Find out what causes Washington, DC, historic buildings’ roof tear offs
Washington DC is a city full of flat roofs. In fact, the city has one of the highest concentrations of flat roofs on individual properties in the entire country. Flat roofing is generally considered to be commercial roofing. Aside from the thousands of Washington DC rowhomes, flat roofing is somewhat unique on residential properties. Washington DC has this inventory of many thousands of historic buildings, but other cities in America and many other parts of the world do not have as many flat roof historic brick buildings as Washington DC. We have something unique here.
Benefits of Flat Roofs
- Washington DC has a largely unique concentration of buildings with flat roofs for a few reasons related to access, height restrictions, and fire separation between attached buildings. Flat roofs are not necessarily better than pitched roofs, both have their advantages and disadvantages. In the case of a densely built urban area with attached buildings though there are a few reasons why flat roofs perform and work better.
When in the never ending expanse of suburbs in America you can see a vast expanse of pitched roofs with various types of asphalt shingles. Pitch roofs work well on free standing individual homes and small buildings. But flat roofs do not work well on attached buildings. When you look across the majority of buildings in the historic neighborhoods of DC including Capitol Hill, Dupont Circle, and Georgetown, you can see that pitch roofs are not common.
Height Limit by Zoning
- The Washington DC, neighborhoods, like many parts of the USA, have height restrictions on buildings. We see high rise buildings in the CBD and other areas as well, but by-and-large the construction of multi-story high rise buildings and skyscrapers is limited by local zoning ordinances and special approvals. Zoning is in place today and has been in effect in similar ways for over 100 years in Washington DC.
When a building under a zoning height restriction has a pitched roof, that pitched roof adds to the overall height of the building in a way that detracts from the amount of available interior space. For example, if a pitch roof has a 12/12 slope, meaning 45ﾟ angle, the the roof could add more than one floor level of height to the overall height of the building. In such a case, the rooftop space would not be fully usable but would actually subtract or detract from the amount of usable space in the overall building. For this reason, height restrictions are one of the contributing factors to why pitched roofs are uncommon in the historic buildings of Washington DC.
Ease of Access
- Along with the limitations required by local ordinances and zoning, there are also limitations from a practical perspective on roof types. For example, over 3 stories high, it’s very hard to access pitch roofs. Pitch roofs require extra provisions for access and safety.Working on a pitched roof is more dangerous than working on a flat roof. When a building is in a confined, restricted or crowded and dense space, it is difficult to access a pitched roof without scaffolding.
Flat roofs on low buildings still require access provisions for maintenance and construction, but in many cases access can be provided via interior pathways. Access hatches and even ladders from the outside of a building can be used to access a flat roof with a relatively light set up, on relatively low buildings. By comparison, pitched roofs on taller buildings are difficult to access without elaborate access systems. This is one of the many contributing factors to why flat roofs are so common in Washington DC.
- Pitch roofs are generally stick framed. Historic DC buildings are almost ubiquitously built with masonry exterior wall construction assemblies. Essentially most of the buildings in these neighborhoods are built with double wythe brick masonry.
A wythe is a single thickness of brick, generally 3.5″ thick in the stretcher bond position. A double wythe wall provides a fire separation rating. It’s understood within building guidelines that a double wythe brick wall can limit the spread of fires for a designated or stipulated amount of time period or resistance factor of fire spread.
- This limitation contributes to a building’s ability to meet joining and fire separation requirements. In a stick or lumber framed roof assembly, by comparison, the framing and sheathing and roof membrane itself provides a comparatively small fire separation coefficient. This means, stick framed partitions and roof configurations are bad at resisting the spread of fire.
At side-by-side unions between buildings, where a pitched roof joins another pitched roof, framed with wood materials, there is an inherent risk of fire spread. In other words, if a fire were to happen in one building it could easily jump from one pitch roof to the next. This type of construction has been banned in many parts and types of buildings in Washington DC. Such requirements or limitations against framed building separating partitions go back over a century.
Avoidance of Valleys
- A roof layout, especially in a pitched roof, has to be geometrically configured so that it has sufficient grade or slope to drain from all points in the roof. DC row buildings were generally built in rectangular or rectilinear types of shapes. If the ridge of the roof, in a gable style type configuration, runs parallel to the front of the building, then with a long run from front to back, the roof will have to be extremely tall to maintain a sufficient pitch. A shingles clad pitch roof generally is required to have minimum 2.5 units of height per 12 units of run. As a side note, single-ply membranes generally require only 1-2% minimum grade or slope.
Roof pitch dimensions are generally described in ratios with a denominator of 12. For example A12/12 roof is 45ﾟ angle, for each horizontal run of one unit there is a corresponding one unit of vertical rise. By comparison a 18/12 roof will have 1.5 units of vertical rise per 1 unit of horizontal run. For a 60′ long DC building, with a ridge parallel to the front facade, the run would have to be approximately 30′ from the ridge in each direction if placed directly in the middle of the building from front to back would require a roof height of 45′. In that same example, a minimum allowable slope of 2.5/12 would require a roof ridge at a height of 6.25′ above the highest point of the roof perimeter. Even in the lowest extreme, it’s still a significant loss of building height.
- Because of the simple geometry and constraints of the size and shape of DC buildings, such a significant rise is generally not practical in terms of costs to build and not allowed because of height restrictions on buildings. So for these practical reasons, because of the rise to run ratio required for shedding water, it’s uncommon to have pitched roofs with a ridge run parallel to the front facade of the building.
By comparison, if the ridge were to run from the front of the building to the back of the building, in an otherwise similar context or layout, there would be a valley between the adjacent buildings. In other words the downward sloping rafters of the roof would terminate at the perimeter of the building where that building adjoins an additional or neighboring building. If the next building would be built in the same way, that would essentially create a valley. A valley is where 2 roof elements join each other in a downward sloping joint.
- Valleys themselves are also required to have a pitch of roughly 2.5 units per 12 units of rise . In a configuration where buildings are placed side-by-side and have eaves that run towards each other, that valley also needs to shed water and therefore be built with a significant slope. This type of layout is not feasible in relatively thin and long buildings, such as those found in DC where property plats are commonly divided into the shape of long and narrow rectangles.
Historic Roofs vs Modern Roofs
- Historic roofs can be built from a variety of materials. One of the oldest types of historic roofs is a standing seam sheet metal roof. At the time of original construction, roofs of this type were generally built with a ferrous lead alloy. Today lead alloy metal roof materials have been taken off of the market and banned for environmental reasons. In many cases lead is banned from use in building materials for health concerns. In the case of sheet metal roof materials, a lead alloy is a relatively stable form, but nonetheless has been taken off the market or banned from production and use because of environmental harm associated with material production. By comparison modern alternatives generally do not perform, in terms of oxidation resistance, as well as lead alloy materials, on a unit per unit cost comparison.
Built up roofs are also a common type of historic roofing material, generally abbreviated as BUR’s. These types of roofs generally lack mechanical attachment and therefore require a heavy aggregate or stone ballast of crushed rock or gravel on top of the built up roof membrane. Because of the heavy load of materials and the overall large volume of materials required for multiple overlaid layers of roofing plys, and difficulty creating dependable terminations, this type of roof membrane is increasingly less common.
- By comparison, there are many types of common modern roofs. Many of these modern roofs are single-ply membranes. Single-ply membranes with modern synthetic materials are generally very strong, have higher reflectivity and therefore better from environmental impact, and also much lighter and have the ability to be fastened to a roof deck substrate. These multiple reasons generally make modern roof systems less expensive with better overall performance. While being less expensive from a material perspective and installation perspective, they still nonetheless require a lot heavy work and technical expertise
The picture below shows an example of a high reflecity modern single-ply roof system.
Roof Lifespan and Double-life Before Tear-off
A typical modern roof membrane can last over 20 years. With good maintenance, modern roofs can even last for a much longer period of time. Nonetheless, at points throughout the building’s life cycle, old membranes will need to be removed and replaced.
Limit on Layers
- The building code allows for a roof membrane to be overlaid one time. That means that a roof system can actually have an underlying abandoned membrane in place below the useful roof membrane. The building code also requires that the underlying membrane is torn off and removed at the time of replacement of the second membrane on top.
In other words, if you already have two roofs in place, you cannot just remove the top layer and overlay the underlying bottom layer for a second time. At the time of removal of the top membrane, in a case where there’s two membranes, both membranes must be completely removed.
Poor Industry Oversight
- In Washington DC, DCRA is charged with responsibility to apply oversight to the rules of the rules of building construction. These rules are generally set and adopted through the International Building Code. The building code requires that contractors are licensed and follow a minimum set of requirements in line with quality standards and resultant concerns with building safety. The building code basically views a building built outside of code compliance as unsafe to users and occupants.
However, the building code itself does not apply as a method of enforcing oversight, the code essentially just sets the requirements or standards. In the case of Washington DC, DCRA is the source of oversight, yet DCRA is notoriously under-resourced or ineffective at enforcing the rules of the building code. This is one of the many reasons we recommend working with a quality focused contractor such as Infinity Design Solutions.
Typical Materials in a Roof Assembly
- Roof building assemblies may vary from building to building, but generally they are built with a similar set of materials. Commonly, roofs in the Washington DC historic districts are framed with a 10″ roof rafter. Often those roof rafters will run with overlapped or sistered tails, bearing on larger king joists which run from demining wall to demising wall.
Generally, on top of the rafters there will be a wood sheathing. A sheathing is a decking that is laid and fastened on top of the rafters. The word deck in this case is used to describe a system similar to a floor system or subfloor. In the case of a roof deck though, it’s generally pitched and intentionally not actually perfectly flat. There’s always generally a little bit of slope built into the roof deck so that rainwater will shed properly by gravity alone. The roof sheathing or roof decking will provide the substrate for the roof membrane but generally between the roof deck and the roof membrane there is a recovery board or insulation board.
- Insulation board works similar to the fiberglass batt insulation which is ubiquitous in wall and roof framing, Insulation board by comparison is generally created and shipped in a 4′ by 8′ pieces. These individual pieces will range in thickness, generally from about 1″ to 2″ thick. This insulation board has an insulative value, generally somewhere around a thermal resistance rating of approximately R4 to R8. The resistance level itself though is generally not the main purpose of the board.
The insulation board has two main purposes, it limits condensation between the deck and the membrane and the insulation board creates a substrate for the roof membrane which is more consistent and smoother for the installation of the roof membrane. Insulation board or recovery board will generally be fastened down with long screws and plates. Plates are often zinc galvanized metal disks up to approximately 3″ in diameter. Those plates prevent the fastener head from tearing-through the insulation material as the roof membrane faces heavy uplift forces from wind in typical Washington, DC weather patterns.
Yet the picture below shows an example of polyisocyanurate insulation board fastened wis fastened with galvanized steel plates and long screws.
Single-ply membranes, in some cases can be fully adhered on top of the insulation board. In that type of arrangement there’ll be four main materials, starting from the bottom working up will be the roof wood framing, roof deck, the insulation board, and the single ply roof membrane.
Below we discuss topics related to delamination and leakage at penetrations in roof systems, Delamination can also occur between linear joints and transitions between elements of roofs. The image below shows an example of delamination at a lap joint between a vertical counter flashing and a horizontal roof membrane.
Surprises Often Found
- At some point, in the life of almost every historic building in Washington DC, the roof has already been torn-off once and or multiple times. When a roof is removed in a process we often call tear-off, sometimes we find surprises. All concealed materials or assemblies in building construction should be perfect, but that’s not always the case. Sometimes we find that concealed substrate materials are deteriorated or damaged. Some examples follow.
Rotten wood at a parapet wall, a result of failed coping metals follows below as an exampñe of common rotted decking.
Loose Brick at Parapet
- Demising walls generally form the perimeter of the roof system. A demising wall is essentially the partition that sets the boundary of the building. Within multi-unit and or multi-use buildings, parcels or properties within the building will also often be separated by demising walls. The demising walls are sometimes referred to as party walls. These walls essentially have a fire rating that will limit the spread of a fire, to an extent. Forward thinking building owners and engineers will often also prescribe demising walls to be built with greater values of sound attenuation.
On flat roof buildings, these demising walls will normally be built above the flat roof structure. In a circumstance where the demising wall goes above the roof, we call that wall built above the roof a parapet. Parapets are normally covered with a metal or similar impermeable type material coping. The coping at the demising wall allows for the roof membrane to be terminated where it is covered from vertical falling precipitation. The parapet coping will normally extend slightly downward to cover a bit of both sides of the parapet itself. In Capitol hill and Dupont circle and Georgetown, most buildings are built with a parapet that is around 8.5 to 9.5″ wide. The coping will return downwards on each side of the parapet approximately 1.5 to 2″.
At the time that a roof membrane is removed, before the installation of a new roof, we will often find loose bricks at the top of parapets. It’s a good idea for a building owner to consult with a contractor, such as our company, who specializes in both roofing and masonry.
Rotten Wood at Penetrations
- In building construction, a penetration is anything that passes through a partition, floor, slab, or ceiling. For example, when pipes or ducts are run through floors or roofs, and where the pipe work or ductwork comes through the floor, that is a penetration. In the case of a roof, where there are penetrations for vent pipes, plumbing pipes, refrigerant lines, condensate lines, or air ducts, any of those types of penetrations will require special provisions and special roofing details to seal and terminate the roof around the penetration properly.
In the image below, you can see an example of a refrigerant line set penetration through a roof membrane to a HVAC condenser. The exposed insulation on the low pressure refrigerant line is permeable and essentially fills with water similar to the capillary action of a sponge. That water then slowly drips inside of the building.
It’s very common, at the time of removal of the old roof system, we find damage to water susceptible framing materials at locations coincident with roof penetrations. These locations are generally places where there have been long term slow leaks. Those locations, if there are areas of roofing delamination or openings, will allow a small amount of water to enter almost every time there is precipitation or rain storms and that water will cause rotting in the framing materials.
In most cases, this type of damage is not extremely expensive to repair but it will often be an unexpected cost that cannot be foreseen or fully known and predicted before the start of construction.
Compounding Damage at Ponding Areas
- Technically, roof systems are required to be built without ponding. Ponding is a circumstance where water will pool. If a roof is built so that it has grade or slope at all locations, even a flat roof will not pond or retain significant puddles of water.
The picture below shows an area of flat roof where a roof cricket or tapered build up is required but missing. Without the roof cricket or tapered build-up water ponds against the skylight curb.
Areas of ponding can cause problems. Overtime, ponding at locations of single-ply membrane seams will eventually allow water to enter the roof system. We consider this type of issue to be a compounding problem because it makes itself worse as time goes on. For example, at a ponding location, as water enters slowly in the beginning, over time the materials below the ponding area will begin to rot. As those materials rot, the deflection or depression in the area of ponding will increase so that the surface will depress and sag lower. As that subsurface sags lower, the ponding or water pooling area will increase in volumetric area. That higher volume of water will allow more water to enter below the surface which will cause more rot which will in turn cause more sagging or depression. This becomes a vicious cycle leading to accelerated deterioration in the ponding areas of the roof. Rear terminations are notorious for this type of vicious cycle related to ponding and water ingress. As opposed to most incidences of escape of liquids or typical leaks, this type of issue can continue for a significant amount of time without the building residents or occupants even ever being aware that the problem is happening. In the meantime building deterioration and rot accelerates to the point that damage can become significant.
Another image showing an example of ponding at an area around an internal roof drain follows below.
Failed Rear Terminations
- One of the most common problems associated with rear roof terminations is very similar to the issue of ponding and compounding leak problems described in the section above. However, usually the rear termination, when we see problems of this type, will be caused by a unique circumstance. Since most of the historic properties in Washington DC are over 100 years old, most of the roofs have been changed or replaced in the past.
When a roof is removed by a less professional contractor or driven to make cheap decisions by a low budget client, often the lower quality contractors will leave the existing rear termination in place.Removing the existing rear termination and replacing it is more work and has an associated additional cost, but we recommend all of our clients have the rear removed if the roof membrane is removed.
- When the rear termination is left in place, the replacement roof will be lower than the rear termination and therefore cause a backgrade in the plane of the roof. That backgrade causes ponding, which leads to a portion of the roof to be under water for a longer time period than is natural. That will in turn lead to accelerated deterioration of the roof at that location. This type of problem is extremely common at flat roofs in Washington DC.
Consulting With an Engineer and Flat Roofing Specialist
- We recommend building owners consult with a licensed, solution oriented, contractor or engineer. For complex and large roof systems, in certain circumstances, we may specifically recommend a client consult with an engineer, depending on the circumstances. A good quality contractor should be able to examine options and provide alternatives that can work in a customized sort of scenario. All roof systems are different, and it’s helpful for a client to be able to trust a contractor to provide options which might work best for their unique situation. This is one of the many reasons relationships matter.
From a conservation and preservation perspective, several approaches can be taken to improve conditions related to flat roofing and masonry repairs. 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 building elements can be significantly extended. Hire a professional contractor which specializes, appreciates,, and understands historic construction elements and buildings.
- We recommend seasonal or annual routine roof check ups and condition surveys. Identifying and diagnosing problems early can reduce the cost of upkeep and repairs, significantly. Often, when a building system suffers from deterioration, the associated need, and their cost of repair, grows exponentially. In building engineering terms, we refer to the exponential increasing damage, as a result or reflection of the principles at play in the natural nonlinearity of the deterioration curve, a common engineering concept.
In this article we talked about the following terminology and concepts, follow the links below for more related information from the IDS website:
- Flat roof
- Commercial roofing
- Height restrictions
- Fire separation
- Pitched roofs
- Asphalt shingles
- Free standing buildings
- Attached buildings
- CBD (Central Business District)
- Access and safety
- Access hatcheS
- Double wythe
- Stick framing
- Rectilinear geometry
- Roof ridge
- Roof gable
- Single-ply roof membrane
- Rise to run ratios
- Roof valleys
- Roof deck
- International Building Code.
- Roof rafter.
- Sistered framing
- King joists
- Insulation board
- Fiberglass batt insulation
- Roofing screw plates
- Substrate materials
- Demising walls
- Multi-use buildings
- Property parcels
- Sound attenuation.
- Roof parapet
- Building assembly penetration
- Slow leaks
- Roof ponding
- Roof cricket
- Tapered roof build up
- Skylight curb
- Vicious cycle
- Accelerated deterioration
- Internal roof drain
- Rear roof penetrations
- Geometric planes
- Nonlinearity of the deterioration curve
These concepts are part of the fundamentals of flat roofing, waterproofing, historic masonry restoration, tuckpointing, and brick repair.
The links in the list above will take you to other articles with more information on defects, failures, preservation and repair of historic masonry. 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 our contact form or give us a call. We will be in touch if we can help.
Roof Tear Off Before & After Pictures