roof height and angles

[jow104]

I have a double garage with an asbestos corrugated type flat roof. I would like to change the roof structure to a tiled pitch style. I need to keep the overall height no greater than 4 mtrs to avoid applying for planning permission. The width of the garage is 5.5mtrs. What headroom would I have at 30 degrees pitch or can I have a lower pitch than this figure?

[glenn k]

Well if you pitched it at 30 degrees
half of 5.5 is 2.75
So 2.75 Sin 30 =2.75 x .5 =1.375m height of center
hope this is what you were after

[jow104]

So 1.375 mtrs is 54" plus headroom of 90" would give me 144" total height. This leaves me around 12" to spare below my 4mtrs maximum height allowance.

Are my reckonings correct?

[glenn k]

Yes but why work in pounds shellings and pence then convert back?
Don't poms work in metric like the rest of the world except for the septics ofcourse.

[jow104]

I was born in the 1930's all measurements an volumns were imperial.
When somebody says 3'9"" I know and feel the size. When someone says 1200mm. I feel like swearing.
I was either born to early or too late.

[Ivan in Oz]

G'Day Woody,
What do you say??
Avoirdupois.........that's swearing to some.

I still think in both but only utter in metric.
What was it really, now :confused:

Count

[Guy]

Well, i was brought up in the UK.
At school we were only taught the metric system, but outside of school everything was imperial which is a good thing coz now i can work in both metric and imperial at the same time. I confuse a lot of customers by measuring something in metric and imperial. Sometimes the imperial is closest measurment.

[duckman]

G'day jow104,

I have a double garage with an asbestos corrugated type flat roof. I would like to change the roof structure to a tiled pitch style. I need to keep the overall height no greater than 4 mtrs to avoid applying for planning permission. The width of the garage is 5.5mtrs. What headroom would I have at 30 degrees pitch or can I have a lower pitch than this figure?

Glenn K is essentailly correct but he left out one very important detail which is the height of the roof structure itself ie the rafter and batten and possibly the roof covering. Roof covering or cladding may or may not have to be included in the height calculation depending on the building/planning regulations in your town/suburb, county etc, etc.

Bear in mind that the rafters and battens have to be measured plumb ie vertically. This dimension is always greater than the width of the timber.

A picture is worth a thousand words so give me a couple of hours, its Saturday morning here and I have go shopping.

All will become clear soon.

HTH,

Mark.

[Rocker]

Glenn,

I think you have got your sines confused with your tangents. The number jow needs is 2.75 x Tan 30. i.e. 1.59 m. See attached diagram.

Rocker

[glenn k]

Yes Rocker you are right
sin=opp/hyp

where tan= opp/adj
sorry woody (and I used to teach maths)

[Barry_White]

There is still hope then for us who are maths challenged then.

[Ian007]

hi jow104,

you dont say if your garage has any overhang ie eves
if it does then the calculations above would be wrong
you should spec if there is any overhang and if so what size
and we can work it out.

I will go and find the math formula and post it hear so you could give it a go at working out

Cheers Ian

[duckman]

hi jow104,

you dont say if your garage has any overhang ie eves
if it does then the calculations above would be wrong
you should spec if there is any overhang and if so what size
and we can work it out.

I will go and find the math formula and post it hear so you could give it a go at working out

Cheers Ian

Ian, the presence of eaves has no relevance to the total rise of any roof. All it does it change the overall rafter length. The span of the roof in conjunction with the pitch and the rafter/batten dimensions, determines the overall height.

In Australia, Hancock's roof book is still the definative way to perform these calculations.

Mark.

[Ian007]

would it not be true that if you had a longer rafter run then the finished height would be changed as the base width is changed?

are you saying that a building 6m wide with 30deg pitch would have the same finished building height as a building 5m wide with the same pitch?

Cheers Ian

[duckman]

would it not be true that if you had a longer rafter run then the finished height would be changed as the base width is changed?

Too right the height would change because the SPAN has changed. Adding an eave however, doesn't change the span and that is the key.

are you saying that a building 6m wide with 30deg pitch would have the same finished building height as a building 5m wide with the same pitch?

Cheers Ian

No, not at all. Eaves are dfferent because the eave overhang is outside the pitching line. Picture a roof in elevation. When I get a chance to draw the diagram I promised jow, it'll become clearer.

Hooroo..

Mark.