Roofing & Repairs

Preventing Moisture From Entering Inside Dwellings

Uncontrolled moisture inside homes and buildings can cause major damage to the building structures, as well as to furnishings and finish materials like floors, walls and ceilings. Likewise, excess moisture can trigger mold growth, which not only damages the building materials, but can lead to health problems for the people who live in the affected dwellings. Mold is usually not a problem indoors unless there is excessive moisture. Newly constructed buildings and those with natural ventilation give off significant amounts of moisture as a result of moisture trapped within building materials such as: concrete, lumber and wet-installations such as plaster and ceramic tile adhesives and grout. The primary causes of moisture problems in dwellings include:

  1. Building materials that are exposed to moisture before the structure is fully enclosed
  2. Poor control of rain water, resulting in roof and flashing leaks
  3. Wet or damp ceiling and wall cavities
  4. Moist air entering the dwelling
  5. Condensation on exterior surfaces, especially metal roofs

In the areas of Costa Rica that experience excessive rain, relative humidity measurements are extremely high, and cellulose based building materials, such as drywall, plywood, OSB board and wood ceilings and flooring can deteriorate from this excessive moisture.

It is recommended to utilize structural fiber cement boards, Plycem, for roof sheathing. These laminates resist moisture and do not rot or breakdown when in contact with or near moisture entry points.

Moisture enters roof and wall cavities in two different paths:

1.The cavity is not airtight and outdoor moisture penetrates. Night, sky radiation causes the trapped humid air in the cavity to diffuse into cellulose based sheathing products.


2.The roof and or sheathing become wet during the evenings, because of night-sky radiation and subsequently, condensation infiltrates into cellulose based sheathings by intense ultraviolet radiation the next day.

Both paths result in moisture accumulation in the sheathing, and that moisture begins to damage the sheathing and any attached wood or metal structures.

The most common roof structure structures in Costa Rica are wood or metal rafters and battens and then corrugated metal roofing laminates attached with screws.

standard corrugated laminates, ridge caps

The gaps between the rafters and the exterior metal laminates allows for air and moisture to enter. Additionally, under the gaps of the ridge caps, driving rain, debris, rodents and insects can enter and cause contamination to the interior ceilings.

roof gaps

In humid climates, the outside air that enters the roof cavity, due to uncontrolled ventilation, diffuses to the exterior surface of the roof sheathing, and when exposed to the air temperature by night sky radiation, condensation forms and moistens the sheathing material. The exposure to humidity is not such a problem when moisture resistant products, like fiber cement laminates, or waterproofing membranes have been installed to prevent the moisture from damaging the sheathing product.

roof sheathing, underlayment1

How to Properly Install a Roof Assembly

Control Rain and Moisture Infiltration – In roof assemblies, an underlayment; such as an SBS Membrane should be installed over Plycem fiber cement sheathing as the primary water barrier. The joints should be lapped properly and the underlayment must be integrated with flashings to direct water off the roof sheathing.

Any type of finished roofing product can be used on top of a waterproofed roof assembly like this.

When metal laminates are to be the finished roofing product, a minimum of one inch of vented airspace should be permitted between the underside of the exterior roofing laminates and the top of the sheathing.

air space roof layer diragram

Insulate Attics and Ceiling Cavities – Install additional insulation to meet the minimum code requirements and/or the specifications of the project. This can be done by installing fiberglass batt insulation.

If you have any questions about the information provided, contact



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