Many owners and engineers ask the same question when waterproofing roofs, terraces, and gutters: Can waterproof coatings be exposed to the sun in the open air and used directly on the exposed roof? The answer is summarized in one sentence:
Not all waterproof coatings can be exposed to the open air. Only waterproof coatings marked with [Exposure] and anti-ultraviolet weathering formula are suitable for direct exposure of roofs; K11, ordinary JS, ordinary polyurethane waterproof coatings are strictly prohibited from long-term open sun exposure and must be protected
。

Why can't most waterproof coatings be exposed to the sun in the open air?

Ultraviolet rays are the biggest killer of roof waterproofing. Ordinary waterproof coating formulations do not add anti-UV stabilizers, and long-term sun exposure will cause aging reactions:

1. The coating film quickly pulverizes, loses light, becomes brittle, and the elasticity decreases significantly;
   

2. Cracking, shelling, falling off, and loss of waterproof function;
   

3. The temperature difference is repeated with thermal expansion and contraction, accelerating the failure of the waterproof layer, and leaking again in as little as 1-2 years.
   

Like K11, JS polymer cement waterproof and ordinary oily polyurethane commonly used in our bathrooms and basements, the waterproof performance is very good, but the positioning is
Concealed waterproof
It is suitable for being covered with ceramic tiles, mortar, and cement protective layers. Once exposed to the open air, the lifespan will be greatly reduced, and the rework cost will be very high.

Second, which waterproof coatings are suitable for exposed roofs and open-air sun exposure?

Mainstream categories suitable for direct exposed roofing, sun exposure and aging resistance:✅
Exposed acrylic high elastic waterproof coating
The most commonly used in the market, with good elasticity, UV resistance, simple construction, and adjustable color. It is suitable for concrete roofs, color steel tile roofs, and terraces. It can bridge minor cracks and is a popular choice for self-built and factory roofing renovations.✅
Aspartame polyurea waterproof coating
Weathering resistance, wear resistance, excellent UV resistance, high and low temperature resistance, longer service life, suitable for high-standard roofing, photovoltaic roofing, and can be exposed and waterproof to people.✅
Exposed silicone waterproof coating
Excellent aging resistance, good air permeability, good heat insulation effect, suitable for roofing in areas with long-term strong sunshine.

⚠️ Purchase judgment criteria: product packaging and test reports are clearly marked
Can be used externally, resistant to artificial climate aging, resistant to ultraviolet rays
Don't just listen to the verbal propaganda.

3. I have brushed ordinary waterproof paint, what should I do if I want to expose it?

Two safe solutions: Option 1: Make a cement mortar protective layer (about 2cm) to isolate ultraviolet rays, the most traditional safe; Option 2: On the original waterproof layer, apply a layer of exposed weathering topcoat/anti-UV overcoat as a whole as sun protection.

提醒：不建议直接在普通防水上面裸晒，后期维修成本远高于前期保护层费用。

IV. Key points for exposed waterproof construction of the roof

1. Base solid clean, no floating ash, no clear water, cracks, Yin and Yang Angle first do strengthen treatment;
   

2. Brush strictly according to the specified thickness, apply thinly multiple times, and it should not be too thick once;
   

3. Avoid construction at noon in rainy days, strong winds, and high temperatures.
   

4. After the exposed waterproofing is done, check it regularly, and repair it in time if there is local pulverization;
   

5. For roofs with long-term water accumulation, priority should be given to water-resistant and weather-resistant dual-performance materials.
   

V. Summary

Simple distinction:
Concealed waterproof (K11/ordinary JS/ordinary polyurethane) cannot be exposed to the sun, but a protective layer is required; special waterproof for exposure (acrylic exposed, asparagus polyurea, silicone exposed) can be directly exposed to the sun in the open air, used for roof top
. The roof waterproof leakage has been repaired repeatedly, mostly due to the wrong matching of materials and scenes. Choose the right UV-resistant waterproof coating to save the trouble of making a protective layer, and the waterproof life is more stable.