Existing wood roof design?

[SteelPE]

I’m not really a “wood guy” but I have been asked to look at a project for a relative (my parents). The roof on a house that they own is sagging. It has been sagging since I can remember. They are looking to insure the home with a new company since the old company does not operate in this state anymore. The new insurance company is looking for a letter stating that the “roof is not going to cave in”, whatever that means. The problem is that the home was built in the 1720’s and as I remember, there is quite a bit of charring on the underside of the roof.

My first step was going to be taking some field measurements, but what values should I use for the wood…. And what values should I use for the loads to prove that the roof is “not going to cave in”? I suppose I could use the values for snow load in today’s code as a beginning point but I am not even sure I am going to get it to work.

The client is already very irritated with the request from the insurance company. What an odd predicament?

 

[msquared48]

A few things:

1. You are probably dealing with creep in a structure of that age.

2. As for the grade of wood, go to wehat species is common in the area for the time, use the best grade and back off a couple of ratings to use the stress for the analysis unless you can justify a higher value from testing. Your local jurisdiction may have guidance in addition to that they require. Check with them.

3. Charring obviously implies fire in the past - just deal with the section remaining, obviously.

4. To make points with the plans checker, tell him this is not a "Cave" that you are analyzing, but a wood home. The roof will analyzed for the potential of failure, but not "caving in" as that is not appropriate.

Is tghis a log structure with a long log ridge beam?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[SteelPE]

msquared

Thanks for the advice. I was kind of blind sided with this when I went to visit them this morning. My knowledge of the structure comes from a renovation I did with my father 20 year ago when I was 14. I seem to remember the decking being 3/4" sawed planks with main rafters at about 2-4' o.c. The house is balloon framed with 1" thick planks on the outside walls as sheathing. The renovation consisted of a complete gut and construction of virtually every aspect of the house except the exterior skin and roof. The historical society in town would not have been happy when we were done. Also, the roof at the time had 3 layers of shingles. They were removed and the whole roof was sheathed with plywood with a new layer of shingles at the time.

I do some work with a company that specializes in wood structures. We tend to team up when projects are outside of our area of expertise so I will ask them for some help as well.

 

[dik]

If sagging, it usually means that the horizontal load capacity has been compromised. Check the collar ties in the attic, or possible ceiling rafter to roof rafter connection.

Difficult to correct because of the large forces required to 'raise the ridge'.

Dik

 

[SLTA]

1720 = lots of time to creep. I know no one wants to admit "defeat", but get your parents to hire a structural engineer who specializes in historic preservation work. Are they in the USA? If so, would you post where they are? I can get on APTI.org and make a few suggestions for local folks, if you don't know anyone there.

 

[msquared48]

Although I agree with the creep issus, it might be well to point out that there were a lot fewer creeps in 1720 than there are now.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[SteelPE]

Admit defeat? I haven't even started yet. It would be pretty embarrassing for me (I am a licensed structural engineer) to recommend hiring someone else. Although as noted above, I am going to seek advice from a company the specializes in wood construction.

I'm just asking where to begin with regards to wood grades..... and what does "not collapse" mean to others.

I have dealt with the IBC and IEBC many times. This time it is a little confusing as the structure technically doesn't need to comply with today's building code requirements as it was built 300 years ago and we are not adding any load to it. So the structure may not technically comply with the snow loads required today.... but it doesn't have to.

 

[SLTA]

SteelPE, this is exactly the issue. Working with structures that are 300 years old is TOTALLY different than working with new structures, or even those that are 100 years old. Please know I have nothing but the highest respect for your abilities and mean absolutely no disrespect by recommending you get a hist pres engineer involved.

Be careful with the wood construction company - if they don't regularly work with historic structures, they won't know what's up at all.

Wood grades - usually the wood used back then was amazing old growth and very solid (hence the fact that it's still up!). There are two approaches I'd suggest - take a core to get analyzed, or conservatively use the current doug fir values. I'd be less worried about analyzing the existing structure for today's requirements and more interested in making sure the existing structure is still in good shape - no rot, insect damage, water damage, connections not pulling apart, etc. "Structural Renovation of Buildings" by Alexander Newman has a great chapter on wood building analysis and renovation and would be a great place to start.

 

[woodman88]

The request by the new insurance company sounds like they want to collect the fees and have someone elses insurance pay for any damage. I would think it would be best to carefully read the new insurance policy and then talk to a lawyer about it.
Or better yet tell the owner to look for another insurance company.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[SteelPE]

woodman

I said the exact same thing. If I write a letter and something happens they will just go after my insurance. It seems like the insurance company is reaping all of the benefits (yearly premiums) while not taking as much risk.

 

[dhengr]

SteelPE:
To satisfy the insur. co. I would argue that that 290 year old roof system has withstood the test of time. Your new roof and plywood sheathing likely weighs less than the three layers of shingles did, and has strengthened it in other ways. And, certainly within the life of that roof it has seen some local record snow and wind loads, maybe investigate this history a bit. I would like to know when the fire damage was done, thus the years since that damage, any record snows since then, and still doing well. You will invariable have some swayback (sag), deflection and creep with that much age. You will also find some very dry brittle wood in those rafters. Some analysis of the existing structure is quite appropriate, and maybe some sampling, but a good deal of engineering judgement is appropriate too, and I would certainly not expect (or need) this roof to meet today’s code, to be grand-fathered in, as a reasonable insurance risk and reasonably safe roof. To some extent you are always sticking your neck out a bit on jobs like this. In this case it might be an unseen existing material condition, but it has withstood the test of time. In new construction it is the misplaced rebar or missing connection bolt that you didn’t see, but shows up ten years later when that beam is finally fully loaded for the first time. You are being paid for your good, reasoned, engineering judgement, not to be all knowing and almighty.

There are many structures which should be seriously questioned before they are insured under normal insurance terms and conditions. But, it doesn’t seem to me that this building which has been standing for 290 years should be one of them unless something serious is uncovered. Something more serious than some permanent set in the rafters. I would inspect the entire roof system for general condition, and want it to be sound. I have argued for years with insurance company clients, after the fact, when we’re inspecting a failure, about why they insured a building which was so obviously structurally deficient. They tend not to want to answer that question, they want the premium and the business, and will fight the claim later, if needs be. You should also make it clear that you are not the insurer of last resort, but rather, you are giving your engineering opinion to a reasonable degree of certainty, based on such and such investigation, testing, analysis, etc.

 

[TLHS]

A roof that's stood up through a few hundred years has likely lived through a couple of events that would be design level events to a modern code. The fact that it hasn't failed in that time would be enough for me to realize it's solid unless there's obvious distress or there have been some modifications. It may not be up to modern code, but it should meet a reasonable definition of 'safe'. The question then becomes how you convince the insurance company of that without taking on the liability.

 

[SteelPE]

TLHS,

I would classify last winter as a really bad one. We had a bunch of structural collapses in the area including a bunch of metal buildings and wood trussed roofs.

I will make a site visit sometime this week (it is only about 10 min away) and post some pictures about what the insurance guy is concerned about. Obviously my inspection is only going to be visual and I will not be able to see the rafters/ceiling joist connection as that will be covered. I just get a little worried about these things as I know the insurance company is trying spread the risk.

 

[graybeach]

I agree with dik that the sagging could be due to a problem with the ceiling joists or collar ties. If possible, you should consider making an opening in the ceiling at the worst sagging location and inspect the rafter to ceiling joist connection. Check any splices in the ceiling joists as well.

 

[MiketheEngineer]

Even though you are a structural - that doesn't mean you know or understand everything. Example - I am a structural but would probably never tackle a major bridge!!

So it might make sense for you to get some help with someone who is more familiar with this type of thing - like me. But I live in a part of the country where maybe one or two buildings were built in this era!! I am assuming you are on the East Coast..


Get with a truss engineer - they should be able to help and good luck.

And YES, creep is the the most likely cause of what you are seeing!! Unless the fire did more damage than you describe.

 

[connect2]

Don't forget to check connections. They are most problematic. Recently did a church where the bell tower spire blew off, 150 feet at the cross. Had been there 125 years. But the wrought iron nails had corroded, cause of corrosion was the wood.

 

[SteelPE]

Update,

So, the client (my parents) decided to get one of their friends to take responsibility for the existing roof. He signed a statement that I probably would have changed the wording around a little. I don't know if he actually did any inspections or not but he was present during the house renovation 20 years ago so he is familiar with how the structure was built.

 

[BAretired]

If anything goes wrong with the roof, he is not likely to remain a friend for much longer.

BA