H$LBroker:
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HsLBroker: A home is gotten when you look for comfort based on your fantasy, but a shelter is a necessity, and a building becomes a shelter when it can prevent rain,water,sun, strong wind and other climatic elements from affecting the occupants- human,animal, living and dead loads with personal effects.

As far as I am concerned the moment you have your roof placed on a building, other things are important but they are basically needs. You can’t move into a house without a roof , but we have seen people living in a house with no window or makeshifts windows covered with ordinary bags and cardboards.

So it is very important to calculate the dimension of the true length of a roof members. These dimensions are essential to determine the accurate span of Rafter, Wall plate, Tie beam, King Post, Overhang, Purlins, fan struts etc.

Residential timber-framing construction clearly differentiate between spacing and span Spacing the centre-to-centre distance between structural members, unless otherwise indicated.

Span the face-to-face distance between points capable of giving full support to structural members or assemblies. In particular, rafter spans are measured as the distance between points of support along the length of the rafter and not as the horizontal projection of this distance.

Single span the span of a member supported at or near both ends with no intermediate supports.

Continuous span the term applied to members supported at or near both ends and at one or more intermediate points such that no span is greater than twice another.

Span/length calculation for roof members.

As can be seen the span of the rafter in the figure below is not quite in agreement with the Code but we will use the figures for the rafter span as calculated below.

Wood/ timber are sawn into various sizes amongst are

2” by 2” i.e 50mm by 50mm

2” by 3” i.e 50mm by 75mm

2” by 4” i.e 50mm by 100mm

2” by 6” i.e 50mm by 150mm

3” by 4” i.e 75mm by 100mm

3” by 6” i.e 75mm by 150mm

HsLBroker: From the picture above you can see the height of the roof of building and the length and breadth of building is already shown in the floor plan, so left to be known is the rafter or the fall ‘x’ as shown in diagram above.

Remember the Pythagoras theorem in secondary school days used in calculating the longest part of a right angle triangle.

The numbers of wood to be calculated also depends on spacing for the purpose of this exercise, we are using the format in diagram above as our specifications for our calculations.

For ceiling noggins of 600mm centers it simply means the wood are nailed 600mm apart, so add the breadth of plan and 600mm as overhangs both sides i.e 7,800mm or 7.8m +1200mm or 1-2m=9meters

Then divide by 0.6m=15pcs of 16.975m of 50 by 50.

For the longitudinal portion, 16.975+1.2m(the overhangs of 0.6m both sides)=18.175m then divide by 0.6=31 pcs of 9m of 2” by 2” ( Note how the length on one side is used to calculate the pieces/numbers of wood required for the length of the other side)

So to know the number of 2 by 2 to buy in the market for the ceiling noggins-

18.175m x 15pcs=273m of 2” by 2”

9m x 31pcs=279m of 2” by 2”

Total=552m

In the market, the wood comes in 3.6m per piece

So 552/3.6=153 pcs of 2” by 2” timber to buy in the market for ceiling noggins.

WALL PLATE is the timber/wood that are placed on top of blocks or roof beams as the case may apply, so for the fastest way to calculate this, is to add the entire walls length and divide by 3.6m to know the numbers of pieces you need. I am using approximately 100m.

So for 100m, divide by 3.6=28pcs of 3” by 4” or 75mm by 100mm to purchase in the market for timber named wall plate.

For FACIAL BOARD, remember the length and breadth of the plan of the building with its overhangs of 0.6m each, so add the perimeter

9m+9m+18.175m+18.175m=54m

Divide 54m by 3.6m=15pcs of 1” by 12”

For TIE BEAM, going by the section, it was recommended that we lay the timber named tie beam which is 2” by 6” or 100mm by 150mm at 1.2m centers apart. So what we need to do is to follow the same format used in calculating the ceiling noggins but it is going to span only the breadth of 7.8m only.

So divide 18.175m by 1.2m centers (spacing)

=15pcs of 7.8m =117m, then divide by 3.6m to know the numbers to buy/purchase in market.

117/3.6=32pcs of 2 by 6 of wood termed as tie beam to buy in the market.

STRUTS are a bit complex to calculate, they are the members placed diagonally as shown in diagram to connect the rafter with tie beams. We can use the numbers of tie beams and multiply by 2-

i.e 32 pcs multiply by 2=64 pcs of 3 by 4 or 75mm by 100mm termed as struts to be bought in the market.

For RAFTER, check the height of roof and the breadth of plan to determine the rafter which is the falling portion of the roof (except is already given from the section)

Lets take the height of the roof as 3.5m and the breadth of plan plus overhang is 9m.

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HsLBroker: What is a Purlin in Timber Trusses Roofing Members.
A horizontal structural member which supports roof covering or a longitudinal member in a roof frame, usu. for supporting rafters or the like between the plate and the ridge.
In architecture, structural engineering or building, a purlin (or historically purline, purloyne, purling, perling) is any longitudinal, horizontal, structural member in a roof except a type of framing with what is called a crown plate. In traditional Timber framing there are three basic types of purlin: purlin plate, principal purlin and common purlin.
Purlin plates in wooden construction
A purlin plate in wooden construction is also called an "arcade plate" in European English, "under purlin", and "principal purlin". The term plate means a major, horizontal, supporting timber. Purlin plates are beams which support the mid-span of rafters and are supported by posts. By supporting the rafters they allow longer spans than the rafters alone could span thus allowing a wider building.
Principal purlins in wood construction
Principal purlins in wood construction, also called "major purlin", "side purlin". Principal purlins are supported by principal rafters and support common rafters in what is known as a "double roof" (a roof framed with a layer of principal rafters and a layer of common rafters). Principal purlins are further classified by how they connect to the principal rafters: "Through purlins" pass over the top; "butt purlins" tenon into the sides of the principal rafters; "clasped purlins," of which only one historic U.S. example is known, are captured by a collar beam. Through purlins are further categorized as trenched, back, or clasped; butt purlins are classified as threaded, tenoned, and/or[clarification needed] staggered.
Common purlins in wood construction
Common purlins in wood construction, also called a "major-rafter minor-purlin system". Common purlins are typically "trenched through" the top sides (backs) of principal rafters and carry vertical roof sheathing (The key to identifying this type of roof system).