Supporting Garage Rafters

I have an garage built postwar in the late 1940s that I share with the neighbor since it sits on the property line. There is a 2x4 framed stud wall that acts as the dividing wall for the two properties. Each side is about 21' wide by about 21' deep. The neighbor's side has an additional stud wall dividing her 21' side.

My 21' side has no dividing wall or support for the joists, the bottoms of which are 11' above the concrete floor. There is very little sagging, but I would like to add a plywood floor so I can store fairly light items up there.

But here's my question. I would like to keep the center open rather than build a 2x4 stud dividing wall so I can store an additional car in the garage. I am thinking of a 21' wide I-beam supported at each end by a 6/6 or greater. But I am a novice at such things and thought I'd see what the experts here have to say.

First picture shows the garage. Second picture shows the two gray colored joists that run through the center of the garage front to back and gives an idea of how the joists are spaced.

Thanks.....

vince

I’m far from an expert but will give you my thoughts based on the two photos of what you have now and possibly what you could do.

As it is now it looks like many buildings I have seen built around that time and by todays codes I would say its lacking structure. The rafters seem to be undersized and spaced too wide and the joists the same. The joists were put in there to be in tension and act as a collar tie to keep the front and back walls from spreading and never intended to carry a floor load much more than what you have up there now. The rafters worked well all these years spaced wide in part because the sheeting is those 1” thick planks.

When you talk about a center wall like your neighbor I’m picturing it going front to back is that correct so you can still get cars in.

Either way if it was mine and having 11’ of head room I would want to keep it all clear span inside I hate walls and poles breaking up the area. The front and back walls are the load bearing walls now for the roof and roof load and they have been doing that job for 75 years. I would do something like you were thinking but I would use wood beams and instead of one I would do two right to left inside the building. Roughly divide the depth of the garage into thirds and in your first photo set a post just inside the two small windows on the inside of that wall and two up against the neighbors wall. Make a beam out of a couple 2X’s and then add in new ceiling joists between the ones you have all resting on the beams. Deck it and even add a pull down stair etc.

What I’m picturing is almost like building an outside deck construction only in the building. That way you can calculate the loading based on all the new construction and not add loading to the old. The old building will only be strengthened by the new.

The reason I suggested the fabricated beams is for myself it’s easier to place them up one at a time and screw them together than trying to get one big heavy beam set alone. That and with wood you can screw and drill bolt holes etc a lot easier than with steel.

However you go you should run some numbers based around how much weight you think you will put up there. I don’t know if you have a footer on the end wall to take a post or the center wall I doubt you do so some footings might need to be dug.

Just my morning thoughts looks like a fun project.

3 or 4 ply 2x12 might be enough for a beam across 21 ft.
After that you and your neighbour should consider a fire wall between you. 2 layers of fire rated drywall on one side all the way to the peak.

If (4) 2x12 seem about right for a beam that’s why I was suggesting break it up into two beams of (2) 2x12 each. That would shorten the span above because those look like maybe 2x8 with the shorter span they could be put on 24” centers it looks like, adding one between each joist. The end posts now being 4 could be sized down also.

The fire stop is a great idea.

Bud,

Thanks for the great write up and assessment of the situation. Yes, the garage was built with frugality in mind, as some of the joists and other wood has concrete on it from being used as forms. And the owner of the properties at that time was a noted cheapskate.

You are correct in that the neighbor's dividing wall runs front to back. It really cuts down on the room in each side. I should have mentioned that some of the joists have boards nailed into them that run up to the rafters at the peak of the roof where they are also nailed in. Adding in additional new joists is also a good idea that I was thinking about, just makes sense.

I agree wholeheartedly about the fabricated beams being much easier to use/make, and also like the idea of dividing the garage to space out the two.

I don't have any footers to use. I'll try to find out how thick the concrete floor is.

And Neal, thanks as usual for the beam sizing and fire rate drywall tip, very good!

vince

Oops forgot about bearing. the dividing wall as it was built later will not have a foundation and would need some help to carry the load.

Vince
The kingposts that were added up to the ridge or to the rafters will transfer the weight of the stuff you have on the joists up and back down thru the rafters to the walls. Simplest of truss. To my way of thinking if the roof structure is already overworked when covered with snow you wouldn’t want to add to that with a second floor filled up with stuff. The beam and joist design we are talking about takes the second floor load and puts it out to the non-load bearing sides.

I guess what I’m really suggesting is more of a mezzanine than anything else.

As to support I would want something more than just point loading against the poured floor. Just the framing will be a lot of weight and then whatever you plan on for a floor load rating 100 lbs per sq ft ??? The experts could better tell you if you could do steel plates or something like that to spread out the force. You don’t want to go to all the trouble to build it and have it sink and pull the roof down with it.

Neal, Just to clarify, the dividing wall between the two properties was built when the garage was. The second dividing wall between the neighbor's two units was built later.

Bud,

The garage now has less stuff up on the joists that it had for years before I bought the house. I cleared out a lot of old doors, shutters, roofing, etc. I'll have to get a good idea of the thickness of the existing concrete floor and try to figure out the weight. My thought is that the additional joists and beam are basically to further strengthen the existing structure as there is no visible sagging of the current joists, which are already supported by the walls.

vince

generally if the wall is bearing on a foundation there will be evidence of it as it would have been put in at the time the foundation went in, maybe level with the floor but you should be able to see a different concrete at the wall, more often in older construction the foundation curb for an interior wall would be somewhat higher than the floor.
Looking at the construction, there would be no need for a bearing wall and it is safer to assume it is not there unless you can see it.

There are other tricks to get bearing to that point with out digging up the floor.
We alway put post in under a point load to force the load down to larger footing that was designed to take the load. With out the post the load will try to spread out on a 45 degree angle on each side of the load, which would be fine except for crush factor and other forces that don't allow the load to spread evenly but in concrete low walls may require extra footing for loads but high walls will spread the load over a longer section of normal footings
So with that in mind you could cut like rafters with a birdsmouth cut to lock on the front and back foundation and notched on top to take the beam, say a double 2x8 on each side .
It would be much like Bud's favorate, home made truss.