116 year old roof is inadiquate??

[Prestressed Guy]

I am working on a historic project that was built in 1903. I realize that for you folks in the eastern US this is just a pup, but in Washington, this is about as old at it gets. My work is on the floors and lateral to bring it back to level and up to seismic standards. I suggested that the original roof be removed and replaced with new MPC engineered trusses. the AHJ does not want to raise the roof pitch or change the profile due to the historic nature of the building.

The owner asked "Why can't I just jeep the original roof (no rot or decay, good straight grain lumber). After all, it has stood up for 116 years" The original roof is 2x8 ceiling rafters lapped with 2x6 roof rafters at 1 5/8":12 pitch. the total depth at the ridge is only 28.5". I ran a collar tie design and it says 300% of capacity.

Any thoughts.

 

[Prestressed Guy]

Here is the calc for the roof.

 

[JAE]

Is it the upper rafter that’s failing?

For 1900’s era wood you probably have better than modern day DF No 2 but that won’t get you past 311%.

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[Prestressed Guy]

That was how I was looking at it but then there is 116 years of it working fine. They would like to remove the roof down to the framing in sections and replace with 5/8" plywood and metal roof. The resulting DL would be less than it has seen for the past century. All tails are in very good condition with no rot.
The analysis that I did was for tied rafters. In this case, there is a center support from the lapped 1x4 king-post. It sits on top of the 2x8 and is nailed to the 2x6 top chords to make a (sort of ) truss. It is certainly not up to modern standards but has stood the test of time.

 

[Prestressed Guy]

The question that the owner is asking is can the roof structure be grandfathered without modification.

 

[TehMightyEngineer]

I also agree with JAE, that wood is going to be a lot closer to select structural or better. My house is built with old growth timber from the 50-60's era and it's so dense I can't drive a 16d nail into it without the nail bending over. You also likely don't have dimension lumber sizes but rather rough sawn sizes.

I ran a RISA 3D model quickly and I came up with different values. Even using select structural and rough-sawn sizes I'm getting the rafters at 517% utilization. However, for dead load only it's just under 100%.

Do you fall under the IEBC? I suspect you can justify leaving this roof alone given the historical performance. I'd make sure it's tied down sufficiently for wind/seismic and leave it. I assume the 25 PSF "live load" is actually snow load in your model?

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[Prestressed Guy]

Teh: It is old nominal but not rough. They measure 1 5/8 x 5 5/8 and 1 5/8 x 7 1/2". They are very nice looking lumber and very hard. I would not want to nail anything into it and would only plan on using self drilling screws like the Strong-drive SDWS for the seismic and wind hold down. This has worked very well for the work lower in the building.
Yes, 25psf is snow.
IEBC is controlling code. Due to insect and ground-water damage the entire 1st level floor structure needed to be replaced. The second level remained largely intact with new beam line down the center of the 30' building. The studs are straight-as-an-arrow 2x6 x 24' tall balloon-framing from ground level to 2nd floor roof. For at least the past 60 years, the upper level has been clear-spanning 30' with 2.5"x11.5" DF floor joists @ 24" oc.

 

[TehMightyEngineer]

Any chance the original roof was meant to have an interior bearing wall at the centerline? 300% or 500% overstrength seems unlikely to last 100 years. I still say it should be able to be kept based on historic performance but I'd definitely figure out just how it is that it's working. Either it's not really seeing the dead+snow loads you calculated or it's got some other support path.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[Prestressed Guy]

There is no evidence of interior supports for several large rooms in the upper level. The ceiling appears to be original T&G 1x4 pine and does not have any of the tells of removed structure.

On the lower level, I found lines of nail holes were old top plates had supported the upper level joists with a pair of internal bearing lines. In other areas, there are ridges in the ceiling paint that suggest a centerline beam. Photos dating back to the 1950's do not show those supports so they were removed a long time ago. I found that the upper level was used as a civic theater until sometime in the 1980's so they would pack half of the townsfolk onto the floor with a 30' clear span on 3x12 joists on 24" centers. Run that calc if you want to loose some sleep! says a lot about the reserve capacity of good quality wood framing. I will admit that these joists are some of the cleanest wood I have ever seen. You cannot find a knot and the growth rings are very close. It makes our current VG fir look like scrap wood.

 

[Prestressed Guy]

Teh,costal WA has 25psf design snow loads but we sometimes go for several years with no snow at all. When we do get a major snow storm that makes national news like this year, it is 6" that stays on the ground for more than 24 hours. If you are dumb enough to drive into Seattle during one of these events, you will fine hundreds of cars abandoned on the freeway for days afterwards. It is pretty hilarious for someone who grew up in real snow country.

 

[TehMightyEngineer]

I suspect that's your answer there then. Under dead load only this roof is perfectly adequate. It's likely not seen 25 PSF of snow or even if it has it's likely that the dead load really isn't 15 PSF.

If that is the case then I'd say it meets the requirements for historical performance of existing structures. Especially if you're removing the roof and replacing it with a slippery metal roof and new sheathing. Just make sure you're 100% clear to the owner that the roofing contractors cannot store any materials on the roof and must bring their sheathing and roof panels up a few at a time. If it was me I'd also provide a clear report to the owner that the building roof doesn't meet current minimum building loads, that you recommend replacing the roof framing but don't requite it to meet the IEBC, and explain that a large snow event may cause damage to their roof.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[Prestressed Guy]

Teh, excellent suggestion. I have had this basic conversation with the owner but will memorialize it in writing. There are valid logistical reasons to not replace the roof given the length of time that the structure will be open and our uncertain weather. An untimely rainfall could considerable damage to the interior whereas the replacement of the sections of sheathing and a layer of felt for protection can be done in a day. Added to this is the historic council doesn't want the roof profile or structural height changed.

 

[dhengr]

Haydenwse:
I don’t think you really mean “inadequate.” You may mean it does not meet our current design codes and design and analysis methods. Talk with your local weather people for some snow, wind and EQ records data. What are the max’s. and the top 10 records and how do they compare with our current design loads. You may see that the roof has seen loads greater than the current codes would mandate. Is the roof swaybacked along the ridge line, or along the length of the rafters? Are the bearing walls plumb, and straight up at the top pls? Or, do the walls show they have been pushed out due to the rafters settling/spread over time. Look at the connection conditions at the ridge and the wall top pls., are they well cut and well fitted, and tight, and, if they are in good shape and don’t show any slippage or other wear-n-tear, that’s another condition in your favor. I would be inclined to argue that the roof has withstood the test of time (proven itself, over a long time), under some pretty severe loads over many years, and it does not show any significant structural distress. It should be allowed to remain, unaltered.

Some of those old roof framing carpenters really knew what they were doing, from years of experience and apprenticeship. When you consider the evolution of bldg. codes, lumber grading and rules and the changes in the quality of the materials; clear, virgin timber vs. quick growth factory forest materials, or at best some pretty tough looking second/third growth lumber, the allowable stresses or strengths from the two eras will certainly differ by a factor of two or three. They were more concerned with strength and things not failing, than they were with a reasonable amount of deflection or vibration, and they were generally content with somewhat lower design loads. I would have more faith in that roof, assuming its excellent conditions as you imply, and which has been standing for 116 years, than something built by todays builders and their general experience and construction quality which had been designed to using todays codes and materials.

 

[msquared48]

Haydenwise:

I have also seen many of these older structures in the Puget Sound area that do not compute using our current understanding, but still work.

Remember that wood develops a memory and gets tougher as time progresses.

I have seen an unclad 8 foot high wood stud wall from the turn of the century function as a Verendiel truss with only toe and end nailing providing the shear moment connections. That wall bent a steel cantilever beam that was designed to support it.

Bottom line is that the structure ultimately knows more about itself than we do. Some things you simply cannot explain, they just work.

And as far as I am concerned, this turn of the century material is far better in classification than DF Select Structural. I bet it would test at 2400 to 2600 psi, easy. Personally, I would classify it as prehistoric reinforced concrete!

You might want to pull a sample, if available, and have it tested in a lab.

For the record, what was failing? The material or the collar tie connections?

Mike McCann, PE, SE (WA)

 

[JAE]

You might want to pull a sample, if available, and have it tested in a lab.

I think that is your ultimate best option here if you want to be sure.

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[msquared48]

I strongly agree here JAE. I guarantee that there is more to this animal than meets the eye...

Mike McCann, PE, SE (WA)

 

[Prestressed Guy]

Msquared, both. members are showing overstress and nowhere close to the number of nails.

 

[JLNJ]

Is the deck continuous? Could it be helping as your compression element so that the rafter doesn't have to do all the work?

(I.e. the rafters bend and the deck compresses.)

Just grasping here since there seem to be some other forces at work.

 

[JAE]

With wood framing there always seems to be many and varied secondary things going on that we structural engineer-types, who like succinct, distinct, and singular load paths all traced out.

In reality the decking might be holding up the rafters! (just kidding...I think)

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[kipfoot]

One thing to consider in a situation like this: If you're adding insulation as part of this renovation, then you may be changing the loading on the roof. An old roof may have never experienced a full snow load because the surface has always been too warm for snow to accumulate. If you decide to go ahead with reinforcing, LVLs at the ceiling level may be a good way to go.