Assessing existing timber cut roof - assumed timber grade and deflection of purlins

[liam1369]

Afternoon All,

I am assessing an existing property to determine if the roof is suitable for retiling and new sheathing etc.

The property was constructed circa 1860 (In Kent, UK), solid wall construction with a timber cut roof supported on purlins and wall plate.

The purlins are 145 x 75mm and span up to 6m gable to gable.

Does anyone have any advice on what is the assumed timber grade to assume for the softwood? - C12.

In addition, is there a typical defined deflection limit for roof purlins? - Similar to 14mm / 12mm without noggings for joists. What is a typical member deflection limit for supporting an interlocked slate roof? - 15mm?

Some pictures attached.

Any guidance will be hugely appreciated.

Kind regards,

Liam

 

[1503-44]

A soft pine wood bending stress in the US might have a bending stress allowable of 1200 psi
Do you know the wood type?

6m is a very long span for 145mm (6") beam depth. My Rule of thumb for any span is 1" depth per foot span. Of course that is assuming a typical loading.
But even if your beam stress is OK with a very light load, deflection and vibrations could be excessive.

A ball park deflection limit could be span/240.

Just my non-expert, but I think pretty reasonable thoughts there to get you started.
That said, I don't think what you have there would pass a modern building code inspection.
I do recommend you wait to see if you get some other opinions.



Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[XR250]

If there is no deterioration and if the new roof does not weigh more than the old one, I would call it good. I doubt you will ever get the numbers to pan out on this.

 

[liam1369]

I don't know the wood type.

New roof will be heavier as new insulation and insulated plasterboard on ceiling joists below. (Current collar tie).

If numbers don't work then will need to double up rafters or strengthen purlin.

 

[XR250]

There is lot more to this than just individual rafter or purlin strength and deflection. How are your valleys supported? How are the rafters kept from spreading in all locations? Old roof framing can be a can of worms.

 

[dik]

Can you make the new ceiling surface support independent of the existing roof framing. Unless you are in for a lot of investigation (and, the original may not have worked) you don't want to add any additional loading. Even remediation can be tricky and consider unintended effects.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[1503-44]

I'd be seriously thinking of adding new roof framing inbetween the existing elements. What's there now is really on the skinny side.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[XR250]

Does the insulation actually weigh that much? Maybe not worry about that and just support the ceiling boards separately on new framing if needed.

 

[GeorgeTheCivilEngineer]

I think you'll get stuck trying to assess it in depth quantitively.

Performance wise presumably it has done very well. 160 years of surviving storms etc.

Lath & plaster can be 25 - 30mm thick. That's now been removed and 15mm plasterboard is going back up. Insulation is very lightweight but can be calculated.

Old timber will often be using much better stuff than C14. (check out articles on this site

https://www.buildingconservation.com/articles/gradingtimbers/gradingtimbers.htm

&

https://www.buildingconservation.com/articles/timber-roof-structures/timber-roof-structures.html)

Rusting of wrought iron hand made nails would be my main concern. Consider specifying new nails or fixing plates, especially for tension members or where acting in shear with minimal bearing areas. Doubling up timbers if there is a specific concern over deflection or decay.

You could undertake a few isolated beam calculations but the load paths are so likely to be complex it's an exercise in futility. Not saying it can't be done, but doesn't make economic sense.

Edit - there's a water tank that could be cut out to relieve some load.

 

[liam1369]

Hi George,

Thanks very much for your response.

Noted on the nails.

Yes, water tank will be removed and everything fed in from ground.

Concern is a 6m span purlin which sags. I think I am going to propose PFC strengthening beams with steel EA corbel either end to take a bit of deflection out.

Kind regards,

Liam

 

[EdStainless]

Back many years I worked for a structural and a similar case.
House built in 1860's and they wanted to make some modification.
We did some basic calculations and using any modern rating for lumber it wasn't close.
We had a section removed and tested and the properties of course were fantastic (old growth Hickory or Chestnut I think).
As George said, that relieved us from concern about the wood and let us focus on connections and load points.

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