Why Silicone Roof Coating Turns Yellow Over Asphalt
Short answer
It's not your roof coating. The yellow comes from the asphalt underneath your seal. It can change how the roof looks, but the waterproof barrier is still there. Using Bond It Bleed Blocker eliminates this issue completely.
Why it happens
Asphalt and modified bitumen roofs contain oils. Over time those oils can migrate up into a silicone coating and tint it yellow or brown. Roofers call this bleed-through, or tobacco staining.
It is not a one-season problem. Coatings Pro reports granulated modified bitumen roofs 10 to 15 years old or more that still show tobacco staining, most likely because no primer was used or it was under-applied.
The cause sits below the coating, which is why a better topcoat alone does not solve it.
What yellowing does and does not affect
Yellowing does not hurt the waterproofing or the bond. It changes how the roof looks, which is why building owners notice it and call.
For an owner who paid for a bright white roof, that matters. Color is part of what they bought.
How to prevent it
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Make sure the roof is ready. Start with a clean, dry, tight roof, a moisture scan to find wet areas, and an adhesion test.
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Prime before you coat. Apply a bleed blocking primer to encapsulate asphaltic bleed-through before the silicone.
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Apply the right amount. Too thin a coat can still let oils through, and too heavy a coat can cause delamination or cracking in the topcoat. Follow the manufacturer's rate.
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Cover every inch. Gaps in coverage can lead to product failure.
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Topcoat with silicone once the primer is fully dry.
Where Bond It Asphalt Bleed Blocker fits
You don't want unexpected yellowing on the job, so contractors put a first coat of Bond It Bleed Blocker down to take that worry off the list. It also helps level the existing substrate and bridge the gaps between the granules, which gives the final coat a more even look. Its Hi-Vis blue color is visible in bright daylight, which helps you avoid gaps and missed spots during application. It's also one-part and water-based with low odor, so there's no two-part mixing and fewer fumes on the job.
Application details:
- Coverage is 1.0 gallon per 100 square feet (16 wet mils), applied as a single coat. Apply evenly until no bitumen or felt shows through.
- Apply between 40°F and 100°F, and not when rain is expected before the primer has fully cured.
- It is tack free in 2 to 4 hours. Allow 24 to 48 hours to cure before topcoating, depending on temperature and humidity.
- Topcoat with Bond It Roof Seal within 5 days.
On the job
On a recent modified bitumen job, the roof was coated without Bleed Blocker. The work was done under the roofing contractor's previous ownership, which did not always require Bleed Blocker on mod bit, and the tobacco staining is easy to see in the photos. After talking with the building owner, he understood what caused it and said he wished he had gone with the added Bleed Blocker. The contractor now strongly recommends it.
Frequently asked questions
Will yellowing hurt the roof?
Not the waterproofing or the bond. It changes how the roof looks.
Do I need a primer on every modified bitumen roof?
Yes, if you are coating it with Bond It Roof Seal. Modified bitumen needs Bond It Bleed Blocker first, and skipping the primer can cause adhesion problems as well as yellowing.
How long can the primer sit before the topcoat?
Topcoat within 5 days of applying Bond It Bleed Blocker.
Would you recommend Bleed Blocker on a mod bit roof?
Yes. It keeps tobacco staining from the asphalt vapors venting off the roof, and it gives the roof a more uniform look by filling in the gaps between the granules.
Would you use Bleed Blocker on a repair?
On a large repaired section, yes. For a small deficiency in the substrate or a small area that needs sealing, no.
On a mod bit roof, would you use silicone to repair a small leaking area?
Yes. It is easy for the crew to do, and it takes little time or material, so it is a good value for contractors to offer building owners and property managers who have a leaking area.
Sources
View Gripbond PRO
View Roof Seal Reinforcing Fabric
View Rescue Tape
View All Weather Silicone
