A Mushroom Shaped Skylight

The Curious Case Of A Mushroom Shaped Skylight

We were on a rowhome flat roof recently, and we saw something that was a bit interesting. The picture below shows an example of a flat skylight with a flare at the top of the skylight.   At first glance, it’s not white obvious why this skylight has such an interesting and odd shape and form. Looking closer though, the reason for the oddity becomes clear.  In some cases, the ceiling under tge skylight could be framed at a flared angle to allow a skylight, limited inside, to emit more light.  That’s not the case here though.  The same principle does not work in reverse to allow the skylight to emit more light by being restricted at the curb, as shown in the photo below.

mushroom shaped skylight
Here, in the photo above a square skylight is installed over a significantly smaller flat roof curb.

 The reason or calls for this odd shaped installation is because  This skylight is not the first skylight installed at this location. The first skylight was smaller in shape and instead of procuring a skylight that fit the existing curb or reframing the curb professionally, a skylight of a non-compatible larger size was used.  At that point, the contractors still had the choice to reframe the curb to enlarge the outside dimensions of the curb to fit the new larger skylight. The contractor though, likely in an effort to save money, just framed the top rim of the curb wider to fit the wider skylight.

mushroom skylight shape using taper
The taper is repeated at all four sides with the skylight flared outward like a mushroom.

From the underside of the ceiling below, the differences are likely not noticeable.  From a strictly functional perspective, if the newly modified odd curb flashing does not leak, then This weird makeshift type of work might be good enough and if it saves money, that’s always a good thing for the client.  Most professional roofers though, would not do this.

shape restricting opening size skylight
Looking from a lower angle, you can see that the original existing curb steps inward significantly and restricts the opening size.

From a best practices perspective, it’s generally better to create a less complex rectilinear shape to roof spied surfaces.

angled contour of the curb
The angled contour of the curb requires the overlaid modified bitumen membrane to bend in a way that creates a loose and uneven overlay, requiring reinforcement.

Smart proactive replacement, construction, upkeep and maintenance of low slope roof and mansard roof systems requires an enthusiastic interest and understanding of historical methodologies, waterproofing principles, and building science.   Here in Washington DC, historic and modern residential and commercial buildings are extremely expensive and the roof and related systems provide the shield that preserves the building. 

We encourage all of our clients, and all readers of this article and to our blog in general, to prioritize the value of quality construction and building maintenance, and develop a relationship with our company.  You can learn a lot more on our blog.  Feel free to check it out.  If you have questions about the roof and related systems of your building in Washington DC, fill out the webform below and drop us a line.  We will be in touch if we can help.

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