Solving a Split Pitched Roof or an Odd Shaped Roof.

On this Split Pitched Roof page.
The old way of solving roofs | Definition of split pitched roofs. | A very odd shaped roof. | Same walls, better roof. | Getting the true surface shapes | Overhangs at eaves

The old way of solving roofs

What we have on this page is basically the way that I was taught when I first started learning about roofing in the late 1950's  It is the way that I would recommend for a young apprentice carpenter to learn about the basics.

I go into this method on my roof basics page and even if you have already read it, read it again because we are going into some stuff here that you may find tricky.

Just to reinforce my point, visualise if you can any of the buildings that you know of, that are over say 60 years old.  I'll be willing to bet that you can picture some grand old places, with some magnificent roofs that would test the skills of experienced roof framers today.

Every roof in the whole of history that has ever been built prior to around 1950 was built without the use of computer technology, scientific calculators, or any one of the dozens of quick fix, super duper, no brains required, pre-programmed gadgets that promise "we will work it all out for you at the amazingly low price of $------".

So, by all means, when you know how to draw it out by hand, and have grasped the basics, then, and only then use a calculator if you want to speed things up a bit.  End of rant, let's get into it.

Definition of a split pitched roof.

  • When two roof surfaces sit on two walls that are at right angles to each other, and they are both sloping at the same angle they join at a regular hip of valley, and show up in a plan view as being 45 deg.
  • When the angle of the walls is less or more than 45 deg. and the slope angles are still the same, then they still join at a regular hip or valley.  This time the hips or valleys when drawn in plan view show up as lines that bisect the angle of the walls.
  • When the the two roof slopes are different, on either side of the hip or valley, then a split pitch or a bastard hip or valley is created.
  • A bastard hip or valley when drawn in plan view does not bisect the angle of the walls, but swings towards the steeper sloped roof.
  • The framing members, jack rafters, purlins etc. will have different side and top cuts

A very odd shaped roof.

If the brickie or the wall framer can build the walls, the roof framer can roof it.

For the purpose of this page though, I am not going to be talking about roofs sat on curved walls here.  It is certainly possible to do it, and I have seen some beautiful examples, but it is a bit out of the ordinary.

An odd shaped plan and roof
Split pitched roof - An odd shaped plan and even odder roof.

Now here's a thing that your average roof framer or truss detailer would scratch his head at and probably think you are mad for designing.

  • Any shape can be roofed as I have said, but we try to do it it economically.
  • Every roof surface here is drawn at the same angle, which makes the eaves overhang all around the perimeter constant.
  • It also means that there is only one level ridge, but a series of interconnected hips and valleys.
  • This means that just about every single rafter or truss used in this roof is a different length.
  • Apart from the complexity of the construction, it will look odd if it is ever built.   People in general, not just builders tend to expect certain things like level ridges, so one that is close but not quite level just looks wrong.

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Same walls, better roof.

So what we have to do is simplify the roof, which will make it a lot better looking, and a bit easier to build.  A roof like this would never be easy to build, because of the number of bastard hips.  In Aussie terms it would still be "a right bastard" to build.

Odd roof tidied up
Split pitched roof - Same wall plan with the roof tidied up a bit.

If this was a real job, I would play about a lot more withe the design of the roof, to try and solve some of the problems that will crop up later when I go into the eaves overhang details.

  • When walls and the roof surfaces that come from them are parallel, they will meet at a level joint, either a ridge, or more uncommonly if they are sloping towards each other a flat valley or box gutter.
  • Roof surfaces 1 and 6 have a level ridge between them.
  • The wall under surface 4 is also parallel to 1 and 6, but because of the lot greater span it was easier to group it with the others.
  • The idea is to get the roof surfaces as similar to each other.  With the apex try to get it as central as possible.
  • I start off by drawing the plan, and in this case I have shown the outside perimeter in purple, which represents the fascia line which is level.
  • From the plan I extend construction lines sideways to get a side elevation, and downwards to get a front elevation.
  • If I wanted to get a view looking towards say roof surface 3, I would draw my construction line square off roof 3's fascia line.
  • I need one of two things to to draw an elevation.
  • The height of the roof at a certain point.  Which is the way I did it, I just picked a height "A" and went from there.
  • Working from someone else's drawing, they may state that say, roofs 1 and 6 have a 30 deg. pitch, so I would draw my side elevation first, which is square off the 1 and 6 walls, and draw a line at 30 deg to my baseline until it meets the construction lines (coming from the plan) of the ridge and apex.
  • Let's say that I was given the fact that roof surface 7 had a rise of 4 in 12.  I would start with an elevation square off 7 in plan view, (the front elevation) draw 12 units sideways and 4 units up to get my angle.  I would then extend it to the construction line dropping down from the ridge.  Bingo! I have my height at "B".  From that I can solve the roof.
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Getting the true shapes of the roof surfaces.

If we can get the true roof shapes, they will give us most of the other things we need to build the roof.

true roof shapes
Split pitched roof - Finding the true roof shapes.

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Overhangs at eaves

The effect that odd roof angle have on the eaves overhang
Split pitched roof - The effect that odd roof angle have on the eaves overhang.

At the top of the sketch is a side view of a couple of rafters shown at different angles.  Showing that as the angle changes, so does the overhang of the roof.  The way I see it is we have a few choices when face with this problem.

  • Alter the wall height. It is easy to see from the top sketch, that if we changed the wall heights then with a bit of drawing, we could find two different wall heights, that would give us a level fascia, and a same eaves overhang.   In the instance shown this could be impractical, but in the case of a new addition it could work.
  • Where it does work is when the difference is quite small, and it may be just a case of adding an extra or thicker wall plate to one wall.
  • Create small sloping runs to change the fascia heights.  I have seen this done a few times and it is a good way out of the problem when the pitches are very different.
  • Where the difference is very small, like between roof surfaces 5 and 4 in the sketch above, I would move the hip away from the exact corner of the building a touch so that it intersects the point of two equal overhangs.
  • Doing this of course slightly alters other things, which takes us back to wall heights.  What I would do here is probably adjust the depth of the birds mouths.  Of course not making either one too deep.

In just about every case I've seen where a mistake has been made on a roof (and I'm including a few I've done myself here) it is a result of not thinking it out first, or worse still, not even having the basic knowledge to be able to recognise where problems can occur.

The absolute best tool you have for working out roofs is not a book of tables, or an electronic calculator, it's that thing sat on your shoulders!

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Please Note! The information on this site is offered as a guide only!  When we are talking about areas where building regulations or safety regulations could exist,the information here could be wrong for your area.  It could be out of date!  Regulations breed faster than rabbits!
You must check your own local conditions.
Copyright © Bill Bradley 2007-2012. All rights reserved.
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