Softwood grades

I'm trying to gain an understanding of the various grades. There are H-grades (which relate to treatment - hazard levels). Then there are F-grades. Somewhat confusing.

Here's a snippet from our span tables. My interpretation, in the highlit example, is that if you use F5 then the largest span must not be more than 1100. With F7 and the same size timber, the largest span can be no more than 1200. And similarly, if you have MGP10 the largest span can be no more than 1400.

This implies F7 is stronger than F5, and MGP10 is stronger than both. To put it another way, if the span is identical for all three, the load-carrying capacity of F5 will be the least, with F7 being better, and MGP10 being able to carry the most load.

Tell me if I've missed something.

Here's a doc I found while searching the web - it says F5 and MGP10 are the same. Huh?

https://www.melbournetimber.com.au/p...-treated-pine/

Attachment 537758Attachment 537759

In the basics yes you are correct.

Here is another resource worth having a read of.
https://barwontimber.com.au/pages/st...DR9eAczJgjeTz_

F rating originates from many years ago but is still in use.
MGP stands for Machine Graded Pine, it is a newer standard.
Both tell you the amount of force or load they can sustain for a given amount of deflection, the higher the number the higher the force or load they can with stand. As they are two different standards measuring similar properties of the timber beam you can interchange one for the other where the rating is equal or greater than the one it is replacing. A timber beam will only be measured and graded with one of the standards.

Have a read of this https://timberlinkaustralia.com.au/n...-in-australia/

It will give you a bit more nuance.

Thanks for the links. They add to my knowledge, but they also throw some mud on the issue.

It’s interesting to know that F and MGP represent different grading systems, and that MGP is a more comprehensive system.

The Timberlink article compares F7 with MGP10 in regard to span. It says F7 will span further than MGP10 when used as a bearer, but the reverse is true when used as a joist. That seems a peculiar thing to say. The timber doesn’t know whether it’s being used as a bearer or a joist. The properties remain the same.

Quote:

---

Originally Posted by ErrolFlynn

Thanks for the links. They add to my knowledge, but they also throw some mud on the issue.

It’s interesting to know that F and MGP represent different grading systems, and that MGP is a more comprehensive system.

The Timberlink article compares F7 with MGP10 in regard to span. It says F7 will span further than MGP10 when used as a bearer, but the reverse is true when used as a joist. That seems a peculiar thing to say. The timber doesn’t know whether it’s being used as a bearer or a joist. The properties remain the same.

---

Because you want the thing with the highest break strength for the bearer, it carries the most load. You want the thing with the most stiffness for the joist, it carries a lighter load but you want it to be stiff rather than springy, higher modulus of elasticity.

In here

https://www.ubeaut.biz/reno/spans/Allseasons%20Span%20Table%20Set.pdf

span tables can be found for both joists and bearers. The tables for joists seem straightforward. I’m having a few problems with the bearer span tables. Here's a snippet.

Attachment 537762

1. What does the “2/” mean in the first column? If I were to guess, I’d say this spec requires two lengths to be used, parallel, representing a bearer of 90 x 70 mm?

2. How are the 1200, 2400, and 4800 measurements used in the table? I’m having trouble relating the Span specs to each of those values. For example, the first one is 1500 but I can’t see how that relates to 1200.

Quote:

---

Originally Posted by ErrolFlynn

What does the “2/” mean in the first column?

---

Yes two beams, usually nail laminated against each other.

With point (2) above, I found the following.

To correctly calculate the minimum strength and placement of the timber that you should use on your deck, you need to first establish the floor load width of your bearers, and then the spacing of your posts. The image and text below is a simple guide to the concept of floor load width.

Attachment 537765

The FLW terminology suddenly makes sense. It's not the width of the entire floor; just the relevant portion the bearer supports.

Quote:

---

Originally Posted by ErrolFlynn

...
1. What does the “2/” mean in the first column? If I were to guess, I’d say this spec requires two lengths to be used, parallel, representing a bearer of 90 x 70 mm?
...

---

Yes. Look up "vertical nail lamination (VNL)" for the correct method.

Also ensure you understand single span and continuous span, per the tables

Thanks for that, I will check that out. Though, rather than nailing two bearers together, bolting the pair on either side of a post might be as good. Possibly better as it would spread the load over a wider area and reduce the span slightly. If you work it that way.

The single vs continuous spec is clear, and logical. Though, I was surprised to see the same pattern for span distance doesn't apply to bearers. There are different span distances between single and continuous in the bearer spec table, but you have to look carefully to find them (toward the larger sizes).

Check out "double bearers/spaced bearers" for blocking requirements

Also remember AS1684 is a "minimum safe" standard, so while safe, building to its limits may not provide, for example, the stiffness in a floor you might expect to achieve. "Over engineering" somewhat is often better practice

Yes, good point.

That over engineering can be obtained by a number of ways.
You could use the next size up in timber sizes for instance or you could add an extra bearer to reduce spans, use a higher spec timber [ say MGP12 or F17 rather than MGP10] or use a tighter spacing or all of these if money is no object.

Quote:

---

Originally Posted by Moondog55

if money is no object.

---

...if only.

The span tables in the following example suggest a pair of 90x35s as a bearer. My initial thought was to ignore that and go for 90x70. That has to have equal properties. At least that was my plan until I spotted the price difference - two of the smaller sizes are less than half the price of the larger.

Attachment 537785