crown land, down private ATV trails so the likelihood of them ever finding out about an addition is slim... again this is what I am being told.

2) I will be going there in a couple weeks. I will be digging some "test" holes to see if I hit bedrock. I am currently running on the assumption I am not on bedrock as a worst-case.

3) The passage way is an interesting idea... would you connect the roof?

MountainDon · March 29, 2020, 10:57:29 PM

Well, whether or not to approach the building dept is your decision. I don't know what your officials' departments are like but here in northern NM, sparsely populated in places, crowded in others, they have the most amazing maps. On the assessor's website, I can find details on all the tax parcels, complete with photographs, both aerial and terrestrial. The permit department is in a different building, but they can access the same GIS maps and info that the general public can.

I can understand not wanting to disturb the status quo. If the property is taxable I find it hard to believe they don't know there is a cabin. They may be just as happy to "let sleeping dogs lie". Who knows what may happen if you double the size of what is there? I'm not sure what I would do given that you state a new building would likely be too close to the shoreline. Proximity to bodies of water can spur enforcement. I know it does here in NM. I have seen an unpermitted project stopped and a removal enforced. That may be the exception as I know of another here that is simply being ignored as no complaints have been filed and another where they permit office simply assessed a double size permit fee, then left the owner alone.

Best of luck. With the uncertainties, I'd probably go cheap as possible with a balance to keep it safe.

How old is the present cabin? How level? Doors and windows open freely? Any noticeable tilt to the floor? Does a ball roll to a low spot? If it is not badly out of kilter maybe it would be best to replicate the foundation. I don't believe I just said that; I am usually all for building for the long haul and being legal or at least within the range of what the permit departments and zoning (if any) will let slip by. I do worry about waterways and sanitation though.

As for the roof, I would not join one to the other. I would look at having one overlap the other by a foot or more in height... enough room to swing a hammer and paintbrush.

MountainDon · March 29, 2020, 11:32:50 PM

With a width of 16 feet and a single-story height it is possible to run a beam down each long side and forego the center beam. Less digging and when it becomes necessary to shim and relevel it would be nicer not having to deal with a center beam. Use the AWC calculator for sizing floor joists. 2x12, #2 of most common species will work at 16" OC. The normal floor load is considered 40 PSF with deflection of L/360. See what species you can get locally; if you get lucky and can get hem-fir (northern) you could get by with 2x10 #2.

When building the beams keep in mind to place the splices over the posts. A three layer 2x10 beam is just okay with the 16 foot width. You could bring the beams inside the 16 foot width by up to 11 inches (with 2x12 joists) on each side. Then place the floor joists on top of the beams. That would stiffen the joists some and possibly make it easier to square up the floor.

jregier · March 30, 2020, 08:02:51 AM

I haven't been in the existing cabin yet, so I am unsure how off-kilter it is. In a couple of weeks I take possession and will assess but based on the pictures in the link I sent (did you see them?) the existing "supports" have moved out of level.

I will look up the calculator, I will have snow load to deal with too. I can get douglas fir rough-cut locally, I used that to build a pergola last year. In regards to the joists on top of the beams, is that a must if I forgo the center beam? My only concern with doing that is there will be a step up to the addition as the current cabin has the joists inside the girders/beams. Not a huge deal but might be nice to keep it all level. An advantage I see to building the joists on top is that they will be raised off the ground higher, current cabin joists are only 1 foot in some spots to the ground.

March 30, 2020, 08:06:01 AM

In regards to the GIS maps, I looked this place up and you can clearly see the existing cottage. It falls outside the property lines (too close to the shore) but the real estate agent said the accuracy of those maps is not great and shouldn't be trusted. He also said the cabin was build prior to the township incorporating so it is grandfathered in.

MountainDon · March 30, 2020, 09:25:19 AM

Quote from: jregier on March 30, 2020, 08:06:01 AM
In regards to the GIS maps, I looked this place up and you can clearly see the existing cottage. It falls outside the property lines (too close to the shore) but the real estate agent said the accuracy of those maps is not great and shouldn't be trusted. He also said the cabin was build prior to the township incorporating so it is grandfathered in.

Yes, if the existing building was there when the township was incorporated it would be grandfathered. There will be a limit to what can be done as far as renovations and additions though. The AHJ (authority having jurisdiction) should have a list of what sort of work must have a permit or a list of those things that can be done w/o a permit.

The agent is correct about the accuracy of GIS maps as boundaries can be off. Was no survey done before the purchase?

Regarding my suggestion to look at joists on top of the beams. That is not necessary; just suggested to eliminate the need for hangers and to allow moving the beams inwards a little to reduce span, which may help make a non-bouncy floor. On top vs hung; advantages and disadvantages either way.

The general rule regarding the ground clearance is if the bottom edge of a floor joist is less than 18" above ground PT lumber is required. And if the lower edge of a beam is less than 12" above ground PT lumber is required.

Having a local source for rough sawn lumber can be good. Sometimes the problem can be that a code built structure may need graded lumber. Many mills do not have a grader. Rough sawn won't fit the commonly available hangers. Simpson does make a rough series though.

Don_P · March 30, 2020, 12:24:21 PM

I would talk to the AHJ and find out what you can and can't do. It looks like the existing needs foundation work first, at a minimum, I'm not sure what the black on the joists is, are they sound. I'd think about lifting it to a better height and correcting its issues first. If it is barely over a line it is not large, move it and set it on the new foundation in bounds. It can be lifted and slid on 4 timbers it looks like, if it is close. If there is a sawmill closeby I've cut lots of beams and cribbing for that kind of work.

If you have rock available and moving stuff is onerous do large reinforced stonework, if sand is available locally as well then you are just moving mortar.

I'll take issue with posts/ girders and joists on top, we just walked through 4 hinge points in that scenario, currently you have 2 and looking down that row of "posts" you can see those hinge points have moved.

If you do want to use treated posts get foundation grade treated (UC4 class) and run the from footing all the way up to top of wall, the wall then braces them.