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Maths Question!!!

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Rich_N

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Morning all,

I have a simple question. I need to calculate how much timber I need to clad the underside of a pitched Gazebo Roof. Maths is NOT my strong point 😉

The gazebo is 3m square (it is actually square) and from the centre of the joist to the top of the underside of the rood is 1.7m.

Im going to use 120mm or 150mm T&G.

Can anyone advise on either how much lumber I'll need or how do I work out what I'm going to need?

Thanks in advance,

Rich
 

pe2dave

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From a while back.
roof sloped side (h) = sqrt(1.5^2 + 9)
Area one side of roof = 3h sq m
Whole underside = 6h sq m

Divide that by area of one length of t&g (ignoring the tongue size) and you have your number of lengths.
Roughly 20 sq m?
If 2.4m lengths, What effective width?

Over to you Rich.

HTH
 

Jameshow

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120mm = 75m

150mm = 60m

Flat panel but as it's steeply pitched I would add 50% for both the pitch and for the off cuts you will create at the corners.

Cheers James
 

Rich_N

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I'm looking at 3m lengths so the first board will be fully used, 2nd will be cut and so on.

What confuses me is once I've gone up a few boards then cut offs will fit....if I used 100mm widths then I'd need 17 from bottom to top, but at about board 9 it'll go to cut offs.......ahhhhh, this blows my tiny mind......lol.
 

pe2dave

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Suggestion: Work out the area you need (what is the 'usable' width of your boards?) then divide into 20 sq metres. Add a bit for sods law and use that number, assuming you can use offcuts (hence sods law).
How far apart are the ??? to which you are fastening the boards...

(and I'll ask, why are you not using 8x4 boards? Marine ply or some such?) A bit easier IMHO
 

Rich_N

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Yeah, agreed. And a distinct lack of knowledge to accompany the extra cost.....lol
 

Jameshow

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I'm looking at 3m lengths so the first board will be fully used, 2nd will be cut and so on.

What confuses me is once I've gone up a few boards then cut offs will fit....if I used 100mm widths then I'd need 17 from bottom to top, but at about board 9 it'll go to cut offs.......ahhhhh, this blows my tiny mind......lol.
At half way up the boards will be 1.5m wide so the boards beyond that will be able to come out of the offcuts except that the angle of the edge means you'll need another board or two..

Cheers James
 

Stanleymonkey

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It does make your head hurt. Just an observation if number crunching isn't your thing:

At 1/10 scale you could draw the triangle on a sheet of A4 paper 3m (300cm) becomes 30cm, 1.7m high (170cm) becomes 17cm and then your planks can be drawn on with lines 12mm or 15mm apart, depending on the size of plank you use.
The angled cuts are going to be wasteful as you can't flip the pieces over but you might be able to visualise how many you can fit out of a three metre length when you have a picture in front of you to look at.
 

PhilTilson

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I have obviously misunderstood the original problem! When the OP says he wants to "clad the underside of a pitched ... roof", I understand that to mean the two rectangular areas that make up the two halves of the roof. So I don't understand these comments about the boards "...getting shorter as they go up". Are we actually talking about the underside of the roof or the 'gable ends'?

Working on my original premise, if the roof centre height is around 1.7m then the slant height - ie the length of each of the two halves of the roof - is 2.27m. This gives a total area of 3m x 4.54m or 13.6m². As T&G measurements are normally stated as the fitted width (ie 150mm T&G means the pitch between adjoining slats is 150mm) then for 150mm T&G you would need 13.6 / 0.15 = 91 lengths or for 120mm T&G it's 13.6 / 0/12 = 114 lengths - a 'length' being 3m, of course.

Naturally, I have been talking about a gabled roof. If the OP actually meant a tented or pavilion roof, where the centre goes to a point with four triangular sections making up the roof, then that's a different story! Full information helps!
 

J-G

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As T&G measurements are normally stated as the fitted width (ie 150mm T&G means the pitch between adjoining slats is 150mm)
That is not my experience. Sellers - marketers - usually want to fool you into a false sense of 'value' so advertize the gross size - they sometimes even state the size of the rough-sawn stock since that is larger - The last T&G - or rather Shiplap - I bought was stated to be 125 x 18 but the 'cover' was 110mm and the finished thickness was 12.
 

pe2dave

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Working on my original premise, if the roof centre height is around 1.7m then the slant height - ie the length of each of the two halves of the roof - is 2.27m. ular sections making up the roof, then that's a different story! Full information helps!
sqrt(1.5^2 + 1.7^2) = 2.267 - quite right. Apologies to the OP, I can't even drive my calculator now!
thanks Phil.

Interesting point about T&G measurements. Live and learn.
 

pe2dave

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That is not my experience. Sellers - marketers - usually want to fool you into a false sense of 'value' so advertize the gross size - they sometimes even state the size of the rough-sawn stock since that is larger - The last T&G - or rather Shiplap - I bought was stated to be 125 x 18 but the 'cover' was 110mm and the finished thickness was 12.
Seems Jewsons agree with this. 19 X 125MM (ACT SIZE 14.5 X 120MM)
 

PhilTilson

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and that 120 is not the 'cover' - ie. the 'pitch' of the boards - you'll likely get 105 or maybe 110.
Yes, it would make sense for the OP to check this with the boards he is intending to buy and adjust the calculations accordingly. For example, my calculation for 150mm boards came out to 91 lengths. If the actual measured 'cover' turned out to be 135mm, then he would need 150 / 135 * 91 = 101 lengths. (Plus an allowance for 'gotchas'!)
 

Terry - Somerset

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To judge from the OPs original post where he talks about offcuts, I assume it is a "tented" roof. Gabled roof would use boards of the same width.

The triangle formed by each side of the gazebo would be an isoceles triangle with a base of 3000mm and a "height" of 2267 mm.

Assuming the intention is to fit the boards horizontally, the longest one would be 3000mm and get progressively shorter towards the apex.

With (say) 150mm boards, 16 lengths are required (2267/150). The width of one length may overhang or be reduced - aesthetics may rule!

Very crudely the length of each board will reduce by 188mm (3000mm/ 16 boards).

Most (but not all boards can be cut down -

1 length - 3000
1 length - 2812
2 lengths - 2624 + 188
2 lengths - 2437 + 375
2 lengths - 2250 + 563
2 lengths - 2063 + 750
2 lengths - 1875 + 938
2 lengths - 1688 + 1075
2 lengths - 1500 + 1313

Depending how you join at the corners you may need a compound mitre saw. Also note that 3000mm length may be marginal for the bottom row.
 
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