Rooftop Decks – Part II

In a recent article, we took a look at a rooftop deck and the mounting locations of the deck. The deck is supported by wood posts and the posts pass through or penetrate to low-slope roof membrane. You can see that not just the deck girder, but all of the roof deck joists, are supported by just the sole four posts at this roof.

Importance of Strong Roof Deck Joists

By putting such a heavy load at each of these posts, it reduces the overall amount of penetrations in and through the roof membrane. However, it is concerning that since the bearing locations do not sit on top of the structural wall, they have to have a transfer or separate connection to the roof joists below. And at this location or each of these four locations that connection must be strong enough to support the load of the deck. Historically, at the time of the original construction, ceiling framing was slightly undersized.  There’s a common old adage or belief in our collective conscience that maintains ‘that things are not built as good as they were in the past’ but that’s not always true with all parts of historic row homes or historic row buildings. We find that ceiling framing was not as strong as the framing systems we would use today.

low slope rooftop decks joists

Right away though, at this building assembly, the biggest concern, from the visually observable conditions, is that the post penetrations through the modified bitumen low-slope roof membrane appear to be poorly installed and we are generally concerned about connections in the waterproofing or roof membrane which solely rely on The connection of low slope roof systems and framing lumber.

roof termination bar missing

The next picture below shows the same area more closely. You can see that the modified bitumen roof membrane is used as a base flashing.  This application of base flashing meets the building code requirements in DC.  However, roof termination bar or counterflashing is omitted, and that fails to meet the requirements of the building code.

This type of omissions is very common in Washington, DC.  We are not saying it’s correct, but we are constantly fixing or improving this type of construction condition as built by other contractors.

material type of rooftop deck

Why Material Type of The Posts Matter

Although it’s not correct to just install the base flashing by flame torching the modified bitumen roof membrane directly to the wooden post of the deck, the biggest problem is actually the material type of the post.  In this case the four posts are 6×6 pressure treated Southern yellow pine.   The biggest problem with this type of material is that wood is porous, it swells and shrinks consistently throughout its life, and it will move differently and separate from the modified bitumen roof membrane.  This type of installation specifically relies on the connection between those materials, and it is therefore guaranteed to fail.  The failure will be further complicated by hard to detect and discern small areas of delamination and inconsistencies.

Maintaining Rooftop Decks

Smart proactive replacement, construction, upkeep and maintenance of low slope roof systems requires an enthusiastic interest and understanding of 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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