I build in stone for a living, mostly outdoors, and the detail I get asked about most on houses is the one nobody thinks about until it fails: what happens at the top and bottom of a window. On a Dwight two-family last spring, a $300 sill problem had been left long enough to become a $4,000 lintel problem.
The lintel carries the wall, the sill sheds the water
An opening interrupts the load path. Something has to carry the masonry above it across the gap — a stone lintel, a brick arch, a steel angle, or a concrete lintel. Whatever it is, it needs proper bearing at each end, usually four to six inches onto sound masonry, and it needs to be stiff enough that it does not deflect and crack the courses above.
The sill has a different job. It has to throw water clear of the wall below. Three features do that: a slope, typically 10 to 15 degrees; a projection past the wall face; and a drip groove on the underside so water cannot track back to the wall by surface tension. A sill missing any of the three delivers water to the masonry directly beneath the opening, which is why so many walls show a damp patch in exactly that spot.
How each one fails
- Rusting steel lintel. Steel expands to several times its volume as it corrodes, jacking the courses above and cracking the joints in a characteristic horizontal line at the head of the opening. This is the most common lintel failure in twentieth-century brickwork, and it is progressive.
- Sagging or cracked stone lintel. Usually from a lost bearing at one end, or from a lintel that was undersized for the load.
- Dropped arch. A brick or stone arch loses a keystone’s worth of support when the joints deteriorate and it drops in the middle.
- Sill with no drip. Water runs back under the sill and into the wall.
- Sill cracked at the ends. A sill built solidly into the masonry at both ends and unsupported in the middle acts as a beam and cracks.
- Sealant instead of flashing. Caulk across the sill-to-wall joint hides the water path rather than draining it.
What repair costs
Replacing a rusted steel angle means needling the wall above, cutting out, installing new galvanized or stainless steel with proper bearing, and rebuilding the courses: $1,400 to $4,000 per opening. Catching it early enough to clean and coat the existing angle is $400 to $900 and is often possible if the jacking is minor.
A stone sill replacement runs $400 to $1,600 depending on size and stone. Adding a drip groove to an existing sill that lacks one — cut in with a grinder and a guide — is a couple of hundred dollars and prevents a recurring damp patch permanently.
Because head-of-opening cracking looks alarming and settlement cracking also concentrates near openings, work out which mechanism you have before pricing anything. The distinguishing features are set out in reading a crack in a brick wall, and the Brick Industry Association technical notes at gobrick.com cover lintel sizing and bearing requirements.
Frequently asked questions
How do I know if my lintel is steel? Look at the underside of the opening head. A visible painted metal edge, often with a rust line, is a steel angle. A flat course of brick with a fine joint pattern above the opening usually means a soldier course supported on steel behind. Radiating brick or a stone slab means an arch or a stone lintel.
Can a rusting lintel be treated without replacement? Sometimes. If corrosion is early and the jacking is under about an eighth of an inch, the steel can be exposed, cleaned back to bright metal, treated and coated, and the joints repointed. Once the expansion has displaced the masonry, replacement is the only real answer.
Should sills be sealed to the brick? No — the joint at each end of a sill should be left open or filled with a soft, permeable mortar so any water that gets behind can drain out. Sealing it traps water inside the wall, which is the opposite of what a sill is for.