Most historic buildings in Capitol Hill, and other parts of Washington, DC have exterior demising walls, above grade that were originally just built with a double brick wythe wall. Normally there is a 3 step application of non-portland traditional plaster coating on the inside of those walls.
At specific locations at interior and exterior parts of double wythe brick facades, there are often wood elements built in, embedded directly into the brick work. You normally can’t see these wood elements. The wood elements are generally called nailers, normally they are covered by things like plaster on the inside of the wall and metal flashings on the outside of the masonry walls.
Recently, we repaired the roof and parapet masonry and roof coping on a historic Capitol Hill row home. In the pictures below, you can see an example of one building assembly built with embedded wood nailers.
Cut brick slips are used to increasingly lower the level of the parapet top and coping towards the back of the building. In Capitol Hill, most roof systems slope from the front towards the back, here in this case the roof is built the same way. Brick slips are thin, and in this case tapered pieces cut from full size clay bricks. In this case they are used to provide a gradually tapered and sloping level to the parapet wall. The brick slips are not central to understanding the embedded wood nailers, but an interesting element that you can learn more about and take a closer look at in the pictures in this article.
The top of the brick parapet was and in this case was rebuilt to be covered with a metal coping. Coping metals are a type of flashing, and very similar to other flashings, made from relatively thin sheet metal. In historic times, a few types of materials were used to make the metal flashings. Today prefinished aluminum is a common type of metal flashing, also referred to as coil stock, if field-bent. If the metal is ordered already shaped, it is called pre-bent or shop-bent.
In historic times, when this building was constructed, modern masonry anchors had not yet been invented and small wood plugs work ok in some cases in interior walls, but they do not work well in exterior walls, due to temperature differential driven condensation and building micro-movements. At the time, the best way to have a fastener stick into masonry was to install a wood nailer strip. Those nailer strips were set into wet mortar, similar to how a brick was built into the wall.
That sort of technique worked pretty well to get the job done, especially considering the technological limitations of the time.
There are a few problems with this sort of installation though:
These masonry buildings can last for hundreds of years but in many cases the wood nailers have been rotted out already for decades.
- Nailer boards of that time were not treated with the wood preservative type of chemicals we use today to prevent decay and consumption by insects. In some cases wood of the time and. Was treated with creosote, but most kneelers were not because they were small batch type implementations created on site in an impromptu manner.
- Similar to a wood plug, even though this is a larger portion of wood, the nailer strips are also susceptible to moisture from condensation.
- Yet in many cases these wood nailers, especially installed at a parapet causes a point of decoupling. For reference, expansion joints are intentionally created to decouple one section of a wall from another. In this case, decoupling allows for a section of the building to be isolated from a different section of the building. That can be a good thing because it can allow each area to independently bear on a substrate, for example. In this case though it would be better for the parapet to be continuously monolithic and connected to the wall below. At this particular job, our company rebuilt that parapet because the nailer caused a decoupling point which led to delamination and separation of the parapet bricks from the substrate. In this case that was a problem which needed to be repaired.
From a conservation and preservation perspective, several approaches can be taken to improve conditions related to substrate wood nailers. Primarily, waterproofing elements at coping and flashing should be inspected and checked on a routine maintenance schedule, either seasonally or at least annually. If coverings are kept in good condition, the life of embedded wood elements can be significantly extended. Hire a professional contractor which specializes, understands and appreciates historic construction elements and buildings.
In this article we talked about the following terminology and concepts, follow the links below for more related information from the IDS website:
- Brick Wythe
- Non-Portland Traditional Plaster
- Metal Flashings
- Embedded Wood Nailers
- Brick Slips
- Field-Bent Flashings
- Pre-Bent and Shop Bent Flashings
- Masonry Anchors
- Temperature Differential Driven Condensation
- Building Micro-Movements
- Wood Preservatives
- Expansion Joints
- Build Material Decoupling
- Continuously Monolithic Building Assemblies
These concepts are part of the fundamentals of historic masonry restoration, tuckpointing, and brick repair.
You can learn a lot more on our blog. Feel free to check it out. If you have questions about the historic masonry 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.