Advise on milling fir for building materials

[pgkevet]

Just to poach a bit of this thread (if I may).

 

What preservatives are suitable for green wood? I ask 'cos assume won't soak in.. tar or pitch below ground and borax above where it can be recoated? Anything that can allow plants to grow up it?

[tommer9]

For cladding it will be fine in the green state, however for structural frame members be more of a problem, you will have to factor in shrinkage into design and whilst green the timber will be less strong. As timber seasons it dries out and loses water from it cells and shrinks, during this process the structural strength, stiffness and hardness of the timber increses which is at its best when it has become dry and stable.

 

The shrinkage you need to be concerned about is the across the width of the timbers, where that dimension is important to your design sizes.

Watch out for timber containing tensions from reaction wood, this is generally obvious because it will bow and twist as soon as its been cut from the sawlog as the tensions in the timber are released.

 

If you do build a green frame with green cladding give it chance to dry once the frame is up and externally cladded, wait before you fit door and window openings to allow for shrinkage of top and bottom wall plates.

Don't fill in anywhere with insulation between the members, there 's nowhere for the water to go that is released during the seasoning drying process, it would just get trapped and the timber will rot and grow mould....

 

If I was you, would at least dry your framework timber and just use green on the external cladding.

I have supplied well over 100 cubic metres of softwood (larch) that has been used throughout a green oak building that has taken 4 years to build. There will be shrinkage and movement, but as long as you allow for this then there wil be no problem. The building I refer to i sbeing lived in and has huge glass panes. There will be no issues as it is factored into the design. There is insulation everywhere, underfloor heating etc etc, and 12 months on no problems have arisen.

Dare I ask if leylandii would be any good??!! Just happens I have a job tomorrow in which I'll be felling lots of straight lengths approximately 18 inch stems which I could keep for free otherwise it'll be burnt. Thanks for most interesting comments, i take on board comments about the disadvantages of using green softwood - I presume the same would apply for oak? I have around 40 tons of oak which could be used for the framework it just seems a shame to use it for the frame which will be covered over with plaster board - on the otherhand it'll only go for firewood so it's not as bad as it sounds! The oak was standing deadwood so the moisture content will be lower than normal.

Leylandii is a very good timber which will have excellent durability in its hertwood, and is very stable. It is cypress, so will not need treating either. Do not be put off about using green softwood at all BTW.

 

Different softwoods have different properties, both strength and durability. Strength is unlikely to be relevant here, but durability will be highly relevant. You can't pressure treat and are unlikely to be able to buy anything that works in the long term. Don't assume that non-durable species automatically rot - think of the inner layer of straw on thatched houses, which is often original and centuries old. But, if they get damp, either through poor design or temporary exposure, they will respond differently. Larch is reasonably durable, Douglas fir heartwood is good (but you won't get much out of cord and it's a pain to separate), other species are less durable. Larch is long grain, so tends to warp more, but is strong.

 

What sort of sawmill does your friend have, and have you specifically considered how you'll make the feather edge cladding? Traditionally, this is made by milling square edge boards, planing the outer faces (if painting) and then ripping down near the diagonal, leaving 1/4in one side, 3/4in the other, so you usually need a planed thickness of 1in plus kerf. If not planing, this is slightly easier as you can cut the boards feather edged directly. But, even if you can adjust the height of the mill head over the log independently on each side, you still need to cut your cant square edged before you start cutting, and be very careful about how much you move the head down on each alternating cut - it won't be the same amount unless your log is centred on the bed.

 

Speaking from the experience of having milled enough 8in wide oak feather edge this year to cover my two storey extension, it is tedious.

 

The fixing technique for feather edge of this type is a single fixing through each board, just above the overlap with the board below. The bottom edge is therefore fixed down, but the top edge is free to float slightly (although trapped by the board above so it can't warp). This stops it cracking as it expands and contracts depending on the weather.

 

Alec

 

I disagree with several points here- it is perfectly possible to buy very good quality, durable untreated softwood.......such as larch or douglas, both of which are EXTREMELY resistant to decay. If you buy good quality larch sawlogs, they dont warp.....unless your sawyer doesnt reallly have a clue about millling, in which case find another sawyer/ sawmill.

As for making feather edge, a simple way to do it is to cut you cant (square block) and on the first pass you raise one edge of the cant with a piece of 1/2" timber, and take it out for the next cut, and carry on like that throughout the cant.

[agg221]

 

I disagree with several points here- it is perfectly possible to buy very good quality, durable untreated softwood.......such as larch or douglas, both of which are EXTREMELY resistant to decay. If you buy good quality larch sawlogs, they dont warp.....unless your sawyer doesnt reallly have a clue about millling, in which case find another sawyer/ sawmill.

As for making feather edge, a simple way to do it is to cut you cant (square block) and on the first pass you raise one edge of the cant with a piece of 1/2" timber, and take it out for the next cut, and carry on like that throughout the cant.

 

Regarding durability, BS EN 350-1 recognises five classifications of natural durability for timbers. These relate to the resistance of the heartwood to attack by wood-decaying fungi:

 

•Class 1 very durable

•Class 2 durable

•Class 3 moderately durable

•Class 4 slightly durable

•Class 5 not durable

 

You can find a more complete description at Preserving Confidence

 

Spruce, Scots Pine (aka European whitewood and European redwood) and Douglas Fir are Class 4. Oak and sweet chestnut are Class 2. Larch is Class 3. As such, they can't be classified as extremely resistant to dtcay, although it's true that with good detailing they can last a very long time - Douglas Fir was used for a lot of C19 window frames which are still going strong.

 

Perhaps 'warp' is too strong to describe what the larch will do - more accurate to say it will do what it wants to. If it's got any internal stress, it will move, in a way that short grain timbers won't. It will, however, be no worse than oak, which is perfectly tolerable, I just don't expect it to stay dead straight.

 

Nice technique on feather edge

 

Alec

[tommer9]

Interesting stuff there Alec. I can only speak from 'experience' as it were- what i mean is that i havent come to it from an academic point of view, more from what i have picked up over the years, and what has come to me from woodworkers and other sawyers etc. I take it there are few softwoods that are class 1? Western red cedar perhaps? Or is it just hardwoods that can achieve this?

Thanks for the link. I have opened and bookmrked it for later perusal....the malbec is kicking in now......

Its not my idea on the feather edge....but its a cracker isnt it?!

I know what you mean about larch. IME it definitely depends on the original grade of sawlog.......for example I once had a load which I ended up getting most of my money back on as about 90% of it was unuseable......straight off the mill. I was cutying 5m lengths, and by the time i was about 3m in, the wood had either lifted off the log by an inch or more, or gone the other way and trapped the band. It was atrocious.

Generally though I have found it to be VERY stable indeed.

[tommer9]

I wonder what sort of timescale BS en350-1 is based on? I know of 50 year old cedar shingles that have come off roofs, for over 3/4 to go back on again....(cleft shingles not sawn that is)

[agg221]

US - grown WRC, Cedar of Lebanon and yew are Class 2. UK-grown WRC is Class 3, which are the highest softwoods. Note that oak and sweet chestnut are Class 2, robinia is probably the highest performing UK-grown species as it's Class 1 or 2. To get real Class 1 you need something like teak or opepe.

 

BS EN350 is a test standard. It's based on a highly aggressive, accelerating environment and monitoring onset of decay, hence you can get different results from the same species grown in different environments (you'd probably get different results from sweet chestnut grown on chalk to grown on an acid soil, and you'd definitely get variation between softwoods grown in the south compared to Scotland).

 

Environments also have classes (hazard classes - BS EN-355), so by combining the two standards, what you get is a sense of how long a particular species will last in a particular class of environment. For example, exterior cladding is a Hazard Class 3 environment. If you use a Class 2 timber such as oak or WRC you get an estimated life of 40-60yrs. If you use a Class 3 timber such as larch you get an estimated life of 30-40yrs, but for Class 4 timbers you get 25-35yrs. Class 5 would give you 15yrs estimated life. Go to a Hazard Class 5 environment though, such as a fence post, and these times drop dramatically.

 

Apologies to the original poster if this is more than they ever wanted to know! However, the one point that may be useful to them to add is that detailing can make an enormous difference on cladding. This includes not taking it right to the ground - use a masonry plinth a minimum of 6in and preferably 10in high (just as they did with cill beams in timber framed houses) and think about detailing the eaves with a good overhang, providing free air ventilating space behind the cladding, and about how the openings for windows and doors are detailed so that the water runs away and not into the cladding. Also, seal the end grain of all the cladding boards if you want them to last, e.g. by painting them.

 

This has got rather long and complex!

 

Alec

[tommer9]

Fantastic post Alec, thank you:thumbup:

. That is REALLY interesting stuff, which I imagine there is far more to, but even on that level it really will help (me) when asked about timber by customers. For example i have a chap who wants to build a 12m by 8m swedish style log cabin. He was asking me about durability and i could only advise on 'anecdotal' (I mean not backed up through the sort of stuff you have detailed) evidence.

[agg221]

You're welcome. I've been doing work on extending durability lately, working with TRADA amongst others, and have been absorbing a lot of information from them. They have an enormous amount of data available, but they make their money by selling it so it's hard to get hold of in a collated form. I think I may also have accidently become the UK representative on the European panel for surface timber treatments, as the previous one left and there are only two other UK-based people involved. I'm hoping that by keeping quiet the other one will pick it up though!

 

Alec

[Big J]

Stunning workshop there Timmy!

[aswales]

Just like to make the point , not trying to put anyone off using green timber... Alec is right in his posts regarding timber durability classes etc.. How you use green timber and whether or not it is successfull in your project with regard to it lasting for a significant period of time, being durable against fungal attack etc is more often than not about the environment where that piece of wood is used and how it is treated by that environment together with its natural durability.

 

External Larch claddings, cedar shingles are successful in their use partly because of the natural oils in the woods and also the fact that the designed environment allows the rain water that hit the surface to get away and does not get sealed in and remain trapped.

 

If you use green would appropriately it will be fine providing you stick to a few basic practises/rules.

 

Trada do an afternoons seminar on green oak in construction, think they run it a few times a year. Not expensive and well worth the investment, for those serious about green timber building. Oak frames, claddings and how you use the various elements in a building are discussed plus you get some very useful written material.