Flat Roof Analysis
Almost all flat roofs in Washington DC are constructed with main support beams (2×10′s) that are pocketed into the side brick walls. On top of these walls sit rafters (2×6′s) that are fastened to these main support beams. Then, there is wood sheathing (1×6′s) that are fastened into the rafters. The roofing materials are then installed over top of the sheathing.
Current building code requires that this roof structure be able to hold a live load (LL) of 30 lbs per sq foot, a dead load (DL) of 15lbs per sq foot with a total load (TL) equal to 45 lbs per sq foot. The issue at hand is that almost all of these flat roofs have the rafters spaced at more than 24″ apart. This means that the code will require this roof structure assembly to hold 90lbs per sq foot. The problem is that this assembly will only have an allowable load of 60lbs per sq ft.
What does this all mean? It means that your roof structure does not have the structural support to handle the necessary loads that current building codes calls for. Even though structural engineering requirements are conservative, there is a risk here. In January 1922 the roof of the Knickerbocker Theater in Mt. Pleasant, collapsed under snow from the storm subsequently named for this same collapse. January 14, 1996, during the “storm of the century”, Potomac Mills Mall was evacuated due to snow causing metal beams to sag in two places, which caused the ceiling to droop about three feet. Inspectors declared the mall unsafe at that time. These stories have a tendency to be forgotten.
During the year, we find that 10-15% of the new roofs we install, require structural work due to sagging, cracked, or broken main support beams. Another 10-20% require rafter repairs due to sagging or rotting conditions. Many homeowners are quick to say, “my house has been here for 80 years and hasn’t collapsed, why would it now?” My answer to that is, “What is your life worth? Do you really want to take that risk?” This risk gets worse when negligent contractors start installing heavy solar panels or other roof top equipment without taking the structural loads into account.
In order to meet code, the structure would need to be composed of 2×8′s 24″ on center or 2×6′s closer together (less than 16″ on center). So, in order to install solar panels on roofs (since roofs in your neighborhood are composed of 2×6′s) you will need to modify the structure to support the new loads on the roof.
If contractors are not getting in the attics and looking at the rafters, then they are putting residents at extreme risk. If they are looking and tell you that it’s okay, they are either not being truthful or they just don’t know the facts. Don’t let anyone install any solar panels or other heavy equipment on your roof without having the structure visually seen. This may not be possible most times due to access. In those situations, it is better to assume that the structure is not built to support the load and will need modification.
This means that the only way to install solar correctly (and not put residents at risk) is to install one of the following methods:
1. Remove the roof and install additional main beams (either sister laminated beams or install new)
2. Install new 2×6 rafters sistered to the existing or new 2×8 rafters.
3. Install new sheathing.
1. Install steel I beams that rest on the parapet walls
2. Install solar panels on racking that are attached to I beams