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

There are many things in life that will sag as the years pass. I for one am not concerned about such things. That said, it is not uncommon these days to hear of people paying big dollars to reverse this common side-affect of age.

Well put, Thane!

Jim

This made my day. Thanks Thane!


Too bad acupuncture and meditation do not work on old structures...

Sounds like in the end it's a balance between how much risk versus cost versus desire to preserve. Who owns it? Who would be liable if something were to happen? Because if it can be shown that the risks were considered, and a decision was made to retain the status quo, and something failed and hurt someone, who is starring down the barrel? What is the risk? Is there insurance? Has the building been insured? Can it be? Maybe not if the insurance company's engineer says it's deficient. You and I might decide to retain the status quo, but it would seem to depend on who is liable to make that decision, eh? It's seems more in the realm of perceived integrity by an insurer, with an engineer who may or may not be familiar with timber framing, and may or may not be overly conservative. I'd use my own judgment to an extent. If strength improvements can be made without being readily visible where they'd detract aesthetically, I'd then probably try to work with the proper engineer to make some improvements in order to get the proper insurance. Take what I say with a grain of salt as I am just being an armchair quarterback and am not an "expert". I'm just hoping to give some ideas to think about.

Jim, even without seeing it I believe it is fine. You have looked at it and I know your character, somewhat, and it leads me to trust your judgement.

I don't think it is broken, it has a clear track record, let it stand, as is.

Perhaps the process Jim is leading the building through is a form of acupuncture and meditation. Keep it out of the hospital and away from the doctors. Has the building been checked with picks to test for rot?

Tim

Thanks guys;

Does anyone have an opinion about how much sag (deflection) is acceptable in an old building?

Jim

Just my opinion but I don't think sag of any amount has any (zero) effect on the strength of a piece of wood.
Whether the sag/warp/bend took place over a hundred years of hot, cold, humid, again and again... or a few hours in a steamer then bent into a shape by a skilled hand. The worst thing you could do is to try and straighten them after it took 100 years to make them the way they are now.

Hi Jim & Thane,

As sag deformation increases then a rafter will be subject not only to axial forces but will also be subject to bending due to the sag offset. This type of creep can be progresive and might eventually result in buckling of the component concerned.

I think that quantitative measurement and calculation will help demonstrate the suitability or otherwise of the deformed rafters.

Regards

Ken Hume

Regards

Ken Hume

Ken,
Are you theorizing or have you seen this happen in the real world. I have seen a lot of really saggy rafters but none that have failed as a result of this.
Jim,
The roof is not going to collapse tomorrow or next year but to make some people feel better, maybe an annual inspection where the sage could be measured and recorded would be warranted.

Hi Thane,

I have seen plenty of progressive roof collapses which have been the result of all sorts of failure mechanisms though generally it takes a combination of effects to be at work in order to to give rise to catastrophic failure.

What is really important for the long term survival of a roof is that secondary load paths are available to allow for load shedding and sharing. This will help prevent a localised failure of an individual component from progressing to become a disproportionate failure.

Its also worth keeping in mind that most rafters will contain defects such as knots and these are more likely to be the cause or focal point for the start of a localised failure.

I have taken some shots of roof, rafter, crown post and purlin failures and I will try and look some of these out to post.

Regards

Ken Hume

Well, in terms of modern deflection codes under L/180 limits for a roof with no ceilings, most deflection limits would range from 0.50" to 0.75" based on ground snow loads of approximately 40 psf.

It's doubtful larger timbers would exceed these limits, unless their spans are quite long.

Hi Patrick,

We are probably talking more about "creep" here and that is not quite the same thing as a new build "deflections". "Creep or sag" is a long term effect that will more likely take centuries to manifest itself in a negative kind of way. For example suppose you have to replace a rafter in a centuries old church that has a badly deformed roof - would you use a dead straight rafter in full conformance with code requirements or would you try and use the same scantling and deflected form as was currently present in the roof ?

Regards

Ken Hume

Jim –

Some thoughts, never mind the proposed work, has the recent work perhaps already caused more problems than it portends to remedy. I'm guessing the roof was originally wood shingles, which tend to let the snow go with some regularity (mine on the house here, do so two or three times a winter, always with a roar that has first timers thinking the whole house is coming down) keeping the roofs liveload minimized to some degree through the winter. Granted the three-tabs the architecturals are replacing have probably been holding snow the whole winter long for some years, but the laminated construction of the architecturals suggests to me that they also add more dead load per square than any type of roofing the roof system has ever before carried.

Is the new sheathing lighter or heavier than the old ? How thoughtfully was it applied ? I have too often seen historical work poorly re-sheathed, I'm guessing because material takeoffs are done before it is understood how the framing is configured and false assumptions based in ignorance are made. And at times, all the rules of reasonable stagger are thrown aside to reduce the unexpected waste found when purchased lengths do not match real world on-centers, and no thought is spent on how break on break sheathing is concentrating loads to members, members which prior to these “improvements” have to date never been asked so much of.

It may well be the churches frame is carrying more load than it ever has before, and perhaps doing so in ways that it never has before.

And I have to ask, why does the cost of repairing the damaged plaster in the nave fall to the church and its congregation ? It seems to me it belongs to whomever employed those who dropped the boards, or their insurance carrier should they choose to claim it. With potential profit comes potential risk, one of the reasons folks hire professionals is to avoid such risk. Unless there was some negotiated agreement to lower the price, I believe it should not be the congregations problem.

This has been an interesting exploration of engineering in old buildings for me. I need to look into the meanings of creep, sag, and deflection to make sure I understand them in the same way an engineer would think of these terms. I have never defined a tolerance for how much sag should be acceptable in an old frame.

Will;

The reseathing was done with 1 x 6 pine, spruce or eastern hemlock boards and probably all of the joints were broken on the principle rafters rather than being staggared, which I have no problem with in this case. The edges of the boards are tight together months after the work was done so the boards must have been suitably dry when applied.

You make a good point about wood shingles shedding snow and asphalt shingles keeping the snow on the roof. I think the new sheathing did not change the loading on the roof. I have found that roofing nails pull out much easier of new wood than old wood, so I still prefer marginal old boards to new boards.

Being in the trades I hate to see another tradesman take a loss on a job, but I have wondered if the Church committee has discussed holding the roofing contractor liable for the ceiling plaster repairs. I will ask them.

I have been looking into the history of this building and the architect who designed it designed about eight others in the area in the 1830s. Now I am curious to see if any of the other buildings were built the same way.

Thanks;
Jim

I googled some key words and found this paper which gives a good discussion of "creep".

http://www.prugarinc.com/Prugar_my_roof_is_sagging.pdf

Jim

Jim,

If the damage was caused by them they are liable. That's exactly why we pay so much for insurance. I hate it for them, but the ceiling would not be damaged if they hadn't been there...

I learned something important!

I was completely unaware of the engineering term "creep" and so my initial argument that a building that has been around for 100 years or more has proven itself and will keep on surviving, unless some deterioration happens such as insect damage or rot, was wrong.

Creep can continue to happen over time so a sagging rafter can continue to get worse and eventually fail depending on a number of factors in addition to rot or insects, therefore I can't argue that a sagging rafter has equalized or bottomed out and will be fine.

A sagged rafter may be structurally acceptable, but it should be monitored to see if the sag is growing. Of course, there are the asthetics of sagging rafters as another issue, too.

Many thanks to the professional engineer Janet Kane for returning an email I sent her with a lengthy email explaning creep to me.

Many thanks to you too!
Jim

Some follow up questions on creep. It sounds as if it is progressive.

At what rate does it progress? If you went back 50 years, or half the life would you find half the creep? Or does it get worse as it ages, it creeps more as it ages, if you went back 25 years would you half the creep or 3/4's. Is it going to get worse quicker now that it has deflected? Or is it going to creep at a slower rate?

The cause of creep, an undersized member, live and dead loads, gravity, the slope of the roof, anything else? I am curious if heat has anything to do with the cause, roof members are subject to greater heat, does the majority of creep happen during the summer or winter with snow loads?

I presume creep can be seen in other areas as well such as in a post with a long knee wall, bending the post above the tie. You would also see this in the outward bow of the top plate in the same building.

Tim

Hi Tim,

You pose excellent questions and its not surprising that only a few really long term steady state studies have been undertaken in this respect. I know that TRADA in England did a 10 year continuous load test on a truss and then tested it to destruction and found that there was no appreciable loss of strength in the truss.

The members that are most vulnerable to creep are the long skinny ones like rafters, posts or beams where loads applied to same result in measurable deflections. A measure of this effect is determined by dividing deflection by length giving rise to an approximate measure of "strain" (providing the beam is elastic).

Any structure subject to creep is in effect trying to stress relieve itself and as mentioned by me many times on these pages it is important to have secondary load paths available that can take the strain as a structure relaxes upon itself. This is sometimes referred to as "beneficial creep".

In simple terms deflection is dependant only upon load in an elastic structure whereas creep is a measure of deflection that is also time dependant for that same load.

The various variable loads and conditions that you mention above will play a part though it would be very difficult to quantify the effect of each of those individual factors. Unfortunately really old structures (500 years +) that have deformed badly also tend to have had other factors at play in arriving at their current day deflected form in addition to simple steady state loads.

This would form a good dissertation topic for Methusalum.

Regards

Ken Hume

I left out another aspect, a green stick is more noodle like than a dry stick. It seems like a stick of wood goes through various stages, green/flexible, dry stiffer, old and under load/creep. Creep won't spring back, it comes across to me as a memory which is installed.

I think this statement bundles it nicely, Ken.

"In simple terms deflection is dependant only upon load in an elastic structure whereas creep is a measure of deflection that is also time dependant for that same load."

Things just get tired.

Tim

This is some related cut and pastes, Chapter 4 of the wood handbook also has a good discussion of creep under "duration of load".

Wood is considered a viscoelastic material. This viscoelasticity explains the creep phenomenon in which a given load will induce an immediate deformation, and if that load is allowed to remain on that piece, additional secondary deformation (i.e., creep) will continue to occur over long time periods.

Mechanical properties deal with stress and strain relationships that are simply functions of chemical bond strength. At the molecular level, strength is related to both covalent and hydrogen intrapolymer bonds. At the microscopic level, strength is related to both covalent and hydrogen interpolymer bonds and cell wall layer bonds (S1–S2 and S2–S3). At the macroscopic level, strength is related to fiber-to-fiber bonding with the middle lamella acting as the adhesive. Thus, any chemical or environmental agent that affects those bonds also affects strength.

Ordinary climatic variations in temperature and humidity
will cause creep to increase. An increase of about 28oC (50oF)
in temperature can cause a two- to threefold increase in creep.
Green wood may creep four to six times the initial deformation
as it dries under load.

Me here,
Water is the most common environmental agent affecting chemical bond strength. If the interpolymer hydrogen bonds (these are the linking arms between strands of cellulose within the cell wall) are busily occupied with H2O...bound water, then the number of hydrogen bonds holding cellulose molecules to each other is reduced. The cellulose molecules a much more free to slip and slide in relation to one another, like a wet noodle. Green wood is about half as strong as dry for this reason.

Chemically when you bend a stick of wood the polymers of cellulose slip by one another. The hydrogen bonds linking one molecule to the next are drawn tight, break, align with the next bonding site and rebond, when you release the stress the process reverses. Get the water out of the mix and the cellulose is more thoroughly cross linked.

Keep bending the stick. When the carbon-carbon and carbon oxygen bonds begin to break the deformation is unrecoverable.

Tim;

I saw a graph in my reading about creep that seemed like it gained at a steady, very slow pace if the temperature and humidity variations remain consistant. One of the warnings of a "creep rupture" failure is an accceleration of the creep. In other words, if you have a saged rafter, monitor it. If the creep is accelerating, evacuate the building.

Creep also happens to metal and concrete.

Jim

And old glass.

Tim

I think this image both exemplifies Creep and the forces we sometimes ask of our framing greatly -





This Post had been two panel points in from the end of this bridges span, (so the load was not even as great as asked of the Posts farther from midspan) yet 150 yrs of constant load imparted from the Brace (compression diagonal) had slowly created this much deformation. Glare has the ruler unreadable but that string is at 1 ¾” at the Bottom Chord dap, (just above where it's pictured) and the braces CE is only 12 ¾” (½ way up that “joggle” that is the Braces abutment) above the Chords reciprocal dap which will buttress the load. Of course this ones problem is exacerbated by neglect and other extenuating circumstances, so this example is not representative of all Burr or MKP trusses. But it is super creeped out

Sorry about the techno-speak, can't think of a better / simpler way to describe it.