CARCASSING TIMBER

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CONTENTSWhat is carcassing timber?

Sizes of carcassing timber

Timber grading

Strength grading

Visual strength grading

Machine strength grading

Comparing visual grades and mechanical strength classes

Stamping timber

Pricing carcassing timber

1. WHAT IS CARCASSING TIMBER?Carcassing timber is timber used for structural building components such as:

Ground floor joistsCeiling joistsUpper floor joists

Wall platesCommon, hip, jack and valley raftersTrussed rafters

Flat roof joistsTimber frame building componentsStud partitions

It is also used for:

False work or framingNogginsBoxing-in pipe work

Or for:

FencingPallets and packagingTile battens

Some customers may refer to carcassing timber by its intended end use:

StudsRaftersJoists

Carcassing timber is available in softwoods and hardwoods. This workbook looks atsoftwoods.

Carcassing timber is the general term for timbers used for the frame or carcass of abuilding; many are load bearing. This means that carcassing timber, normally, has to bestrength graded.

Carcassing timber will often be of a lower quality than joinery timber, which is used forfinishing work. Carcassing timber can:

Contain more knotsHave a sawn finishHave a higher moisture contentBe graded for its strength

That said, some carcassing timber is often of a very good quality.

The two main species of timber used for carcassing timber are:

European redwood, pine or redwoodEuropean whitewood, spruce or whitewood

These are two species of timber often sourced from Europe, the UK, Scandinavia and theBaltic states. Their names can vary depending on which country they are sourced from –Russian redwood, Swedish whitewood, etc.

Which is better? Well, redwood tends to have a smoother finish than whitewood.For structural purposes, they are both of the same strength.

Whitewoods are often available in wider sections. Nothing between the two timbers really when used for carcassing timber.

This timber from abroad is known as “imported softwood”.

British timber is often known as “homegrown”:

Spruce or whitewoodPine or redwood

Depending where the tree has grown does affect the quality of timber. Imported redwood or whitewood reaches higher strength grades – C24, C27 and TR26 - has a smoother finish and often has fewer knots.

Homegrown timber while, suitable for structural work, tends to be more knotty, with acoarser texture and offered as one grade – C16.

Higher grades are available but selection is important.

There are other species of timber used. These are:

Douglas fir – imported or homegrownHemlock – importedLarch – imported or homegrown

Carcassing timber in the past as been offered with a sawn finish but now is generally planned. Some customers will refer to carcassing timber by its finish. For example,they may refer to it as:

SawnOff- sawCountry – cutRoughing timberCLSRegularisedScant

There is no one common term and the terms used vary around the country.

Taking the last three terms, see if you can describe them.

Task 1

CLS

Regularised

Scant

Scant, CLS and regularised timber are often planed to width and thickness. “Regularised”can also refer to timber that has been sawn to width.

The size of the timber can vary. Using 47 x 75mm or 3” x 2” as an example the three sizes are shown below:

Task 2

What are the advantages of using planed or regularised timber for your customers? Listthree advantages.

1.

2.

3.

Task 3

What does the term CLS mean?

75mm

47mm SAWN

70mm

45mm SCANT

63mm

38mm CLS

CLS stands for Canadian Lumber Standard and refers to any timber that has been planedto a standard size with rounded corners.

However, it should be noted that many countries now supply ‘CLS’ type timbers. For example:UKSwedenFinland

So the term CLS doesn’t always mean the timber has come from Canada.It is also known along with scant as ‘planed four sides’ or P4S.

Since carcassing timber is going to be used for structural use, let’s see if you can namethe timber components in the house below.

Task 41.

2.

3.

4.

5.

11.

12.

6.

7.

8.

9.

10.

How did you get on? The answers are:

1. Ceiling ties or celing joist2. Rafter or trussed rafter3. Cladding4. Tile os slate batten or lath5. Decking6. Flat roof joist7. Handrails8. Upper floor joists9. Studs or stud partition10. Ground floor joists11. Arris rails12. Fence posts

To recap

Carcassing timber is timber used for structural timber components. It is of a lower gradethan joinery timber. It can be imported redwood or whitewood or homegrown. It isgraded for its strength. It can be supplied sawn or planed.

Carcassing timber is available in many sizes but, as with most timber, it is offered in standard or ‘trade’:

LengthsWidthsThicknesses

Since most carcassing timber is sought for its strength, the size needed will be determinedby:

The load the timber has to carryThe span of the timberHow many timber members are required for the structureThe strength grade of the timber

Task 5

Using the table below, tick the sizes that your company offers for carcassing timbers.

Width, in mm

If the timber is offered preservative treated, show this in the table with a ‘P’.The main widths and thicknesses for carcassing timber are:

2. SIZES OF CARCASSING TIMBER

Thickness

19

25

32

38

47

50

65

75

100

25 38 47 50 75 100 125 150 175 200 250 300

75 100 125 150 175 200 225

47

63

75 thickness

width

Timber widths and thicknesses are often referred to the sawn size or nominal size –47 x 75 mm.

However, if planed or regularised the actual or finished size can be 45 x 70 mm. Whateverthe finish, customers may still refer to it as 3” x 2”. It can all get confusing, but your job is to know all these size variations.

Using the imperial sizes below, write in the metric sawn, CLS and scant sizes.

Task 6

Imperial size Sawn size in mm Scant size in mm CLS size in mm

3” x 2”

4” x 2”

6” x 2”

8” x 2”

Not all sizes are available in CLS. However, the sizes are:

Imperial size Sawn size in mm Scant size in mm CLS size in mm

3” x 2” 47 x 75 mm 45 or 44 x 70 mm 38 x 63 mm

4” x 2” 47 x 100 mm 45 or 44 x 95 mm 38 x 88 mm

6” x 2” 47 x 150 mm 45 or 44 x 145 mm 38 x 140 mm

8” x 2” 47 x 200 mm 45 or 44 x 195 mm 38 x 184 mm

Timber lengths for carcassing and joinery timber usually start at 2.4 m and increase in 300 mm or 0.3 m increment up to 7.2 m.

You will often find that carcassing timber is supplied in packs of uniform lengths, forexample:

2.4 m3.9 m

4.8 m5.1 m

This is known as length packaged timber.

Task 7

While timber is offered in metric lengths customers will often refer to lengths in feet. Forexample: “8 foot”, “12 foot” or “16 foot”.Using the table below, which shows some of the metric lengths available, write in theequivalent imperial length?

Task 8 Metric length Imperial length

2.4 m

2.7 m

3.3 m

3.6 m

4.2 m

4.5 m

4.8 m

5.4 m

The metric and imperial lengths are not compatible. For example: 2.4 m is 1/8” or 40 mmshort of 8”. Likewise, 4.5 m is 2/8” or 72 mm shorter than 15”.

In most cases customers can work with the nearest metric sizes but this is not always thecase.

For example if a customer wanted floor joists for a span of 16’ what length of joist will they need?

Task 9

To allow for the difference between 16’ and 4.8 m the customer will need a joist 5.1 mlong due to it being 77 mm too short. They will also need to allow 100 mm at each end forthe joist sitting onto the wall.

Another skill of your job is to be able to work out the most economical way of obtainingdifferent timber lengths for your customers.

For example if a customer wanted 20 lengths of carcassing timber in 1.9 m lengths, whatwould be the most economical way of achieving this?

Task 10

Supplying or crosscutting the timber from 3.9 m lengths. Using 2.1 m would create toomuch waste.

Try it out again. Using the customer orders below.

A customer wants the following timber lengths.

4 x 2.1 m x 47 x 100 mm6 x 2.7 m x 47 x 100 mm

Task 11

You could supply 2 x 4.2 m and 3 x 5.4 m or 4 x 5.1 m and 2 x 5.4 m.

Try it out again. The customer wants:

4 x 6’ x 3” x 2”4 x 8’ x 3” x 2”

Task 12

You could supply 4 x 4.2 m or 4 x 4.5 m if full 6’ and 8’ lengths are needed.

To recap

Carcassing timber is offered in standard metric trade:

LengthsWidthsThicknesses

The timber can be offered:

SawnPlaned

Customers will often ask for timber in feet, inches and metre and you need to be able to work with both systems.

3. TIMBER GRADINGMost timber is offered and sold graded. By grading the timber, a price can be determined.In addition, by grading timber the use of the timber is determined.

The grade is determined by the number of defects on the timber. The fewer the defects –mainly knots - the higher the grade and higher the price it can command.

For the most part, the better grades of timber, slower grown and with fewer and smaller knots, are used for joinery timber and it is the lower grades that are used for carcassing.

If you worked at a sawmill and you produced European redwood with few defects, Would you sell it for joinery timber and claim a higher price or sell it for a lower price for carcassing timber?

The countries that supply the UK have long-established grades based on timber quality.

Timber can be graded on:

Its appearance or looks – joinery timberIts strength – carcassing timber

For structural use, carcassing timber is strength graded.

Sometimes joinery timber is strength graded when timber with few defects is needed, or ifa feature is to be made from the joists and they are going to be seen.

Strength grading of carcassing timber can be carried out:

Visually, by a personMechanically, by a machine

Most timber merchants and small sawmills visually grade timber, as it is a low cost method.

Strength grading machines are expensive and range in price from £10,000 to £200,000each. Large sawmills use strength grading machines to get the high throughput theyneed.

The use of machines also allows:

A higher yield of timber as more defects will pass testing by a machineHigher strength grades to be produced

Though a machine can not give a precise estimate of strength, it can do so more closelythan a visual grader.

Which timber defects do you think will affect the strength of timber? Write down five.

Task 13

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2. 3.

4.

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The defects that weaken timber will be looked at in the next chapter.

4. STRENGTH GRADING

Visual grading requires a trained and certificated grader to:

IdentifyMeasureJudge

the defects on a piece of timber. When the defects that weaken timber are known thenrules can be set on their allowed size and number.

A visual strength grader must look for the following:

The rate of growth of the timber, as shown by the width of the growth rings.

The presence of fissures, cracks, check or splits.

The presence and amount of wane.

5. VISUAL STRENGTH GRADING

The pressure of in-growing bark.

The alignment of the grain of the wood.

The amount of any distortion in the piece – twist, spring and bow.

The presence of decay.

The presence of insect attack.

Size and position of the knots

There are many strength-grading schemes in the world. The Americans have their ownscheme, as do the Japanese and Australians.

Throughout Europe, we have adopted one system based on EN 14081.

This standard is often used in conjunction with a British Standard for softwood strengthgrading – BS 4978.

When visually strength-grading timber the grader has two grades to assess. They are:

Special Structural - SSGeneral Structural - GS

Special Structural is a higher and stronger grade than General Structural.The defects allowed for each grade is shown below in the CATG table.

Using the CATG table, what are the maximum defects allowed for:

Defects GS SSMoisture content 20% 20%

Fissures Not through thickness (ignore those less than half thickness)

Through the thickness

Not greater than 1 m or 1/4 lengthwhichever islesser

Wane

Slope of grain

Shape

Knots Margin condition

No margin condition

1/3 maximum

1/2 maximum

1/5 maximum

1/3 maximum

Not exceeding 1/3 Not exceeding 1/3

1 : 6 maximum 1 : 10 maximum

Not greater than 20 mmover 2 m

Bow

Not greater than 10 mm over 2 m

SpringNot greater than 12 mmover 2 m

Spring

Twist Not greater than 2 mmper 25 mmwidth over 2 m

Twist

Bow

Not greater than 8 mm over 2 m

Rate of Growth 10 mm maximum 6 mm maximum

Unlimited

Cup

Unlimited

Cup

Only permittedat the ends and not greater than the width

Not greater than 1.5 m or 1/2 lengthwhichever islesser

Not greater than 1 m or 1/4 length of the piecewhichever islesser. At endsnot greater than2 x width

Not greater than 1 mmper 25 mmwidth over 2 m

Insect and fungal attack No active infestation or wood wasp holes

Task 14

Wane

Task 15

Knots

Task 16

Slope of grain

Task 17

Insect and fungal attack

The rules on knots are more difficult but relate to the area of the knots in relation tocross-sectional area of the timber. Our book on visual strength grading shows this.

Timber which fails to meet the General Structural grade is rejected for strength and usedfor other jobs.

But because some softwoods are naturally stronger than others, the design data – thestrength figures the engineer or architect uses – depend on both grade and type(species) of softwood.

The softwood timbers recognised for strength grading are:

Imported:Parana pineCaribbean pitch pineRedwoodWhitewoodWestern red cedarDouglas fir-larch (Canada)Douglas fir-larch (USA)Hem-fir (Canada)Hem-fir (USA)Spruce-pine-fir (USA)Spruce-pine-fir (Canada)Sitka spruce (Canada)Western white woods (USA)Southern pine (USA)Larch

British grown:

Douglas firLarchPineSpruce

Using EN 14081 and BS 4978 the rules for strength grading softwood timbers are shown

For British Douglas fir over 20,000m2 SS = C24.

Source and species

British pine

British spruce

British Douglas fir

British grown larch

European redwood

European whitewood

Canadian Douglas fir-larch

American Southern pine

Grading rules

BS4978 + EN14081

BS4978 + EN14081

BS4978 + EN14081

BS4978 + EN14081

BS4978 + EN14081

BS4978 + EN14081

BS4978 + EN14081

BS4978 + EN14081

GS

C14

C14

C14

C16

C16

C16

C16

C16

SS

C22

C18

C18

C24

C24

C24

C24

C24

There are a range of strength grading machines. Some:

Load and measure the deflection at successive points along the face of a piece oftimber.

Stiffness is measured at each loading point with the data held in a computer; the weakestpoint in the length of the timber determines the grade.

X-ray the timber and measure the defects.

The number and size of the defects are measured by a computer and the grade determined.

6. MACHINE STRENGTH GRADING

Send shock waves through the timber.

Timber with a high density and few defects will respond differently than one with a lowerdensity and more defects and this determines the grade.

But there are features that machines can not always detect so there are limits for:

DistortionWaneSplitsDecayInsect attack

Each piece must also be inspected visually.

Though a machine can not give a precise estimate of strength, it can do so more closelythan a visual grader and many more grades, or more correct strength classes, arerecognised in European standards.

There are twelve strength classes for softwoods. These are:

C14C16C18C20C22C24C27C30C35C40C45C50

The ‘C’ stands for coniferous and the twelve strength classes apply only to softwoods.

C14 is the lowest strength class and C50 the highest.

There is one other common machine grade and this is TR26. What is the timber mainlyused for?

Task 19

The most common grades offered and sold are:

C16C24

7. COMpARING VISUAL GRADES ANDMECHANICAL STRENGTH CLASESThere are comparisons between the visual and mechanical grades.

Remember the main:

Visual grades for softwoods are GS and SSMachine grades are C16, C24 and C27

The table below shows how they compare.

Strength class

* British-grown Douglas fir graded SS and in section sizes exceeding 20,000mm² rates C24;in section sizes up to 20,000mm², it rates C18.

Standard name

Imported:

Parana pine

Caribbean pitch pine

Redwood

Whitewood

Western red cedar

Douglas fir-larch (Canada)

Douglas fir-larch (USA)

Hem-fir (Canada)

Hem-fir (USA)

Spruce-pine-fir (USA)

Spruce-pine-fir (Canada)

Sitka spruce (Canada)

Western white woods (USA)

Southern pine (USA)

British grown:

Douglas fir

Larch

Pine

Spruce

C14

GS

GS

GS

GS

GS

GS

C16

GS

GS

GS

GS

GS

GS

GS

GS

GS

GS

C18

GS

SS*

SS

C22

SS

C24

SS

SS

SS

SS

SS

SS

SS

SS

SS

SS

SS*

SS*

C27

SS

Task 20

Try the table out. If a customer wants a non-trade size for C24 what grade will the visualgrader need to provide?

Task 21

Likewise, if you only had British grown spruce in stock what grade would your visual graderneed to achive for C16?

Task 22

A customer specifies C24 timber and, though many softwoods graded SS meet this level,five of this grade in the Table do not and another may not. List these timbers and suggestwhy they are unacceptable.

To recap

Carcassing timber is strength graded to provide a building component of a knownstrength. With the strength known, the designer can determine the size of timber needed.The timber can be visually graded or graded by a machine.

8. STAMpING TIMBERThe standards require that strength graded timber is stamped to tell the user and othersthe grade.

A typical stamp mark is shown below.

The following information is given:

Strength class - GS/C16BS Code of Practice containing the Strength Class data - BS4978 and EN 14081License number of the company – A000/G000Certification Body logo - CATGSpecies - WPPACondition at grading - dry graded

Another example of a stamp is shown below.

C.A.T.G A000/G000 WppA

BS 4978 GS

EN 14081 DRY GRADED C16

For machine graded timber the following stamp is used:

The following information is given:

Strength class - C16EN Code of Practice containing the Strength class data - EN 338 and EN 14081Licence number of the company – 0000/0000Certification body - CATGSpecies - PCABCondition at grading- dry graded

Task 23

Some further questions for you now. What is the required moisture content for dry graded timber?

The average is 20% moisture content with no reading greater that 24%.

Task 24

Can wet timber be graded?

Yes, it can but it can only be used externally.

Task 25

What are the species of timber PCAB and WPPA?

C.A.T.G 000 0 pCAB

EN 338 C16

EN 14081 DRY GRADED M

The species codes are:

One last thing now on the stamp mark and the standards. Under EN 14081 gradingestablishments can adopt CE marking.

To recap

Timber can be graded:

VisuallyMechanically

The main visual grades are:

GSSS

The main machine grades are:

C16C24C27

Source and speciesEuropean redwood/whitewoodEuropean redwoodEuropean whitewoodCanadian Douglas-fir larchWestern red cedarUSA Southern yellow pineCaribbean pitch pineParana pineBritish grown Scots pineBritish grown Corsican pineBritish grown Sitka spruceBritish grown Douglas firBritish grown larch

SpeciesWPPAPNSYWPCAWPSMTHPLWPNEWPNCARANWPNNWPNNWPCSPSMNWLAD

GAT

WPPA

EN 14081BS 4978

DRY GRADED

1245

M

GSC10

The grades are comparable:

GS = C16 for most imported timberGS = C14 for most home grown timbersSS = C24 for most imported timberSS = C18 or C22 for most home grown timbers

The timber is stamped to show the grade of timber, the moisture content of the timber,the species, the standards it is graded to and the grader’s or companies regulated number

9. pRICING CARCASSING TIMBERCarcassing timber is commonly offered and sold by the:

Metre100 metresCubic metrePack

The two most common methods are by the:

MetreCubic metre

Task 26

Let’s start with the easiest one first. If 47 x 100 mm costs £1.50 per m, How much would45 m cost?

The immediate answer is £67.50. Now, if you can supply 45 m or 10 x 4.5 m that is good butif you only have 4.2 or 5.1 m then the lengths in stock the customers’ needs are:

11 x 4.2 m = £69.30, or9 x 5.1 m = £68.85

Try some more out now.

Task 27

A customer wants 15 joists which are 16’ long. The cost for the timber is £2.86 per metre.

You could supply 15 at 4.8 m = 72 m at = £205.92 but these joists will be 77 mm too short.So the lengths needed should be 15 x 5.1 m joists = 76.5 m = £218.79.

One more, a customer is erecting a stud partition and needs:

10 x 4.8 m of 38 x 83 mm C16 CLS42 x 2.4 m of 38 x 63 mm C16 CLS

The price is £0.89/m.

Task 28

The cost would be:

£132.43

One method of pricing is to price it 100 m. If the cost of this timber was £84 per 100 mwhat would be the cost of the order now?

Task 29

The new cost would be £124.92.

The one system that requires more skill is pricing timber by the cubic metre.