# Forum Home Renovation Roofing  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?

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## 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

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## 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?

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## 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.

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## 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.

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## 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

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## 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.

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## 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.   :Frown:   
All will become clear soon. 
HTH, 
Mark.

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## 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

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## glenn k

Yes Rocker you are right
sin=opp/hyp 
where tan= opp/adj
sorry woody (and I used to teach maths)

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## Barry_White

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

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## 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  :Smilie:

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## 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. :Smilie:  
Mark.

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## 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  :Smilie:

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## 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.  :Smilie:    

> 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.  :Smilie:  
Hooroo.. 
Mark.

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## jow104

My present garage height at wall plate (heighest) 94" =2.350m, and there are not any eaves present at the moment as per attached photograph.  Therefore overhang can be acceptable to whatever measures that keep me under the 4 mtrs. total.

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## duckman

Hi Guys, 
Promised drawing will appear soon. I'm having the @#$^&* time getting this @#$^&* scanner to work. Hrmmmph...  :mad:  
Not happy Jan!   :Frown:   
Back soon..   :Biggrin:   :Biggrin:

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## duckman

Hi guys, 
Still can't get my stupid scanner working so I've had to fire up an ancient CAD program which I haven't used for about 8 years. The drawing is consequently very basic. 
The red triangle represents what you would get if you just used the formual already given eg 
height = (tan(pitch) x 1/2 span) 
When we superimpose a rafter(rafter is represented by the parallel green lines) of any size over this triangle, we see that to the figure obtained using the formula, we have to add the plumb cut at the pitching line and the thickness of a batten and possibly the roof cladding as well. 
This is a lot easier to show someone out on site that it is to put into words.
If this isn't clear enough jow, get back to me and I'll elaborate. 
HTH, 
Mark.

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## jow104

Your efforts have been much appreciated and showing rafter as a separate green line marking makes things much clearer when coming to the final calculation. 
How many hours would a professional roofer expect to allow for a roof of this size?  Anyone got any ideas!

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## duckman

Hi jow,104   

> Your efforts have been much appreciated and showing rafter as a separate green line marking makes things much clearer when coming to the final calculation.

  Any time. :Smilie:   

> How many hours would a professional roofer expect to allow for a roof of this size?  Anyone got any ideas!

  Depends.  :Smilie:  
Will you demolish the existing structure yourself? What is the law regarding the removal and disposal of asbestos in blighty? That could well be the most expensive part of this exercise. 
Assuming the asbestos is removed 'by others', two chippys would demolish the existing roof and cut and pitch the new one, fascia, barge boards and roof battens in a day. That's what I'd allow. The only caveat I'd put on that estimate is that it assumes unobstructed access to the site eg that they don't have to negotiate a number of obstacles to get from where they cut the roof to the garage, that the timber is dropped outside your property and that the rubbish stays on-site and doesn't have to be carried down the block to a bin. Its only a straight gable after all. 
Having said that, if you demolish the entire roof yourself, I'd still allow a full day. There's nothing one can really do with a spare hour or two at the end of the day. :Smilie:  
HTH, 
Mark.

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## jackiew

question - weight of asbestos roof is ( I would have thought ) quite low compared to the weight of a tiled roof ... is there anything that  jow104 needs to inspect about the way the frame of the garage is constructed/his foundations before putting on the new roof?   It would be a shame to reroof and then the whole thing collapse under the weight.      
I ask this because when I bought a house which had had a slate roof converted to tiles the surveyor took special care inspecting the roof timbers and told me that tiles were much heavier than slate, which I think probably weighs about the same as asbestos sheet. 
cheers, jackie

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## Barry_White

How about using the good old corrugated roofing e.g. a nice Colorbond then weight wouldn't be a problem or maybe it isn't available in the U.K.

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## Ian007

i know that our standards are/ may be different to the pommy one's but this is what we work on for load mass.  I took this from the aus standards tried to scan it in but i couldnt read it so i just typed this little bit.
do they have to allow for extra loading in England for snow? I know that they do in some European country's 
Jow104 you may need to find that out mate, cobber etc etc(spoken in an aussie accent) 
10KG per M2 0.50mm steel sheet 
20kg per m2 medium gauge steel sheet roofing,battens, 12mm softwood ceiling lining, sarking and lightweight insulation 
60kg per m2 Terracotta or concrete tiles and battens 
90kg per m2 Terracotta or concrete tiles,purlind,roofing and ceiling battens, 19mm hardwood ceiling lining, sarking and insulation 
cheers Ian  :Smilie:  
Ps cant find anything re slate

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## duckman

> question - weight of asbestos roof is ( I would have thought ) quite low compared to the weight of a tiled roof ... is there anything that  jow104 needs to inspect about the way the frame of the garage is constructed/his foundations before putting on the new roof?   It would be a shame to reroof and then the whole thing collapse under the weight.

  Tiles on the roof of a free-standing garage Jackie? Geez, you have expensive tastes.  :Biggrin:  
In Aust. specifically Victoria, there isn't much difference at all in either footings or wall framing in SINGLE STOREY construction between the structure needed to support a tin roof and a tiled roof.   

> I ask this because when I bought a house which had had a slate roof converted to tiles the surveyor took special care inspecting the roof timbers and told me that tiles were much heavier than slate, which I think probably weighs about the same as asbestos sheet.

  I haven't had to deal with slate roofs for quite some time, so can't remember the relative loads. I'd have to look it up. Hang on a mo. Ian's done the 'looking up' for us. 
Hope some of this helps. :Smilie:  
Mark.

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## duckman

Ian,  

> 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

  In re-reading this thread I became aware that I didn't finish answering your question re the effect of adding an eave to the height of the roof. 
I've tacked a 'tail' onto the rafter that I scribbled for jow. I think that the revised drawing will make it apparent that the overhang contributes nothing to the height of the roof. 
HTH, 
Mark.

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## jackiew

> Tiles on the roof of a free-standing garage Jackie? Geez, you have expensive tastes.

  I wouldn't put tiles on a garage roof ( unless, of course, I scored some for a bargain price ) because I'm a firm believer in look after the pennies and the pounds look after themselves" or "many a mickle maks a muckle" as they say in scotland    :Biggrin:   but JOW104 mentioned he was planning on tiles.     
There are garages and garages.  I've owned a house in the uk where the lightweight timber and asbestos garage couldn't be used for cars because the timbers spanning the roof had "pit props" holding them up because they'd snapped.   that's one garage I wouldn't be putting a tile roof on even if the tiles were free!!!     And depending on who put the garage up the foundations might be negligible or non existent. :eek:

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## duckman

G'day Jackie,   

> There are garages and garages.  I've owned a house in the uk where the lightweight timber and asbestos garage couldn't be used for cars because the timbers spanning the roof had "pit props" holding them up because they'd snapped.

  Snapped? Yikes! That's one roof I would be demolishing from underneath. :Smilie:    

> that's one garage I wouldn't be putting a tile roof on even if the tiles were free!!!     And depending on who put the garage up the foundations might be negligible or non existent. :eek:

  Foundations are funny things, no really. By today's standards, many of the old wouldn't pass in a pink fit, but, most of the structures built on those foundations are still quite sound. That doesn't mean however, as you've observed, that those same foundations would be suitable for a substantially modified existing structure. 
Here in Melbourne, in what we term 'the sandbelt', bayside suburbs where the soil is just sand, the foundations of a lot of the older homes are just a shallow trench, say 6 inches deep, into which large blue stone blocks were laid. These blocks are about 2 feet long and roughly 10 to 12 inches square, measured at the ends. Solid brick walls with terra cotta tiled roofs were built on top of them. Those houses are for the most part, still damn solid. :Smilie:  
Hooroo.. 
Mark.

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## jow104

You lot have certainly of got me thinking. The asbestos roof has got to go in the near future. In the winter it condensates and frost forms underneath. 
At the present time the two outside walls carrying existing weight are 4" thick concrete blocks with an 18" pillar in the middle and two corners. Front, no wall (lintels will be required but there is an 18 wide x4" thick block dividing).  Back wall I suspect foundations are not up to scratch. I shall most probably have to dig down and have a look. The two outside walls which would I assume carry the roof weight I feel confident that foundations will be OK, and these walls look in very good condition.

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## duckman

> You lot have certainly of got me thinking.

  Fantastic!  :Biggrin:    

> <snip>  Back wall I suspect foundations are not up to scratch. I shall most probably have to dig down and have a look. The two outside walls which would I assume carry the roof weight I feel confident that foundations will be OK, and these walls look in very good condition.

  With a gable roof, only the side walls carry any roof load, so the back wall's footing is really neither here nor there.  :Smilie:  That might save some money. The front wall also doesn't carry any roof load, however, you do have concentrations of load because of the door opening and the weight of the door too. I don't know what type of garage door is most popular in England, but roller doors and panel doors can get a bit weighty if spanning a double opening. 
Hooroo.. 
Mark.

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## jow104

I would prefer single doors. These are the doors used at the present time, but will most probably be replaced with new modern plastic variety.

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## duckman

> I would prefer single doors. These are the doors used at the present time, but will most probably be replaced with new modern plastic variety.

  A pair of hinged doors? Plastic garage doors intrigues me. Can you supply a link to a manufacturers web site so I can have a peek? 
Hooroo.. 
Mark.

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## jow104

Sorry about the confusion.  The doors would  be drop down, but I am certain there are hinged doors available. I will look around and let you know if I can.

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## brekyl

i hope anyone can help me for this question. i have been told that my roof angle has to be 15 degrees on my extension. from the top of the rafters to the top of the roof line what is the measurement? thanx

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## jow104

You will have to provide the measurements of the 2 short sides of that triangle for the whiz kids to give you an answer :Wink: 
Or if you have they are not readable on my browser. 
Or the span involved can most probably supply the height if 15 degree is required.

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## brekyl

i have put some more measurements in, i hope this helps.

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## Ivan in Oz

I make it a Height of 90.95cm 
for the 339.5cm Base of the Triangle. 
I was unable to locate my old Log tables,
anyone remember them:confused:  :Tongue:   :Tongue:    http://www.math2.org/math/trig/tables.htm

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## brekyl

ok thanx, how about an angle of 7 degrees then?

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## Sybarite

Hi Brekyl, 
I was looking at your dimensions and ended up with the attached results.  It looks to me that to get 7 degrees both sides would mean relocating your vertical... 
I can't see if I have gone wrong somewhere - anyone? 
Respect, all.

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## jow104

Why is there a difference measurement on each half of the span:confused: :confused:

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## Ivan in Oz

> ok thanx, how about an angle of 7 degrees then?

  My head hurts after that :Shock:  
Gimme a break 
Was 90.95cm anywhere near correct, Anyone:confused: :confused:   http://www.math2.org/math/trig/tables.htm 
Remember :Redface: 
SINE Some Old Hags
COS  Can't Always Hide
TAN   Their Old Age

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## brekyl

sounds right ivan, two different sizes are because the longer side has got eaves where the other hasnt.

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## jow104

> sounds right ivan, two different sizes are because the longer side has got eaves where the other hasnt.

  But can you then have the same height to obtain 15 degrees either side, unless your apex isnt't central?

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## Ivan in Oz

> But can you then have the same height to obtain 15 degrees either side, unless your apex isnt't central?

  That's what I reckon.
I worked the other angle out to be approx 16.5* 
Close, 
but with no overhang, 
what's that....Eves

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## jow104

We've got some medieval cottages with roofs like this in Europe :Shock:

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## brekyl

thanx for the answers, i am just going on the 15 degrees that i have specified on my plans but the minimum is 5 degrees so anywhere between that will be fine.

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