Learn why slip sheets and vibration isolation are important at mechanical equipment on a roof
In our last informative blog post, we talked about pitch pockets that protect and seal the services which run to and from a rooftop mechanical system. Today’s topic is similar: we are looking at how the roof membrane is protected from HVAC and mechanical equipment movement on a rooftop. This us separate though, and different from the issue of structural bearing and capacity.
Main Categories of Movement With Mechanical Equipments On Rooftops
There are 2 different main categories of movement common with mechanical equipment on a rooftop. The 1st type is passive thermal and moisture expansion and contraction movement. The 2nd type of movement is mechanical vibration. Thermal movement happens consistently and residually in buildings, even where it’s hard to detect that movement by the naked eye alone. Buildings experience significant thermal and moisture related expansion and contraction, even in just a short time period such as one day, in series of micro movements, physically smaller than just one millimeter. This movement is still extremely significant in the overall effect on the building. It happens at a few small movements a day, but it’s tons of building mass moving every day.
By comparison though, mechanical vibration will generally be smaller and limited in size and scale. The impact though can be nonetheless significant, as although scale is reduced the speed is significantly higher. Over time, unmitigated mechanical vibration can be an annoyance to building occupants, but equally important the vibration and movement can damage a roof membrane.
Impact of Low Quality Installation on Roof
The picture below shows a field built mechanical equipment stand supporting 2 different condenser units. The quality and capacity of this particular mechanical equipment stand can be debated. There are obvious signs though of low quality workmanship. but the stand may meet the structural capacity requirements. Particularly important though, slip sheets have been used to separate the mechanical stand from direct contact with the substrate roof membrane.
The picture below shows an up close view of the location where the mechanical stand makes contact with the roof membrane. If you look closely, you can see an extra layer of modified bitumen membrane has been cut and placed under the area where the 4×4 PT SYP wood sleeper has been laid on the rooftop. This extra portion of scrap membrane works as a slip sheet to separate the treated wood from the underlying rooftop membrane. Effectively, this piece of scrap material works as a slip sheet and is required by building code.
The next picture below shows two different types of isolation spacers. The particular one on the left side of the image has been discarded next to the condenser unit on the rooftop and is an example of a solid neoprene waffle form isolator. The waffle form helps to attenuate the vibration which eminates from the condenser unit.
The next pictures below show a similar type of rooftop mechanical equipment assembly and stand. These elements are also separated from the copper roof membrane below by a cut slip sheet, taken from a piece of thermopolyolefin roof membrane.
An up close view of the assembly of those elements follows in the next picture below. This type of separation is important to reduce the impact of vibration of the equipment, but also this separation is extremely helpful because the chemicals used in treated wood can be extremely corrosive to certain types of metals such as copper.
Maintaining Present Day Rooftop Systems
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.