MHIC Construction Methods and Materials 2 — Questions and Answers
Question 1: A contractor is installing a load-bearing wall in a residential addition. Which type of lumber is specifically graded and approved for structural applications?
- Construction-grade dimensional lumber
- Select structural lumber (Correct answer)
- Appearance-grade lumber
- Furring strips
Correct answer: Select structural lumber
Select structural lumber carries the highest structural grading designation and is engineered and tested for load-bearing applications. Construction-grade lumber is general purpose but not optimized for structural loads, appearance-grade prioritizes aesthetics, and furring strips are non-structural thin boards used for leveling surfaces.
Question 2: When constructing a concrete slab on grade, what is the PRIMARY purpose of placing a vapor barrier (polyethylene sheeting) beneath the slab?
- To prevent frost heave from damaging the slab
- To reduce heat loss through the slab
- To prevent ground moisture from migrating up through the concrete (Correct answer)
- To improve the bond between the subbase and concrete
Correct answer: To prevent ground moisture from migrating up through the concrete
A vapor barrier beneath a concrete slab-on-grade is installed to block capillary moisture migration from the soil up into the concrete and finished flooring above. Without it, moisture can cause efflorescence, deteriorate flooring adhesives, and promote mold growth. It does not address frost heave, thermal insulation, or bonding.
Question 3: In masonry construction, what does the term 'wythe' refer to?
- The mortar joint between two courses of brick
- A single vertical layer of masonry units one unit thick (Correct answer)
- The metal ties connecting brick veneer to a structural backup wall
- The decorative cap course at the top of a brick wall
Correct answer: A single vertical layer of masonry units one unit thick
A wythe is a continuous vertical section of masonry one unit in thickness. A double-wythe wall has two parallel layers of masonry. Mortar joints are called beds (horizontal) or head joints (vertical), metal connectors are called wall ties or anchors, and the top decorative course is called a coping.
Question 4: A homeowner requests that OSB (Oriented Strand Board) be used for wall sheathing on an exterior renovation. Compared to plywood, which characteristic of OSB must the contractor account for during installation?
- OSB has significantly lower racking resistance than plywood
- OSB is more susceptible to edge swelling when exposed to moisture (Correct answer)
- OSB requires larger fastener spacing than plywood
- OSB cannot be used in wind-bracing applications
Correct answer: OSB is more susceptible to edge swelling when exposed to moisture
OSB's edges are more vulnerable to moisture absorption than plywood because the edges expose the strand layers directly, causing them to swell. This is why OSB requires proper sealing of cut edges and adequate drainage gaps. OSB and plywood have comparable racking resistance, similar fastener schedules, and both are code-approved for wind bracing.
Question 5: When installing floor joists using the 'platform framing' method, where is the next story's wall framing erected?
- Directly on top of the lower story's wall top plates, before the subfloor is installed
- On a raised sill plate that spans across the floor joists
- On top of the subfloor deck that covers the floor joists (Correct answer)
- On the rim joist attached to the outside of the floor joists
Correct answer: On top of the subfloor deck that covers the floor joists
In platform framing (the dominant modern residential method), each floor level serves as a 'platform' on which the next story's walls are built. The subfloor sheathing is installed across the joists first, creating a flat working surface, then the walls are framed on top of that deck. This is distinct from balloon framing where studs run continuously from foundation to roof.
Question 6: A contractor is selecting insulation for an exterior wall cavity in a climate with significant humidity. Which insulation type is most resistant to moisture absorption and will not lose R-value when wet?
- Fiberglass batt insulation
- Cellulose loose-fill insulation
- Closed-cell spray polyurethane foam (SPF) (Correct answer)
- Mineral wool batt insulation
Correct answer: Closed-cell spray polyurethane foam (SPF)
Closed-cell spray polyurethane foam has a very low moisture vapor permeability and does not absorb liquid water, maintaining its R-value even in humid conditions. It also acts as an air barrier. Fiberglass batts lose insulating effectiveness when wet, cellulose is hygroscopic and can hold moisture, and mineral wool is more moisture-resistant than fiberglass but still less effective than closed-cell SPF in high-humidity applications.
MHIC Construction Methods and Materials 2
A contractor is installing a load-bearing wall in a residential addition.
Which type of lumber is specifically graded and approved for structural applications?