Engineers, both professional and arm-chair;

Is it a reasonable arguement that a relatively simple, 100 year old building (or older) has proven itself structurally? In other words, an old building that has not deteriorated from rot and is not showing any signs of failure (some minor sagging allowed)can be left alone?

Thanks;
Jim

Sounds like a vague question to me. When you say "proven", I think of load testing. In other words what the building was designed to handle as far as loading. Was it loaded to this capacity? Are you planning to add loads for future use? If it's say a barn that will be re-purposed as a dwelling, then engineering calculation should be done to determine it's safe load capacity, including, snow, live, and dead loads. A careful inspection would also be in order. We've seen pics here on the forums of interesting partial failures and old repairs. If it's being moved, every piece should be properly inspected and scrutinized, and use new pegs.

If it's just a structure that you are leaving in place, a proper inspection for safety purposes is in order, if anyone will have access to the structure. From the way you posed your question, it sounds like someone, uneducated in timberframe or building, is afraid of a given structure. Go ahead and have it inspected by a professional timber framer, or an engineer who is experienced in timberframe building. In Maine, you won't have problem finding one as I know there are plenty of experienced people up there. Just ask.

Hi Jim,

A 100 year old building is still a relatively new building.

Brad has highlighted some pertinant questions above which would really need to be answered to advise you further however if we are talking about a status quo situation then why would you want to change it anyway ? What is your underlying concern ?

Back to the armchair !

Regards

Ken Hume

Many buildings never see the design loads we now build to resist. Since I considered it an important building, my outhouse might make the century mark. It would probably be a poor canidate for adaptive reuse though. Is there a change of use. I've been in old buildings that have survived but were not well built, you gotta figure the beetles are getting tired of holding hands and the angels are getting tired of flapping. It wouldn't take much to tip the balance. It sounds like your building has proven itself beyond warranty though.

Kind of on the same line of thought somehow, what is the real warranty period on a building;
The carpenters in my county with less than 15 years experience just saw the first significant snowload on their work and it was about half of design load. I remember the previous big snow well, the roofs of the grocery store and the Nautilus plant collapsed. When the leaking gas in the plant exploded it cleared the snow off the roof. It is entirely possible to work a lifetime thinking one is doing well, something like Haiti or Andrew comes along and proves that thinking wrong. The test can come after we have a whole lot of work out there.

It was a vague question, but I am talking about a specific building and really only about the roof load. My church building just got a new roof. The old asphalt shingles were put on without underlayment and had been leaking for years so the sheathing had deteriorated and all roof sheathing was replaced in addtion to the new roofing. An engineer was called in and recommended reinforcement of the common rafters between the king post trusses. The roof is not perfectly flat but not unreasonably bumpy. There is no evidence of structural strain or failure, except the minor sagging.

I have seen severe sagging in rafters and purlins on old buildings, but this one is not bad. It has survived all of the extreme snowfalls and wind storms since 1832. Why would it now need to be modified? The congragation is small and not wealthy. It seems that "status quoe" is what is "best" for the current situation.

I think it is a common occurance for an engineer or builder who usually work on new buildings to look at an old building and say it need a lot more work than it really needs. I think this is the case here. Sometimes the new work causes problems!

If this frame was being built today I would agree the design was not the best because the common rafters land on a small flying plate and are quite long, with only one purlin supporting them about one-third from the bottom. I am arguing that in this case the risky design is adequate as proven by the length of service.

Trasferring the load to the main plates and anchoring the common rafters to the plates is good advice, but is it necessary?

The replacement of the sheathing resulted in cost over-runs and dropped boards damaged the ceiling plaster in five places. The ceiling will cost tens of thousands of dollars to repair and paint. This is where the congregation should focus there attention and cash.

I have over 25 years construction experience, over 15 years with old timber frames, but I am not an engineer. I am willing to say the status quo is appropriate for this building knowing that another hurricane could come along and tear the roof off, but the chance is so low it is not worth the effort. Roofs of old buildings do not need to be perfectly flat.

Perhaps "status quo" is another name for preservation carpentry, my main interest.

Thanks;
Jim

Jim, as I read, the first thing that came to my mind is a metal roof. You are familiar with them, as I have seen of late, the ridge being a bugger. This type of roof releases snow load and is lighter over all.

Has the church looked for an other opinion? How about a consensus from a few different timber framing engineers and restoration guys, like yourself?

Tim

It could very well depend on the size of the rafters and the spacing between them. If the rafters are, say, 4x8 and spaced no more than 24", then they should be more than adequate to handle Maine snow loads, especially since you say there is very little sagging.

I am having a bit of a tough time understanding the concept of "reinforcing" a rafter.

If the rafters are spaced fairly far apart, I'd suggest a simple solution of adding some rafters in between the existing ones to increase load sharing.

Patrick

Hi Jim,

As allways it would be wonderful to see some pictures.

At the risk of being controversial I would state that in most old buildings the common rafters are not pegged to the wall plates. There is one exception and that is the last full set of common rafters before the start of a hip (the singles).

Sag is to be expected in 100 year plus old buildings and is in itself not generally a cause for concern but tends to happen due to scantings being slight or sparse, loads being relentless, the original design being less than optimal or failure of key parts of the roof causing overload of remainder.

Rather than argue on a qualitative basis why not ask for a quantitative analysis to demonstrate the original defficiency and hence justify the need for reinforcement of the design.

It also has to be recognised that when a major intervention is contemplated then the completed work should be sufficiently robust such that the roof will last for another 170 years.

Roofs are repaired, altered and reinforced over the centuries and generally future generations will be quite accepting of these intrusions especially when today's differences of opinion are no longer a material issue.

Regards

Ken Hume

So they already replaced the sheathing and put new asphalt shingles? So to do any of this rafter work, you'd have to tear that off again? If it ain't broke...if you think it's at all questionable, you could always get creative in reinforcing, as long as it doesn't show. From what you describe, I'm not sure if it has a ceiling at the rafters, or at the level of the ties?

Tim;

The roofing replacement is done and the architectural-style asphalt shingles looks great. I am looking for a consensus from you and the other forum participants.

Hi Patrick;

Most 18th and 19th century buildings in New England have a rafter spacing around three feet or common purlins around four feet apart. This spacing works because the builders used wide, dense boards which can handle that kind of span. It does not work as well using modern lumber since modern lumber is less dense and less likely to be more than 12" wide. I would say that practically no old building meets modern building codes for rafter spacing or roof design load.

The building I have in mind has king post trusses about ten feet apart and two common rafters between the trusses. The common rafters are supported by one purlin framed into the principle rafters. More purlins could be added between the principals to support the commons, thus "reinforce" the common rafters.


Ken;
I am glad you said the following, since I agree: "Sag is to be expected in 100 year plus old buildings and is in itself not generally a cause for concern but tends to happen due to scantings being slight or sparse, loads being relentless, the original design being less than optimal or failure of key parts of the roof causing overload of remainder." Astetically, sag can be unacceptable, but I do not see it as a structural failure in old buildings.

I am not trying to justify the need to fix a building. Just the opposite; I am arguing that a building that was framed "less than optimal", but has survived over 100 years has proven itself and does not need to be reinforced. There is not a change of use for this building.

Brad;
The best time to have worked on this framing would have been when the framing was exposed during resheathing, but it can be done without opening up the new roof. It would be hard to anchor the rafters from the inside, but with some planks in place, someone can crawl to the rafter feet and attach brackets from the inside.

To put the uplift work in perspective, a new code went into effect reciently where new construction within four? miles of the ocean needs to have some serious anchors tieing the building to the foundation and rafters to the frame and every level in between. This requirement is not in effect for existing buildings, but uplift is on the minds of engineers near the coast. Perhaps this is why the engineer wants to add brackets to some of the rafter feet in this case.

My original question was vague because I am looking for a very general opinion. Look at my question in the big picture, not at load calculations or individual pieces of a frame. Let me restate the question in the common saying: "if it is't broken, don't fix it." The key word here being what "broken" means.
If an old roof sags a little but does not have any other problems, does it need to be "fixed"?

Thanks;
Jim