Alternative Types of Roofing – Part II

Earlier this week in our last article, we began a series talking about alternative types of roofing materials.  

In today’s article we will be looking at sections 4-6 of this the closing of this first part of the series:

    1. The History of Clay Tile Roofs
    2. Clay Tile at Mansard Roofs in Washington DC
    3. Limited Spans and the Impact of Structural Roof Capacity 
  • The Overlapping Concentric Nature of Roofing 
  • The Reversibility of Clay Tile
  • Upkeep, Care and Maintenance of Washing DC Historic Roofs

The details of the overlapping nature of roofing materials, overall and in general and the specific reversibility of clay roofing tiles and the upkeep, care and maintenance of historic roofs in Washington DC are topics that will be covered today.

The Overlapping Concentric Nature of Roofing

Almost all types of roofing are installed in individual pieces. There are some exceptions, particularly with fluid applied roof systems but most roofs are installed in individual smaller pieces, and those individual sections meet or join together.

For example, modern modified bitumen low-slope roof membranes are generally roll applied in rolls that are about 3 feet tall. The rolls are actually a little bit in excess of 3 foot, but after the overlap of about 3 inches, the net balance that is exposed is about 3 feet in length or width.   TPO, EPDM, and other types of synthetic roofing materials can be generally purchased in 6 foot or 10 foot rolls. Those rolls, in many cases will extend for up to or even over 100 foot in total length. At each of the 10 foot lengths there are seams though, there is an overlap of several inches where the upper layer of roofing membrane lays on top of the lower layer of membrane.   Concentric applications, generally refer to round or radius elements, but in the area of roofing, sometimes an overlap, whether applied on a convex or relatively flat will be referred to as concentric indicating that each area of the lower portion of the roof is covered, at the top or at the open edge of the material length with an additional material of similar nature or of flashing or coping at an higher area above.

In the case of terracotta roof tiles, each of them is roughly the shape of a half of a cylinder cut lengthwise from top to bottom. Each of those terracotta tiles is covered by another terracotta tile above which means that the otherwise exposed or open edge of the terracotta tile isn’t actually open. It’s covered by an additional piece of material.  

 If you look closely at the pictures below, you can see that there are no exposed or open edges at the upper sides of each tile, instead you can see the lower edge of each tile but each edge is covered above like scales of a fish or like pieces of metal in a suit of arm. This concept of principle is very important for roofing because roofing, like scales of fish or pieces of metal in suit of armor, is required to protect the building, not from damage necessarily, but from water entry which would in fact, without this protection, lead to damage.

roof tiles
In the picture above you can actually see only a portion of each of the roof tiles, the extending lower part.

There is an additional tile that is covered.  For each one that is shown, there’s another one laid upside down, concentrically to channel water away, on a typical and traditional installation.   In the closer view shown below you can see there’s galvanized corrugated roof panel installed underneath some of the terracotta tiles. This additional type of membrane is used to catch the water and work as a more continuous barrier to channel water away from the roof.

terracotta roof tiles
Although terracotta roof tiles are very expensive in the United States, they can be made by hand with low cost labor and fired using organic materials from the wilderness.

It isn’t exactly practical for most Americans to make their own terracotta  roof tiles, most of them have to go back to work on Monday,  and after a weekend of long hard work, they won’t be very far into the process.  The process of making terracotta tiles, especially when done by hand, is very arduous.  There’s tons of repetition involved and it’s simply very heavy work.

The Reversibility of Clay Tiles

Terracotta roof tiles are a bit unique in that the same roof tiles which are concave can be flipped to be installed in a convex position. You can not install asphalt, EPDM, modified bitumen, or TPO membranes in an upside down position, they just wont-work.   Each of those materials have certain characteristics on their upper-side and underside.  One of the main most concerning differences is that the upper layers have the higher resistance to ultraviolet ray exposure. In most cases, the undersides of those materials have a very weak resistance to ultraviolet and would attribute quickly if installed upside down.

Most types of terracotta rooftops though do not have a glazed area and have a consistent almost half pipe type shape which can be flipped upside down or upside right either way to work the same way.

counter opposed tiles
On the underside of the roof, you can see the exact part that cannot be seen from above, the concealed or covered tops and the underlying upside turned counter opposed tiles

Upkeep, Care and Maintenance of Washing DC Historic Roofs

It’s worth noting that while many of the historic methods of roofing do not work well in our climate zone, the principles of this type of building design and engineering highlight the fundamental principles of our roof flat and mansard roof design here in Washington DC. Consulting with a roofing professional, like Dupont Roofing, familiar with the specific climate conditions and the building science of roofing in Washington DC is advisable to ensure the roof assembly is appropriately designed and constructed to manage the flow of water, ice damming, capillary action and condensation effectively.

The upkeep, maintenance, and general care of flat and mansard roof systems should be driven by an understanding and passion for historical methodologies, waterproofing principles, engineering and building science. Washington DC, a city built with both vintage charm and contemporary modernities, residential and commercial buildings of substantial value. 

The roofs of these buildings are their defensive shield from the harsh elements of nature. To our clients, as well as all readers of this article and our blog, we emphasize the importance of quality construction and active building maintenance.  Our website includes informative resources you can use to understand and learn about best practices on preservation of your building.  If you are in need of further guidance on the roof and its associated systems for your Washington DC property, we are here to assist, where we can. Simply contact us or complete the webform below and drop us a line and we’ll respond if we can help.

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