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Sunday, March 9, 2014

A large overhang on a Dutch Hip roof requires a few changes to the normal construction method

In 2011, I was given the opportunity to frame up 4 small houses with Dutch Hip roofs on a remote island in the tropics.  I planned to use the experience to teach western style framing to the contractor's crew.

Panama canal zone Gamboa design
The houses were designed in the Gamboa style of the USA military Panama canal zone architecture.  This style was very effective in the tropics.  It incorporated huge overhangs to provide both abundant shade from the brutal sun and extra coverage around the house from torrential rains (22' average per year).  Temps in the shade during the day were always in the 90's accompanied by skin soaking +90% humidity.  Only at night did the temperatures become bearable in this carpenter's honest opinion.

The buildings were relatively narrow with 10' tall exterior walls.  The ceiling was “open frame” style 2x8 rafters/ridge at 5/12 pitch with 1x T&G decking above.  The Dutch Hip’s tiny Gable end walls were only to be perimeter framed then screened off to serve as roof vents.  During the day, the tropic sun would bake the metal roofing which in turn would transfer some of its heat to the air below the sheets and that hot air seeking to rise would find its way out these vents.   The rising air would draw cooler air in through the windows, facilitating a constant movement of air through the house, somewhat like how a chimney draws air up and out of the fireplace.  The interior partition walls were 8' tall and open above to the roof, similar to work station cubicles in a large office.  All the framing was to be done with imported treated Hondoranian pine, some strange wood that I had never heard of before.  I sure hope it works, since in this climate, mold and termites can knock an untreated wood structure down almost as fast as one can put it.

Reviewing the plans I noticed that the ratio of eave’s overhang distance to common rafter run was near 1:1.  While in latitudes farther to the North this would cause snow loading problems, here in the tropics with lightweight metal roofing as the norm and nothing other than rain (perhaps an occasional coconut) loading the roof surface, one would suffer no dire consequences from the design.  Of course with tails that long, one could anticipate a struggle when running the fascia.   With the birdsmouth located mid length along the rafters and the rafters themselves having varied sized crowns you can easily imagine the extra time that will be involved in jockeying the tails up/down to create a straight fascia line.

Original roof plan

I observed that the architect had drawn the Dutch hip ends to be framed in the standard method where short hip rafters frame to butt into a set of paired common rafters positioned at some distance in from the end of the building.  Normally, Dutch Hip overhangs are relatively short as compared to that part of the Dutch Hip framing set inside the building.  Most architects/engineers like to use a 1:2 or better cantilever ratio favoring the inside in these situations.  In other words, that part of the Dutch Hip framed inside the house is at least 2x the length of the eave’s overhang.  As a bare minimum, they want to see no less than a1:1 ratio unless a counterbalancing force is incorporated.  But here in this case, the cantilever ratio was reversed with much more rafter length hung outside the building than the Dutch Hip framing protruded inside the building.  There was 6' of overhang versus only 4' of rafter length inside the building.  This was a major structural concern since the hip rafter tail alone would serve as the primary support for a 6' x 6' roof section (36 sq. ft.).  Evidently it had slipped by the architect unnoticed.  (Click HERE for short video clip on standard Dutch Hip construction)

What I decided to do was get permission to shorten the overhang distance by 1' making it a total of 5' instead of the called out 6', and then I would run the Dutch Hip’s “hip rafters” full length up to the main ridge rather than butting them into a set of paired common rafters.  I would use continuation jack rafters to frame in the small Gable like extension that is an integral part of the Dutch Hip design.  By doing this it changed the "as drawn" cantilever ratio of 3:2 favoring the outside of the building to a 5:7 ratio favoring the inside of the building.  Personally, with the big eaves, I would have liked to have seen doubled 2x8s used as the hip rafters versus the specified single 2x8 but since I was only there to help with the framing, not redesign the buildings, I would stay out of the architect's realm unless I saw blatant errors. The material for the hips had been delivered per the original drawings so with my change they would now be short to reach the main ridge.  To reorder longer lengths would delay the project better than a month so I chose to "stretch" those boards by structurally joining them to another piece of 2x8 utilizing a dovetail joint.

Step 1 - overlay and mark

Step 2 - cut
Step 3 - assemble

The dovetail joint is detailed in "A Roof Cutter's Secrets" page 115.  Click HERE to view a short video clip from DVD2 of the "Roof Framing for the Professional - the Essentials" video series where I demonstrate making the dovetail joint to join two boards for a long ridge.

While I did not specifically detail how to do this special type of Dutch Hip framing in "A Roof Cutter's Secrets" (RCS), the book none-the-less includes all the background information needed to accomplish the task.  Cutting regular hip rafters are shown on pages 63-67, so nothing new here.  Continuation jacks are nothing more than regular valley jacks (RCS pages 97-98) installed from a ridge to a hip versus their normal usage from a ridge to a valley, so nothing new here either just a different application (continuation jacks are pictured on page 105 of RCS).  The Gable like extension at each end of the ridge could also be framed in layover style using CA valley jacks (RCS pages 97-98) if the full hip end was sheathed all the way to the ridge.  That wasn't an option on this project since the roof surface under the Gable like extension was to be left open for venting purposes.

Click HERE to view a short video clip from DVD1 of the "Roof Framing for the Professional - the Essentials" video series where I actually spend a minute in the classroom presenting this particular "hips to main ridge" Dutch Hip situation.

Edge of hip flushes with ridge

So for these 4 houses, I ran the main ridge well past where the full length hips would tie in so I could incorporate the Gable like extension.  With the ridge run long at each end of the building, there would be no standard "end-of-ridge" king common rafters to position the tops of the hips during stacking so I pulled up a temp “end-of-ridge” king common rafter (reg. common for the main span) to use as a marking guide.  I slapped it alongside the ridge and pulled the birdsmouth tight against the outside wall, then scribed the head-cut when the top corner edge flushed with the main ridge.  After transferring this plumb line up and over the top edge of the ridge to mark the opposite side, I used math to locate the hip intersection plumb lines at the far end of the ridge.  Recall that the distance along the ridge between those two sets of plumb lines would be the same dimension as if the ridge had been cut to an exact length for a full hip situation (RCS pages 63-64).  The math to calculate that length is: building length less building width, plus thickness of the ridge, equals the actual ridge length.  When stacked the top centers of each pair of hips would position at those lines.  RCS page 123 shows proper alignment of the top LP edge of a single cheek hip at the top edge of the ridge.

Hip rafter plumbed and braced at outside corner
When stacking the hips, plumb and brace the bottom of the hip at the outside wall corner.  This will make installing jack fill much easier.

Step hip jacks larger from corner to simplify frieze blocking

After we had the roof skeleton up (a few pairs of setup commons plus the hips), we went back and installed the hip jacks, the common rafter fill, and ran all the frieze blocks.  Since the frieze blocks were exposed they were placed at 90 degrees relative to the top surface of the rafter abutting the outside wall.  As most know from RCS, I start my jack layout from the building corner with exposed frieze blocks as this negates having to install tricky frieze block with a double compound miter at the hip rafter. 

Continuation jacks are used to help frame the Dutch Hip end

Now it was time to finish off the little Gable like extensions at each end of the ridge by installing the continuation jacks.  Because I had run the hip jacks full length on the ends of the buildings to strengthen the dovetailed hip rafter (rather than heading them off at the Gable extension), the 45 degree cheek-cut at the bottom of the continuation jacks needed to be clipped slightly to fit (see photo).  Remember these jacks must plane in with the far side of the hip, so hold them up accordingly.

2x4 cleats are used to straighten the hip and support flyer jacks

Since the overhang was so large we had to straighten the hip rafter tails with long cleats run from the installed hip jacks.  This cleat also facilitated installation of the two sets of flyer jacks located along the hip tail.


Long pieces of fascia along the bldg sides help w/ corner load
We utilized the 2x fascia to help carry some of the eave's corner load by hanging full length 20' pieces along the sides of the building extending out to the hip tail at the corners.  This "diving board effect" works even better if the fascia's crown is placed crown down.

Roof framing completed with 1x decking installed
With the 1x roof decking installed the building really came together.

Scaffolding to work the eaves was set up 6'4" from bldg walls to center
Inside view of the large Dutch Hip roof vent

Copyright 2014 by Will Holladay