I have a house that is about 100 years old. The roof sags like a foundered horse. The "trusses" are 24 o.c. give or take a few inches. The shingles have been replaced. There is currently only one layer of shingles on, probably less than 10 years old. Having put on new siding and windows, the roof now sticks out like an eyesore, so I need to do something. The second floor is currently gutted so I have complete access to the roof on the interior also. From the inside you can see that someone has put OCD (?) board over the original barn slats which were used to construct the roof. I'm thinking about putting a layer of metal roofing over the shingles. Is this advisable? Should I be concerned about a sagging roof, or take the approach that it's been there for a hundered years and will probably last another hundred years more?

First, I doubt that a 100-year-old house has "trusses". It probably has simple rafters. If the sag is sort of saddle-shaped, it's probably because there are no rafter ties, usually also doubling as ceiling joists, at the second-floor ceiling level, so what has happened is that the natural thrust of the rafters has pushed out the top of the supporting walls. The reason for the saddle shape is that the gable end walls brace the end rafters and resist the natural thrust, so the roof only sags in the middle and not at the ends.

All that explanation is fine, but the real question is, what do you do NOW? One answer is to let a 100-year-old house be a 100-year-old house, and live with the sag. Beyond that, correcting the sag opens up a six pack of cans of worms...jacking the ridge back up, bracing the rafters in their new position, and then patching the myriad of cracks in the second floor walls and ceilings that will surely result. A serious question is, what do you jack against? The jack doesn't care whether it pushes up or down, so jacking, unless carefully thought out, might result in more damage to the lower floors than you want.

Metal roofs go well over existing roofs quite well if they are sound. I wonder, though, if your ridgeline looks like a sway-backed mare now, will it look like a metal sway-backed mare if you just roof over it??

I really don't like the idea of shimming the new panels to make up for the sagging, but also don't know of any other way to make it look good without getting into some major reconstruction.

Thanks for your responses. The roof actually sags between the rafters, so it's more like a couple of waves coming into the beach than a foundered horse, I guess. I figured that over time without the proper support that the pressure just sort of pushed the walls out and spread the roof as far as it would go. There are ceiling ties that the plaster and lathe were attached too, but obviously it hasn't been enough to hold the shape. The exterior walls of the house also lean outward a little bit, so your synopsis seems very accurate. One of the contractors suggested a metal roof with shimming, so I guess that is the best course of action to take here. (You might remember this is the same house with all the termite damage in NY state which is now being treated by an exterminator). Again, thanks! Edog

Oh, I didn't recognize the name but now I remember your earlier posting. Best of luck with the termite treatment and repairs! If the walls are already leaning outwards noticeably, I'm not sure that adding more weight to the roof with the metal and the shims would be wise. I like the metal roofs, though, and am considering doing the same for my house in the next few years, but I think I'll rip off the old crap and redeck first since I have similar swags between the rafters to that which you describe.

You've taken on a rather daunting and commendable project. But it sounds like everything you're doing is an improvement. I suppose it's now up to you to see that the structure makes it for the next hundred years (not to put any more pressure on you!)

Hmmm, I didn't think about the weight factor. Perhaps I should put more ties in to try to hold the sides together better???

I don't think there's any way to repair all of the settling and other age and gravity damage that may have taken place over the past 100 years, but as I said earlier, everything you can do to improve and upgrade the structure is another mark towards the good.

The roofs of that age that I've worked on have been surprisingly cheaply built, often just small diameter poles for rafters and short, pieced together 1-bys for sheathing. The OSB overlay done before you bought the house may be the best effort towards structural integrity made to the building since it was first built. I don't know without seeing the house, and I'm not sure I would know much more after a close inspection. But anything you can do to help hold the walls erect and strong will help; after all they are holding up the whole house, and even a small angle outside perpendicular greatly reduces their upward strength and increases other stress factors on the entire structure.

I wish I could be of more help. It sounds like you have your hands more than full already.

Geez, I wish you could see this. It sounds like you have quite a bit of experience with older houses. I'm not sure it's worth sinking more money into, but I can't afford to tear it down and start again, so I'm trying to make it as nice as I can for as little as I can get by spending. If you're ever in upstate NY, let me know!!! The work is going to be slow moving, one step at a time. Baby steps... Thanks for your input.