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How to Waterproof Parapet Walls

MFM Parapet Walls
December 5, 2022 at 6:00 p.m.

By MFM Building Products Corp. 

A guide to protecting your parapet wall from water damage. 

Parapet walls are particularly difficult to waterproof. They are exposed on all sides to the natural world, meaning that these walls fail more often than other parts of the roof. Covering your parapet wall with a waterproofing membrane will help protect it from leaks or other weather-related damage. There are various types of weatherproofing products on the market. Be aware of your how your parapet wall is constructed, as this will determine which product you use 

Typically, we recommend that parapet walls be covered with a membrane that possesses these traits: 

  • Self-sealing around fasteners – the final exterior cladding typically includes some type of fastener – a flashing membrane that meets ASTM D 1970 or ASTM E 331 specifications for nail sealability are highly recommended 

  • Membrane designed for vertical applications. Contact the manufacturer to see if the membrane is approved for vertical installation 

  • Membrane meets ASTM D 1970 for material thickness which is 40 mils. This minimum thickness provides a robust membrane and a solid waterproofing foundation 

Parapet wall and membrane consideration: 

  • If the parapet wall will be capped with METAL, make sure that the flashing membrane is rated for high-heat environments. MFM offers several membranes rated to 250°F for applications such as these. If you use a membrane that isn’t rated for high heat, the heat will melt the mastic. It will run out and cause the membrane to lose all its waterproofing properties 

  • If the parapet wall will utilize brick, stucco or other masonry-based products, ensure that the membrane can withstand these high moisture environments. Membranes meeting ASTM E 96 for vapor permeance are recommended 

  • If the interior section of the parapet wall will be left exposed, MFM offers a UV-stable waterproofing membrane that does not require any coverings to be left exposed indefinitely 

General installation tips and areas of concern when flashing a parapet wall: 

  • Starting where the parapet wall meets the roof deck, install a 12” piece of flashing membrane – half of the membrane up the parapet wall and the other half onto the roof deck. A 3/4" fillet along this intersection point of wall to roof will help with an easier transition from wall to roof  

  • Install a full sheet, 36” wide membrane, overtop the 12” section and extend up the vertical wall. If possible, wrap the sheet up and over the top cap section for best waterproofing practice. If the cap is not removeable, use a termination bar just under the cap to secure the membrane in place, and finish with a bead of sealant along this leading edge. 

  • Always overlap membranes by a minimum of 3” to ensure a secure seal on overlaps. Make sure that water flows over the lap and not against the lap (just like in roofing). 

  • We recommend priming all masonry and weathered surfaces to ensure the best bond of the membrane to the substrate. 

  • As with any self-adhering product, rolling the membrane with a hand roller ensures maximum contact of the membrane to the substrate. Always pay special attention to all overlap areas 

The greatest potential for leaks is where the parapet wall meets the roof deck. Eliminate this by using a 12” piece of membrane then covering with a full sheet. This is a key step in a successful waterproofing application, resulting in years of worry-free performance. 

Learn more about MFM Building Products Corp. in their RoofersCoffeeShop® Direcotory or visit mfmbp.com.



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