Roof intersection


I am in the process of doing a renovation on a ranch house by adding a one room extension. The problem is the intersection of the existing roofs and the new extension which creates a significant problem for proper water drainage. If executed as designed the eave side of an existing gable roof will intersect with the flat wall of the new extension. What do you suggest to control the drainage problem in an inherently poor design? FYI: the wall indicated is either metal or cedar siding.


This is hardly a SketchUp question…
More a design question…

The extension [I assume the ‘orange-parts’] seems to meet the existing roof surfaces and roof eaves ‘horizontally’, so you [as the designer] need to come up with a way of adding things into those areas which will do several things…

Below the new eaves…

You need to provide a ‘valley’ surface that will collect rain-water run-off, and have a slight angle towards the outer area, so it drains to the outside - presumably discharging into the existing eaves gutters - which you have not modeled thus far…
This new ‘valley’ surface will also need ‘flashing’ into the existing roofs so that these junctions drain and are watertight.
Also this new ‘valley’ surface needs ‘flashing’ into the new wall that projects above h existing roof so it is watertight.
The small area between this new ‘valley’ and the overhanging new roof’s eaves will undoubtedly become a haven for roosting or nesting birds, but that’s another issue…

Above the new eaves.
You need proper ‘valley gutters’ formed between the new roof and the existing roofs.
Where they have a natural fall, so that they will ‘drain’, then this is straightforward, but I suspect that although you have not given a clear image of the extension’s very rear part, the new and exiting roof might meet to form a horizontal junction…
So somehow, rather like the visible junction at the front, you need to ensure this is NOT ‘flat’ and that it drains properly to the outside…
Here endith the lesson on building-construction [NOT SketchUp]…


Water runs downhill. IIRC that’s because of gravity.
‘Waterproof’ membranes all fail after a certain period of time. Gravity has never failed.

Don’t build it.


Thank you for the sound suggestions you offered. This project is in the preliminary stage and I am hoping to make contact with people like yourself who have experience and knowledge to offer. I do recognize the need for proper flashing and “valley” for water to drain and do agree that much of that will deteriorate over time. I am considering using “kick out” flashing along the vertical wall but I am concerned that this will detract from the appearance.
Thank you again!


place a 450 (1.5’) wide x 100 (4’) falling to 300 (1’) stain-less steel box gutter around the new build

or even wider, cutting back into the existing building, this will attenuate the new build


I guess you could build some sort of “cricket” behind the addition. Still, it’s a troublesome design.



You can place a roof drain manifold inside the new addition, either exposed or in the wall, spilling out through a scupper in the front.



These are some great suggestions: drain manifold, cricket! I am not sure how a roof scupper works to evaluate the use and benefit but I will surely investigate it further.

Do you think this would have a 15 years life span?:

  • the new extension upper walls were made of metal with overlapping joints over a rubber membrane;
  • the perimeter of the extension has a drain manifold where it meets the gable; and
  • a cricket was placed in the valley formed between each section.

Thanks so much for your suggestions,


here in Australia, cricket is a sport, or something to watch on a sunday afternoon when you want to veg out, a “test” so named can usually guarantee a period of sleep

so despite 20 years in the building game, I had to look up scupper as well, We don’t use half of the colorful naming conventions you do. Much the pity I guess.


A “scupper” is a drainage spout, which projects drain water out and away from the structure. I guess they’re generally used to provide drainage through the parapet of a flat roof, but weird situations call for weird solutions. I’m thinking the OP could just run a drain pipe through the inside of the new addition and then back out again. If the OP is into “industrial chic,” he could leave the pipe exposed inside the structure; otherwise, he could box it in. Something like this, perhaps:



What works in one climate often fails miserably in another.
@frsprings227 You’ve made to mention of conditions.

Proper design and construction of roof, wall and foundation assemblies stems from location specific climate and site conditions. Volumes have been written about the subject by this fellow:
Joseph Lstiburek, Ph.D., P.Eng., ASHRAE Fellow — Principal, Building Science Corporation

Meticulously detailed roof drains, gutters, low-pitch valleys and the like may suffice in a warm climate.
Those same details tend to be avoided like the plague in colder climates.

Fifteen years isn’t much compared to the life of a building.
Keep in mind a roof is an assembly.
Premature failures are generally failures of the assembly, not the materials.

Depending upon quality, an asphalt shingle roof will last 20 - 30 years or more.
Even an old fashion cedar wood shingle roof (not shakes) will last 40 years when properly installed.
Metal, slate and some composite materials can last a lifetime and longer.

Any roof assembly will fail if the design and choice of materials don’t suit the situation.
Hope and guesswork will not keep the water out.


Snow and ice complicate matters. Water melting from the snow and ice on a roof can form pressure that helps the water penetrate any seams that are not 101% waterproof. Good ventilation and sufficient airspace between the thermal insulation and the roof surface is also important in a cold climate as it reduces these risks - they are worst if the outside temperature is below freezing point and heat from the interior leaks to the roof surface and lets the ice and snow start melting under the insulating snowcap.

About the design: Couldn’t the whole clerestory be left out and the addition be topped with a simple lean-to roof, perhaps somewhat more shallow than the main roof?



Gullu_Foyle,thank you for your scupper explanation (and drawing). I doubt that I would be able to use it in this situation, but knowledge is always good to have.

Yes I am located in the northern US where changing temperatures, snow, ice, and rain are common throughout the winter so I will have to address those occurrences into the design.

I am grateful for all the suggestions and insights offered!!! I learned a lot.