A Chicago frame two-flat is a two-unit building with a wood-framed structure and wood or similar siding on the outside, where a brick two-flat has masonry walls. Same layout, different weak points. A frame building fails where water sits on wood: siding, trim, porch posts and the base of the wall. A brick building fails at the mortar, then the brick face, lintels and the top of the wall.
We are not going to give you construction dates or say how many of each type exist, because we can't source that here. This page compares upkeep, from what owners commonly see. It sits inside our guide to Chicago building types, and the basics of the building are in what a Chicago two-flat is. For frame siding, porches and exterior repair, our masonry and exterior team can look at the building and tell you what is going on.
What is the difference between a frame and a brick two-flat?
You can tell from the street, though not always. Here is what we commonly find.
| Frame two-flat | Brick two-flat | |
|---|---|---|
| Walls | Wood framing with siding or similar cladding | Masonry walls, often common brick |
| Weak points | Siding, trim, base of wall, porch posts | Mortar joints, brick faces, lintels, parapet |
| What water does | Soaks into wood and rots it if it stays wet | Gets into joints, freezes, and flakes brick faces |
| Typical upkeep | Paint or other siding care, caulk, wood repair | Tuckpointing, brick replacement, lintel work |
Some brick-looking buildings are wood-framed with a brick face, and some owners have added siding over older walls, so don't assume from the surface. An inspector or a contractor can open a small area and tell you what is behind it. A brick wall often hides how it is built, and a frame building shows its weak points more openly.
What goes wrong first on a frame two-flat?
Wood doesn't rot from humid air. Serious decay needs the wood to stay wet. The U.S. Forest Service's Forest Products Laboratory says serious decay occurs only when wood moisture content is above about 30 percent, and that water vapor from humid air alone doesn't wet wood enough to matter. Rain, condensation or wet ground contact does (USDA Forest Products Laboratory).
In practice, on frame two-flats we commonly find:
- The base of the wall. Siding near the ground gets splashed, shaded and slow to dry.
- Porch posts and stair treads. End grain and joints hold water. Posts rot from the bottom, where they sit on or near concrete.
- Window and door trim. A failed caulk joint lets water behind the trim.
- The roofline and gutters. Overflowing gutters soak the siding below them.
- Where the porch attaches to the wall. Water gets behind a ledger and nobody can see it.
For wood porch parts, the American Wood Council's deck guide says fasteners and connectors used with preservative-treated wood must be hot-dipped galvanized, stainless steel, silicon bronze or copper (AWC DCA 6). Plain steel nails and hangers don't last in wet treated wood.
On frame buildings we often find the damage is a lot bigger than the stain you can see. A soft spot at the base of a porch post or under a window can mean the framing behind it is wet too. Owners get a paint quote for what looked like a paint problem. The fix is the water, then the wood, and the paint last.
What goes wrong first on a brick two-flat?
Mortar. Then the brick face, then lintels and the top of the wall. The National Park Service says repointing mortar must be softer and more vapor permeable than the masonry units, and no harder than the historic mortar (NPS Preservation Brief 2). When mortar is less permeable than soft brick, the moisture and dissolved salts move out through the brick instead, and the face can spall (NPS Preservation Brief 2).
That is why we see hard gray repairs next to flaking brick. We cover the symptoms in when a Chicago brick two-flat starts failing and the material in Chicago common brick.
Which is cheaper to keep up?
We won't give you a number, because it depends on the building, and anyone who gives you one without looking is guessing. What drives it:
- How much has been deferred. Years of open joints or failed caulk cost more than regular care.
- Where the water is going. A fixed roof edge or gutter protects both types.
- Access. A tall wall or a lot line close to the neighbor means scaffolding and public way permits.
- The porch. Both types usually have wood porches at the back, so that cost shows up either way.
Neither type is maintenance free. Frame needs regular paint, caulk and wood repair. Brick needs tuckpointing on a longer cycle, but the repairs are bigger when they come. See our tuckpointing guide for the brick side.
How does the City treat frame and brick buildings?
The same way. The City's code says the exterior of a structure must be maintained in good repair, structurally sound and sanitary. It names siding and masonry joints that are not weather resistant or water tight, veneer, cornices and trim that are not properly anchored, and exterior stairs, porches and balconies that are not structurally sound or properly anchored (Title 14X). Porches must be capable of supporting the imposed loads (Title 14X).
Inspections are largely complaint driven, and an inspector can issue violations for what is visible from outside (City of Chicago).
What needs a permit differs a little by work type:
- Siding. The City's no-permit list covers in-kind siding on residential buildings up to four stories (City of Chicago). Repair is exempt only if it doesn't cut away part of an exterior wall, remove structural supports or change required exits (City of Chicago).
- Masonry. Tuckpointing and masonry wall repair go through the Express Permit program (City of Chicago). The only masonry no-permit repair is a small in-kind replacement.
- Contractor licensing. A Chicago general contractor license is required for most building repair work, including work that is exempt from a permit (City of Chicago).
Rules change, so confirm with the Department of Buildings.
What about insurance and buying a frame two-flat?
Ask your insurance agent. We can't speak to coverage or premiums for either type, and we won't guess. If you are buying, have the exterior looked at before you close. For brick, that means joints, lintels and the roof edge. For frame, it means the base of the wall, the porch framing and the trim. Our two-flat buyer's exterior check walks through it.
On frame, a small leak behind trim or siding turns into rot in the framing, and by the time you see it the repair is carpentry, not caulk. On brick, open joints let water in, it freezes, and the faces flake until you need replacement brick instead of mortar. Either way a violation can follow, because the City can cite what an inspector sees from outside.
Warning signs, by type
Frame
- Soft or spongy wood when you press it with a screwdriver
- Paint peeling in sheets, or bubbling at the base of walls
- A porch post that is dark and damp at the bottom
- Siding that has pulled away from the corner or trim
- Gaps in caulk around windows, doors and trim
Brick
- Mortar you can scrape out with a key
- Brick faces flaking or popping off
- Stair step cracks in the wall, or a bulge
- A parapet that leans or has lost its cap
- Water stains inside below a window or the roofline
If you aren't sure what you have, we'll look at it and tell you what it needs and in what order.
Common questions
Neither is better across the board. A frame building fails where wood stays wet, and a brick building fails at the mortar. What matters is the condition of the particular building and how well water has been kept out. Have either type looked at before you buy.
Often from the street, but not always. Some brick-looking buildings are wood-framed with a brick face, and some owners have put siding over older walls. A contractor or inspector can open a small area to see what is behind the surface.
We can't speak to coverage or premiums. Ask your insurance agent how they treat your building's construction and condition, and what they need from an inspection.
The City's no-permit list covers in-kind siding on residential buildings up to four stories. Work that cuts away part of an exterior wall or removes structural supports is not exempt. Rules change, so confirm your project with the Department of Buildings.
Sources
- City of Chicago: Code Modernization Ordinance, including Title 14X Minimum Requirements for Existing Buildings (PDF)
- City of Chicago, Department of Buildings and Department of Law: Building Code Violation Enforcement Process (PDF)
- American Wood Council: DCA 6: Prescriptive Residential Wood Deck Construction Guide (PDF)
- USDA Forest Products Laboratory: Wood Handbook, Chapter 14: Biodeterioration of Wood (PDF)
- City of Chicago: Guide to Building Permits: What Types of Work Do Not Require a Building Permit?
- City of Chicago: Express Permit Program: Masonry Work
- National Park Service: Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings (PDF)
- City of Chicago, Department of Buildings: General Contractor License
Facts last checked: October 7, 2026.
This article explains general rules and common situations in Chicago. It is not legal advice, and City rules change. For a specific property, check with the Chicago Department of Buildings or a licensed professional.