Thread: F17 hardwood timber

F17 is a stress grading only. F ratings are for timbers that have been visually graded, i.e. not graded using a machine. Machine graded timber is MGP10, MGP12 etc

The stress grading has no bearing on stability in terms of moisture content. The stress grading is dependant on timber species, defects in timber such as knots and their size and location.

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Originally Posted by yoboseyo

I'm sussing out these 200x200 F17 hardwood posts that I'm going to use for my pergola. They are fresh sawn from naturally dried timber. Kilning is offered as a service at cost. I'm just wondering whether F17 carries any kind of meaning for dstability or moisture content, and whether I can expect them to be dimensionally stable enough.

Appreciate the advice!

yoboseyo

What Samo said.

Additionally if that timber is rated at F17 and is green, once dry the stress rating will increase. That F17 may become F21, for example. Stress grading is an indicator of strength. Radiata pine may be rated at F5, but F17 is considerably stronger and will span a larger distance. Having said that your posts will be used in compression and capable of holding up a three storey house let alone a pergola! The stress rating is more important in timber components that are subject to tension such as rafters.

Your description of freshly dried timber, "but offering a drying service" sounds a little contradictory. Perhaps they mean it is air dried only, which is probably down to a moisture content of about 30%. Timber of that dimension will take a long time to be fully dried (around 12% depending on location) without going into a kiln. Probably ok for a pergola but treat the surfaces as soon as possible to slow down the natural drying as this will minimise splitting and cracking (but not eliminate it.)

Regards
Paul

I didn't know green timber can be given a stress grading.

The timber is sawn from old logs. I don't know how old it is - I can ask the vendor perhaps. It may hold quite a bit of moisture because they were sawn from log state.

Are el cheapo moisture meters with a pin accurate? Should I buy one just to test it?

Most hardwood framing timber was traditionally sold "Green Off Saw" (GOS), i.e. in and out of the mill yard asap, so it was up to the chippie / builder to accommodate its drying whims during construction. Stress gradings for seasoned and unseasoned timber are published in construction codes, span tables, timber manuals etc.

Air dried framing timber was a luxury, and kiln dried framing timbers in non-pine species unheard of.

My Dad was a builder in Cairns for many years and we routinely revisited all new homes we had built at 6 months and 1 year after completion to "take up" the shrinkage in cyclone rods and bolts used in framing. It was nothing to see up to 30 mm shrinkage in a low/high set timber frame, battens, rafters, stud, joist, bearer etc. That has to be accommodated for when different forms of construction abut each other i.e. full timber frame abutting a masonry / concrete wall etc. particularly at thresholds such as stairs, balconies etc.

Kiln dried 200 x 200 - no such animal imo. Even 90 x 90 Merbau / Kwilia that is marketed as kiln dried is sopping wet in the center. Try turning a fresh off cut, and you get a free bath.

Originally Posted by Mobyturns

Kiln dried 200 x 200 - no such animal imo. Even 90 x 90 Merbau / Kwilia that is marketed as kiln dried is sopping wet in the center. Try turning a fresh off cut, and you get a free bath.

Are you saying that I should expect a 200x200 to be wet, but it's fine to work with?

Further to what Mobyturns said, on old full brick homes with timber floors on bearers and joists you will often see a gap between the floorboards and the skirting. People often think that the gap is for tucking the carpet under the skirting but it would of originality been installed tight to the boards and the gap developed over time from the shrinkage from the bearers and joists which were all wet sawn when laid

The posts will be fine to work with and would be similar to hardwood paling fence posts and rails to give you a idea

KD is only done on material up to 50 thick

Are they boxed heart, ie, is the centre of the tree (very) roughly in the centre of the post? At that size they likely are, depending on what timber is available to you locally, and I'd expect that if they have been milled any time at all recently they will be quite wet and that as they dry there will be a reasonable amount of surface checking (cracks) on the faces, doubly or triply so if boxed heart. For a pergola shrinkage shouldn't be too much of a problem as longitudinal shrinkage is very minimal with any species, but expect to be tightening bolts in a year's time if you are fastening beams to their sides. They'll be years drying completely.

Originally Posted by yoboseyo

Are you saying that I should expect a 200 x 200 to be wet, but it's fine to work with?

"Wet" is a very wide definition. For external structural framing it will be OK, that is normal. However I would be more concerned about its expected "service life", both in-ground and above ground than its MC. If the posts are to be direct buried in the ground or into a concrete pier footing you will want a species with a good in-ground service life.

The moisture content (MC) of wood is very much dependent upon when it was felled, how long it was on the ground / stored before milling, what dimensions it has been milled to, the species, then how it has been "seasoned" after milling. There are so many variables that can affect MC. A pin moisture meter will only give you the near surface MC of the wood not the heart MC. Saw millers generally don't like cutting "dead trees" because it is harder on their gear.

GOS timber can have a MC well in excess of 20% with air dried seasoned timber it is generally somewhere around 12% depending upon its environment. As the wood looses moisture it shrinks, warps, cups, and may suffer internal checks / voids etc. It is the total change in moisture content from GOS to air dried seasoned timber that determines how much physical change in dimension will occur.

Even at a conservative 2.5% radial shrinkage you can expect at least a 5 mm change in thickness & that is why you will almost always see "loose" timber posts set into concrete pier footings. The post was GOS when set into the concrete, then it dried and shrunk in the process.

The posts are going to above ground, on stirrups. The supplier has several, and in different species. They will need to be cut to size. So I have a choice. I'm leaning towards blackbutt. I'm thinking of bringing a moisture meter. But hopefully I can tell visually and there's evidence of surface checking already

Originally Posted by yoboseyo

The posts are going to above ground, on stirrups. The supplier has several, and in different species. They will need to be cut to size. So I have a choice. I'm leaning towards blackbutt. I'm thinking of bringing a moisture meter. But hopefully I can tell visually and there's evidence of surface checking already

Hi yoboseyo - how did the posts go? I'm currently installing some 150x150 ironbark posts (on stirrups) & the internal moisture is 15%. Did you give the bottom and tops of the posts a few coats of decking oil or something else to slow down drying?