Adding an hexagonal roof to an existing gable roof.

What prompted me to write this page was a question from one of my readers, Sherry who lives in Canada.   Here's what she wrote.

General design thoughts.

I have done something similar, with an elevated octagonal verandah, but that had a short connecting passage to the house.

• A separate room costs more, more walls, more roof etc.
• It does of course offer more design scope.
• It needs a simpler connection to the existing house.
• Separate construction does of course mean that you need more land area to build.

The additions to the house that I have drawn here, with the new room fixed directly to the house:-

• Is the cheapest way, less walls, roof etc.
• Another major advantage for Sherry in Canada is that for a small loss of floor area (all else being equal) there will be a saving in energy costs.
• Another advantage is of course is that the compact design uses less space.

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Hexagonal roof - Option "A", walls at bottom, 3d view and plan view on top.

Option "A"

I have drawn these sketches all with the same roofing angles, the main hip roof is at 45 degrees and the hexagonal roof is at 55 deg.

I thought that this one could be OK, with one of the walls in the new room sharing part of the main house external wall. Almost straight away I didn't like it.

• A basic fact of roofing geometry is that if two roof surfaces are parallel, where they meet will be a level joint.  In most cases it is a ridge, but when they slope down towards each other they form a level joint, that has to be sealed with a box gutter.
• I hate box gutters, and I have hardly ever used them.  Certainly not in my own designs.  Just before I started drawing these sketches, we had a short rainstorm that dumped 44mm of rain (nearly 2 inches) in about an hour and a half.  With rainfall like this we don't need box gutters, clogged with leaves and overflowing into our homes.
• I would guess that Sherry wouldn't be too keen on box gutters either.  Living in Canada, a large amount of snow would just compact into that vee shape.
• Another reason That I don't like this design is because of the acute angle that one of the new walls forms with the existing wall.  Useless dead space, and no view from a window in either wall.

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Options "B" and "C"

Hexagonal roof - Option "B" - Only five new walls, and they are set symmetrical about the corner.	Hexagonal roof - Option "C" - Only five new walls, this time not symmetrical.

Both of these version are similar and I guess I like the symmetrical one best, but the main reason for showing both of them is that if the one on the right is rotated clockwise it would give another option, with the longer join on the back of the house.

So you have in effect three positions here that the new room could be placed.  Any one of which may fit in with existing windows or doors, better than the other two.

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Hexagonal roof - Junction of hip rafters, showing top two rows of roof battens also.

A couple of general details applicable to all pyramid style roofs

At the right is a shot of me with a lot more hair and a lot less waistline than I am currently sporting.  Happy days.

So what we are looking at are the hips meeting at the apex of the roof. You can see the bolts in the hips that are fixing them to a simple steel bracket to hold them all together.   You may say to yourself that a steel fitment like this is a bit of overkill.  What's wrong with nails?

My answer to that is that this roof is still alive and well, doing the same job it was built to do.  Not like 95% of the roofs around it that got wiped out by Cyclone Tracy.

Hexagonal roof - Hexagonal roof - Apex detail using mild steel welded bracket.

The cleats for the hips are offset of course, half the thickness of the hip. With the five at the top, there is not a lot of room to put in another six full length common rafters.   Apart from using a lot larger pipe and more cleats, the best way is to arrange the rafter set out to have two jack rafters on either side of the center line of each roof surface.

Hexagonal roof - rafter set out hexagonal roof

OK so if you don't use a steel bracket at the top, because in fact a roof like this, unless it is so tall that it is starting to look like a steeple, is inherently very stable.   It is self bracing.

So if I was pitching one of these here's what I might do:-

• I would cut the first two with square top cuts and fix them to each other at the top and at The wall plates.
• I would be tempted to nail a couple of ply plates over the faces at the top to stiffen them up.
• Better still would be a couple of multi nail plates either side.
• The others would then have hip top cuts as well as side cuts on them.
• If the roof was not seen underneath, I would connect the opposing pairs with collar ties as high as I could get them.
• If it is going to be an exposed ceiling job, the least I would do would be nail hoop iron straps over the tops of opposing pairs, but if I had the material handy I would like to make up a sheet metal cap, similar to the flashing that will be on top eventually.
• This I'd nail down to the tops of the hips.
• As far as fixing the jack rafters to the hips, I'd be putting a 12mm bolt straight through the lot, both rafters and hip, with 50 x 50 x 3 plate washers to the nuts.
• Elsewhere I suppose nails would fix them, but the addition of nail plates would be nice.

Remember that the strength of a pitched roof is directly related to the quality of the carpentry.  Well fitting joints make for a stiff roof, and this is more so with one of these the taller it gets.

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Connecting to the existing roof. Using Option "C" as an example.

Hexagonal roof - preparation of the existing hip roof.

• Obviously I would leave opening up the existing roof to the last job, getting everything ready for a quick completion.  That would probably mean building the walls and having five sides of the new roof framed.
• I'd take off just enough of the old roof sheeting, covering and battens to do the job.
• The main thing is to leave the original roof framing and fixings untouched. There is no need to affect the strength of it.
• I would saw off any overhanging eaves.  Again, just enough so that they don't interfere with any ceilings.
• I would probably get the roof complete and weatherproof before opening up a wall for a doorway.

Hexagonal roof - the new roof framing.

Here is a sketch of the new roof framing.   The joined surfaces will form what we call scotch valleys.   I have done a sketch of a more conventional one below.

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A scotch valley

Hexagonal roof - Typical Scotch valley

In the layout sketches I have drawn an existing roof that is at 45 deg. pitch, and the new addition at 55 deg. pitch.  I didn't do this for any particular reason, they could have been both the same or a lot more different.  It is purely a matter of choice and what looks good and works well on one house may look different on another.

However when you are making the decisions for a job like this, there are a few other things that affect the final look of the house.  I have done another page on odd shaped and different angled roofs here.

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