Thermal splitting is when shingles split straight through, usually where a shingle bridges a joint in the course below, sometimes in a diagonal stair-step line up the slope. The accepted explanation is that the wood deck and the bonded shingle layer expand and contract at different rates, and the deck pulls on the shingles through their nails until the most stressed ones split, most often in cold weather. Splits can leak, so split shingles are replaced; caulk in the gap is a stopgap at best. This page is general information; your roofer makes the final call on your home.
On this page: Signs · Causes · What to check · Repairs · Basis · When to call
Signs of thermal splitting
- Straight, clean splits through the shingle, often running up and down the slope.
- Splits that sit directly above the joint between two shingles in the course below.
- A diagonal pattern of splits climbing the roof, following the offset of the courses.
- No round impact marks, bruises or dented metals nearby.
- A drip that appears in cold or wet weather below an otherwise healthy-looking slope.
Why shingles split
- Different movement. Wood decking moves with temperature and moisture; the shingle layer, once its sealant bonds, acts like one sheet. The nails transfer the stress.
- Joint bridging. The shingle spanning a joint below has the least support and the most strain as the bonded layer shrinks in cold weather.
- Strong adhesive bonds. InterNACHI notes fiberglass shingles are more prone to splitting than organic shingles because their sealant bonds are usually stronger.
- Moisture in the deck. Some inspection sources argue deck moisture changes drive the movement as much as temperature. Either way, deck movement is the engine.
What you can check safely
- From the ground with binoculars, look for straight dark lines through shingles, especially a stair-step pattern.
- Check soft metals (gutters, downspouts, vent hoods) for dents. Their absence argues against hail.
- In the attic, look for staining on the deck below the area and for signs the deck is damp, since moisture adds to deck movement.
- Photograph the pattern with a date so a roofer can compare over time.
Repairs
| Repair | When it fits |
|---|---|
| Replace split shingles, unsealing and re-sealing the course above | A few splits on an otherwise sound roof |
| Check attic ventilation and deck moisture | Splits keep appearing or the deck is damp |
| Replace the slope and ask the manufacturer about warranty coverage | Splitting is widespread on one slope |
| Replace the roof | Splitting is roof-wide or shingles are brittle |
Fixes that fail: running sealant into each split and calling it done, and labeling splits as hail or wind damage when there are no impact marks or creases.
What the sources say
Primary manufacturer or NRCA guidance on thermal splitting was not found for this page, so this explanation rests on InterNACHI’s Mastering Roof Inspections course (Part 39), which describes the deck-versus-shingle movement, the stair-step pattern and the split-versus-crack test. Treat it as inspection guidance rather than a code requirement. The repair method follows ARMA’s general guidance that damaged shingles can be carefully unsealed, replaced and hand-sealed, and that roofing cement is a temporary fix. Ventilation basics are on our attic ventilation page.
When to call
Call when you see straight splits or a stair-step line, or before an insurance claim if you are not sure whether marks are hail. Comparing splits with true hail damage is covered in hail damage or normal wear. Related guides: shingle buckling, nail pops and the roof problems guide.
Have us look at the splits
Integrity Roofing and Painting is veteran-owned and family-run since 1990, an Owens Corning Platinum Preferred contractor with 650+ Google reviews. We photograph the pattern, check the deck and attic, and tell you in writing whether this looks like splitting, hail or something else. Colorado Springs area: (719) 488-2800 or a free inspection. Denver metro: (303) 209-0152 or a free Denver inspection. Austin: (512) 610-5897 or a free Austin inspection. Same-business-day reply.
Sources: InterNACHI, Mastering Roof Inspections: Asphalt Shingles, Part 39 (secondary source); ARMA, Asphalt Shingle Roofs Are Repairable; IBHS, Is it hail damage?.
