Roofing Sealants: Which to Use, Where, and Where Never
Sealant is a backup and a finishing material. It is never a replacement for metal that laps correctly.
- Intermediate
- 30 to 60 minutes per detail
- Updated
The short answer
Roofing sealant is a flexible material used at laps, exposed fasteners and penetrations as a backup to lapped flashing. Polyurethane and hybrid polymer sealants suit metal, masonry and exposed joints, butyl tape suits metal laps, and asphalt roof cement suits asphalt shingle details. Clean and dry the surface, use the right bead size, tool it smooth, and never use sealant to replace a missing lap or flashing.
Every roof has a tube of sealant somewhere near it. Used correctly, it is useful: it seals the heads of exposed fasteners, closes small gaps at metal laps and finishes the top of a reglet. Used as a substitute for flashing, it is the most common cause of repeat leaks. Sealant does not change the direction water runs. It just fills a gap, and as it ages, it shrinks, cracks and lets go.
The principle behind every detail is the lapping rule: water is shed by overlap, and sealant is a secondary defence. This guide describes the main types, where each belongs, how to prepare the surface and apply it, and where sealant should not go.
Read it alongside roof flashing types and the details where sealant commonly appears: counter flashing, pipe boots and step flashing.
Tools
- Caulk gun, dripless if available
- Putty knife or trowel for roof cement
- Utility knife for cutting tubes and tape
- Wire brush and clean rags
- Solvent recommended by the sealant maker
- Nitrile gloves
Materials
- Polyurethane or hybrid polymer sealant rated for exterior metal, masonry and roofing
- Butyl sealant tape for metal laps
- Asphalt roof cement, plastic or flashing grade
- Backer rod for wide joints
- Primer where the sealant maker calls for it
Safety first
- Many sealants and roof cements contain solvents. Use them in open air, keep them away from ignition sources, wear nitrile gloves and eye protection, and read the product safety data sheet.
- Work tied off. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury. Wet sealant and roof cement are slippery on a slope.
- Do not apply sealant in rain, on frost or on wet surfaces. Most products will not bond and can trap moisture.
Step by Step
Step 1: Decide whether sealant is the right answer
Ask of any joint: if the sealant failed tomorrow, would water still be shed by lap? If the answer is no, the detail is wrong, not the sealant. Fix the lap, then seal.
Watch out: A bead of sealant over a missing lap, reverse lap or rusted flashing hides the leak for a season or two and then fails.
Step 2: Pick the sealant for the materials and the job
Always follow the roofing or flashing manufacturer's list of approved sealants. Shingle manufacturers, for example, specify which sealant or cement they permit, and some sealants attack certain membranes.
- Polyurethane: strong, flexible, bonds to most roofing materials and masonry. Can degrade in sun unless painted or UV stable, so check the rating.
- Hybrid polymer (MS or silyl-terminated): bonds to damp and varied surfaces, stays flexible, good UV resistance. Often a good general choice for flashing details.
- Butyl: sticky and durable, used as a tape or bead under metal laps and fasteners. Stays tacky and does not cure hard.
- Asphalt roof cement: used under and over asphalt shingle details such as tabs and some flashing edges. It is brittle with age and UV. Use plastic or flashing grade cement, not wet patch on dry surfaces unless labelled.
- Silicone: bonds poorly to many roofing surfaces and cannot be painted or recoated. Many roofers avoid it on roofs unless a product is specified for it.
Step 3: Prepare the surface
Scrape and brush away loose material, wipe with a rag and the solvent the sealant maker recommends, and let it dry fully. Sealant bonds to clean, dry, sound surfaces. On masonry, remove dust with a vacuum and a stiff brush. On metal, remove oxidation and oil. Apply primer if the instructions say so.
Step 4: Check the temperature and weather
Most sealants work between about 5 and 35 degrees C, but the range varies, and some cold-weather products work lower. Sealant cold enough to be stiff will not tool or bond. In the Fraser Valley's wet season, pick a dry window, and keep tubes somewhere warm until use.
Step 5: Apply the right bead
For wide joints, pack backer rod first so the sealant depth is about half its width and is stuck to two sides only, not three. A bead stuck to the back of a joint as well cannot stretch and tears. For exposed nail heads, a dab that covers the head and about 1/4 inch around is enough.
Step 6: Tool the bead smooth
Tooling forces the sealant into contact with both surfaces and removes air pockets. Shape the bead so it sheds water and does not hold a ridge. Remove excess before it skins over.
Step 7: Let it cure and check it on a schedule
Sealant is maintenance. Look for cracks, pulling away from the surface, or hardening, and renew it before it fails. Note the dates of sealing in a home maintenance record. See how to check the roof after a windstorm.
Where Sealant Belongs
- Over exposed nail heads and screw heads on flashing flanges.
- At the top of a reglet and along a surface-mounted counter flashing, on top of lapped metal.
- Under shingle tabs at specified points, as the shingle maker directs.
- Between metal laps where the manufacturer specifies a sealant or butyl tape.
- At storm collars and the top of pipe flashings.
- Around the ends of trim pieces and cut edges.
Where Sealant Never Replaces Flashing
- Along a wall-to-roof joint with no step, apron or counter flashing.
- Over a cracked pipe boot collar. See pipe boots.
- Over a rusted or holed flashing.
- Across a valley, ridge or hip gap.
- Around a chimney with no counter flashing. See chimney flashing parts.
- To hold down lifted shingles across a whole slope. See resealing lifted shingles.
Fraser Valley Notes
Sealant in a damp, shaded, mossy location ages faster, and moss roots can lift a bead at its edge. Wind-driven rain reaches joints a dry-climate roof would never wet. Favour UV-stable, flexible products, allow for cool-weather cure times, and recheck sealed details each fall.
Common Mistakes
- Using sealant in place of flashing at a wall, chimney or valley, which fails within a few seasons.
- Applying over dirt, wet surfaces or old failed sealant, so it never bonds.
- Sealing the bottom edge of siding or shingles shut, trapping water behind.
- Using the wrong type, such as silicone on surfaces it will not bond to.
- Bonding a bead on three sides in a wide joint, so it tears as the joint moves.
- Never rechecking, letting cracked sealant stay in place for years.
When to Call a Professional
If a joint has been resealed several times and keeps leaking, the flashing itself is probably wrong, and a roofer should look at it. Sealant over a problem only delays the repair and hides the damage.
Sealants on steep, high or metal roofs, and on chimneys, are better left to a roofer. Wet sealant is slippery, and the correct product depends on the materials involved.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Repair service or the towns we cover.
How to Choose and Apply Roofing Sealants: Questions
What is the best sealant for a roof?
There is no single best. Polyurethane and hybrid polymer sealants suit metal, masonry and exposed joints, butyl tape suits metal laps, and asphalt roof cement suits asphalt shingle details. Choose a product rated for exterior roofing and the surfaces involved, and follow the roofing or flashing manufacturer's list of approved sealants.
Can I use caulk to fix a roof leak?
Only as a short-term patch. Caulk fills a gap but does not change the way water flows, and it cracks and pulls away within a few seasons. A lasting repair corrects the flashing or lap so water is shed by overlap. Sealant then backs it up at fasteners and specified joints.
Is silicone or polyurethane better for roofing?
Polyurethane and hybrid polymer sealants generally bond better to roofing surfaces and can be painted, which suits most flashing details. Silicone stays flexible and resists UV, but it bonds poorly to many materials, cannot be recoated and is hard to remove. Use whatever the roofing manufacturer approves for the specific detail.
How long does roof sealant last?
It depends on the product, exposure and movement. Exposed beads commonly need checking every year or two and renewing within a few years, while sealants protected in a reglet or under a storm collar can last longer. Sun, moss, movement and poor surface prep shorten life. Regular inspection catches cracks before they leak.
Where should you not use roofing sealant?
Never use it in place of step, apron, counter or valley flashing, over a cracked pipe boot, over rusted metal, across valleys, ridges or hips, or to seal the bottom of cladding shut so water cannot drain. It belongs over fasteners, at specified laps and at the top of properly lapped flashing, as a backup.
Related Guides
All Flashing Details guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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