Waterproofing Lightweight Roofs

When people decide to move to Costa Rica and begin to look for housing to rent or purchase, without an experienced advocate to help them sort through the options, sometimes they end up in dwellings they can’t live with.

In this article, I’m going to explain the details of one of the most problematic components of housing construction here.

Lightweight roofs are common in Costa Rica, and after living here for twenty three years, I still do not understand why many people pay good money for substandard roofs. After all, the roof of a home protects everyone and everything under it. Additionally, we live here in a tropical climate, less than twelve degrees from the equator, where there is intense ultraviolet radiation. A cheap roof, like you might find on a barn, is not going to protect you and your home from the heat, ultraviolet radiation and driving rains found in our tropical climate.

Let me begin by explaining the various types of roofing materials commonly installed here.

Without a doubt, the most widely used and least expensive roofing materials are metal laminates, known as “Zinc or Laminas Undulado”; in English they are called “Corrugated Laminates.”


These corrugated laminates are available in calibers (thickness) 26 (.44mm), 28 (.32mm), 30 (.27mm) and 32 (20mm). Any thickness less than .44 mm is dangerous to walk on and the life expectancy is short, especially near oceans and in areas with excessive humidity. The finished surface of these laminates varies and the manufacturers offer various factory applied colors. White is the most reflective, and being less than 12 degrees from the equator, this is a consideration to reduce heat transfer from the excessive ultraviolet radiation. The caliber 26 is available in white. The thinner products are not available in white and would require painting with expensive anticorrosive paint products.

In order to attach these laminates to the roof structure, they require at least 12 screw perforations per laminate. These roofing screws have rubber washers to help prevent moisture from entering in the screw holes, but this rubber deteriorates quickly with the strong ultraviolet radiation in Costa Rica. Once the rubber becomes brittle, all the screw perforations are likely to permit water infiltration. On a roof the size of an average home, there would be approximately 1500 screw holes. You can imagine how much water can enter through this quantity of holes.

Depending on the roof design and the quantity of ridges, hips and valleys, there can be various joints where each metal laminate meets, and these areas are the source of most roof leaks.

Another type of finished roof material, primarily used on commercial buildings, is known as “Laminas Metalock”; in English they are called “Standing Seam”. These laminates are only available in caliber 26, which is .44 millimeters thick. The advantage to the Metalock roofing material is that it requires few screw perforations to attach the laminates to the roof structure.


Most leaks occur where the laminates come together at the joints of the ridges, hips and valleys. In these areas, flat metal laminates, known as “flashings” are cut and installed to prevent moisture from entering underneath, but as with any rectangular building material, the angles and joints are the most difficult to waterproof, as is detailed by the arrows in the photo below.

roof laminates rust & joints

Lightweight roof structures are commonly installed here to save money, and they do not support a quality roof installation that requires heavy sheathing products and waterproofing underlayment layers.

There is only one method to waterproof roofs of this type that can be guaranteed not to leak. I provide one solution that is guaranteed, and this is a flexible membrane that is installed on top of the existing metal laminates.

In order to prevent water infiltration in the vulnerable areas of lightweight roofs, the first step is to clean the existing laminates and then apply an anti corrosive sealer.

Thermotek applied to deck

Once the sealer has dried, a flexible membrane material should be installed on top the metal laminates, over the flashings and into the valleys in order to seal the areas where the water infiltration occurs.

The ideal membranes are modified bitumen products, also referred to as APP or SBS, and they are made from asphalt and a variety of modifiers and solvents.

The best method to apply rolls of this material is a heat welding process that adheres the membrane to the existing roof surface,membrane heat weld

as well as sealing the overlapping rolls of membrane together to create a seamless waterproof surface.

heat weld

The metal roof laminates that are commonly used in Costa Rica are .44 millimeters thick and the membranes are available from 3.5 to 5 millimeters thick. The additional thicknesses of the membranes will provide decades of moisture protection as well as an additional thermal barrier that is very important here, where we are located less than twelve degrees from the equator.  Furthermore, the additional thicknesses of the membranes provide an acoustic barrier to eliminate sound transmission through the roofs and into the inside of dwellings during heavy rains.

This waterproof roof installation is guaranteed not to leak for 8 years when the 3.5 millimeter thick membrane is installed and the guarantee is for 10 years when the 5 millimeter thick membrane is installed.

The entire installation usually takes no more than one week, depending on the size of the roof and the membrane is available in the colors white and terracotta red. Additionally, acrylic sealers in a variety of colors can be applied to the surface of the membrane to further protect the roof.

The writer, Tom Rosenberger has lived and worked in Costa Rica for 23 years, inspecting land, homes, condominiums and commercial buildings for people who want to purchase existing property or build new construction.  If you have any questions about this type of waterproofing installation, send Tom an email at; tom@CostaRicaHomeBuilder.com


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