How to Calculate Timber roof members

Hi, pls, I need detailed explanations on how to calculate roofing materials based on roof plan starting from the roof carcasses to the roofing sheets/coverings. Pls use a typical roofing plan to explain it in details, the no of struts, purlin, tie beam, wall plate, noggings, rafters, kingpost etc, pls prepare the precise calculations of the roofing sheets using longspan aluminium, i want to know it. Thanks, expecting your reply soonest.


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,live 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.

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

What determines the quantity of wood needed depends basically on the height of the roof, how complex the roof is? Which may leads to lots of offcuts e.t.c

To determine the roof members you need to have the floor plan, the section and the roof plan.

From the section a-a above you can see the height of the roof of building and the length and breadth of building is already showned in the floor plan, so left to be knowned 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 calculatons.

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”


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


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 (spacings)

=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.

With the diagram above applying Pythagoras theorem

X2 (rafter length )=4.52+3.52


Rafters are nailed side by side tie beam and struts. So we use the numbers of pieces of tie beams because they are also spaced at 1.2m center to center  and multiply by 5.7m

15 pcs of rafter (same as tie beam) multiply by 5.7 =85.5m

Now divide by 3.6m to know the numbers of 3” by 4” to purchase in market=24 pcs of 3” by 4” required for rafters.

PURLINS are the wood of 2” by 2” or 50mm by 50mm we nail the roofing sheets on.

Purlins are spaced based on designs or type of roofing sheets. But for our calculations, we are using 900mm spacings.

Going by diagram above add 5.7m +5.7m for both sides=11.4m

Divide 11.4m by 0.9m to know the numbers required =13 pcs of purlins of length 18.175m (The purlins has to span the length of the building which is 16.975 plus 1.2m for the overhangs by sides)

So 13 by 18.175m=236m

Now divide by 3.6m to know the numbers requires to purchase in market =236/3.6

=66 pieces of 2” by 2” to purchase in market for the purlins.

KINGPOST is that piece of wood that stands at the middle of two rafters falling at the opposing sides. Is the one that spans vertically from the tie beam/rafter line, so just the same way we calculated for rafter, remember 15 pieces and multiply by  the height of the roof  3.5m=52.5m

Then divide by 3.6m to know the numbers to buy in the market.

Therefore 15 pieces of wood of 2″ by 6″ termed kingpost is needed to be bought in the market.







An architect with decades of experience


  1. This is very accurate. I suggest you can help us with the structural design procedure so we can be more enlighten about building design

  2. Thanks Emma, this presentation is easy to read. Consult professional before you build though. It is always safer.

  3. Name sake, thank you for taking your precious time and energy to illustrate the above stated facts. But i want to advise all that in real practical terms you can not avoid wastes completely and therefore it should not be ignored in your calculations. Secondly, purlins are best calculated one side with one line added to the result b4 multiplying by 2. This is to care for the ridge cap line which does not strictly obey the specified purlins spacing. 3rdly, timber cannot be joined head to head without support & lengths in the market may be less than 3.6m.

  4. thanks emma. do let me know how the rise of a roof of a building impact on the building and its suitability for human health and settlement

  5. Thanks a million times for the info; GOD will bless you awesomely for sharing this info and many others. Happy New Year in advance.

  6. @emma; a job weldone, kudos! I observd dt your calculation exclude kingpost which is necesitated by roof height. Secndly, hw do calclate laying tiebeam tru d whole walls instead of wallplate, pls diff. Wallplate frm tiebeam pls? Tnx

    1. @Mackz, Yes I excluded the kingpost before because of the numbers required but I have added it to the post. And the tie beam against wallplate were interchanged for each other, that has been corrected. Thanks
      Meanwhile thanks for the compliments.

  7. @emma;Yap, well corrected now. Pls kindly analyse the quantity(bags) of cement required to plaster 3.6sqm room. Curious to see your quick response.

  8. Please in Naira value what will it taje to roof a 3 bedroom flat including wood,roofing sheet,-i.e common long span, what average labour will the carpenter and the person the fill the roof charge. For those who don’t know the technicalities this will give us ideal. Thanks

    1. Hi Mr Kola,
      Thanks for your comment on my blog.
      To do calculations without a plan can be difficult and very vague. The specifications, quality etc even the thickness of the aluminium alone will change the cost. However I have sent a mail to you.


  9. A job welldone…but pls I will love it more if d drawing can show all d roof Carcassing members very bcos it’s not everybody dat actually knows d position of dis members…thanx and God bless

  10. weldone…….plz emma, am writting my 400L I.T report. can u kindly tell me how many cubic meter(M3) & Tonnes are there in a full trip of sand sold?

  11. weldone…….plz emma, am writting my 400L I.T report. can u kindly tell me how many cubic meter(M3) & Tonnes are there in a full trip of sand sold at about #15,000?

  12. nice 1
    u are doing a great work
    clue are what you need to give
    people will always want to ask for more
    this is great
    love the job you guys are doing

  13. @ emma, weldone guy… u ve explained d roof carcassing, i want u to do same to a steel truss roof. Pls make it urgent. More grease 2 ur elbow.

  14. Jst have a request; pls I don’t know if you can kindly work on the calculation of concrete slab materials elaborately with diagrams to back it up. Thank you and God bless..

  15. thank you sir,may God bless you.i will to like know how much will it takes me to build a 3bedroom flat.thanks

  16. Hello sir,

    well done on the explanation of the roof calculation.

    One question though concerning the ceiling noggings.

    Please I would like to ask on how to calculate pcs of 2″ by 2″ used for the batten and bracing with the tie beam of the ceiling noggings.

  17. my name is ola am a alluminium contractor if u want to instal any alluminium roof pls call this num 08087813916 i will response thanks

  18. Great job from you sir, in the case of the rafters, i think it should be in two ways (water fall from the roof plan ) you calculated for just one side which gave us 24pcs of 3 by 4. I knw it was a mistake, pls check.

  19. Thank u so much 4 takin ur time to analyse roof design as simple as a b c like ds, ha u are 2 much sir. Keep it up . God will continue increase ur knowledge.

  20. Dear Sir,
    I would like to tank you for enlightening us through your articles and experiences. May God bless you richly.
    By the way I would love to know more about structures especially on loads transfer and determination of section of rings. I will appreciate
    Warm regards.

    Djasro Naibi

  21. Hi
    Thanks a lot for enlighting me on roof member calculation, but for the rafters, you have just given us on side which is 5.7m. since they are two on both sides, you need to multiply 5.7m*2.

  22. I truly appreciate your time,effort you are putting to explain these procedures on how to calculate estimate for building materials. continue this good work. Don’t let critics stop you.

  23. Emma, you’re a rare specie of a man; some of us pay through our noses and crack our brains for nights to get a little of the information you pass on generously. I’m greatly impressed.

  24. Sake, Please give an estimate on all sizes of casement aluminium windows cos all these contractors are cheats. Do this to save us from them.Thanks for being there for the masses.

  25. Please help me for the procedure in quantifying or calculating of take off sheet in roofing materials such as purlins 2 x 2 and rafters 3 x 6 with 40 percent mark up.

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