The Exterior Wall Assembly for Trades Who Mount Things on It
Why this matters
Electricians mount disconnects and meters on it, HVAC crews run line sets through it, security installers hang cameras, satellite installers bolt dishes, handymen add hose reels and house numbers. Every one of those fasteners goes through a wall that was built in layers to keep water out, and the outer layer, the siding, is not the one that does most of that job. Drive a screw through the wrong spot or in the wrong way and water gets behind the siding and into the sheathing, where it rots quietly for years before anyone sees it. The repair is opening the wall, and the cause is a lag bolt nobody remembers.
Safety first
- Scan before you drill. Behind any exterior wall there may be wiring to an outdoor light or receptacle, a hose bib supply line, a gas line to a range or dryer, or refrigerant lines. See related: How to Scan a Wall Before You Cut or Drill. If you hit a wire, stop, do not pull the bit, and have the circuit switched off at the panel before anyone touches it; if you hit a water line, close the house main. Either stays off until the cable or pipe is repaired; then the circuit goes back on with the device on it confirmed live and not tripping, or the water goes back on with the repair watched dry. If you smell gas, everyone leaves, and call the gas utility from outside.
- Cutting siding releases dust. Fiber cement contains crystalline silica: cut it with shears, or outdoors with a saw fitted with a dust collection system as OSHA's construction silica standard lays out for that task (29 CFR 1926.1153, Table 1). Stucco and brick release the same hazard when drilled or cut; use a drill with dust collection or wet methods.
- Painted siding on a house built before 1978 may carry lead paint. Drilling, cutting or scraping it makes lead dust, which OSHA's lead in construction standard (29 CFR 1926.62) addresses for workers. Keep the work small, mist the surface, catch the dust on plastic and clean with a HEPA vacuum. Larger disturbances bring the EPA Renovation, Repair and Painting rule into play.
- Ladders set on firm footing, extended above the work, and kept well clear of the service drop and any overhead lines.
The layers, outside to inside
WALL SECTION, LAP SIDING ON A WOOD FRAME
OUTSIDE INSIDE
|cladding |gap |WRB |sheathing|stud bay |drywall
| lap | | | OSB or | framing |
| siding, |air/|wrap| plywood | 2x4 or |
| vinyl, |drain or | carries | 2x6 at |
| fiber |path|felt| fastener| 16 in on |
| cement, | | | loads | center, |
| brick | | | | insul- |
| or | | | | ation |
| stucco | | | | |
rain -> stopped mostly here
some gets past -> runs DOWN the WRB
and out at the bottom
(starter strip, weep
screed, brick weeps)
Cladding. The siding, brick or stucco you see. It sheds most of the rain and blocks the sun, but every lap siding, vinyl panel and brick wall lets some water past at joints, laps and fasteners. That is normal and designed for.
The gap or drainage path. Between the cladding and the next layer there is a small space, sometimes a deliberate air gap (brick veneer has an inch or so), sometimes just the irregular contact of lap siding. Water that gets past the cladding runs down it.
The water-resistive barrier (WRB). Housewrap, building paper or a coated sheathing with taped seams. This is the layer that actually keeps the wall dry. It is lapped like roofing, upper over lower, so water running down it stays on its face and exits at the bottom of the wall. Window and door flashings are lapped into it.
Sheathing. OSB or plywood (sometimes foam or fiberboard) nailed to the studs. It is what most fasteners actually hold in, and it is what rots when water gets behind the WRB.
Framing and the inside. Studs, usually 16 inches on center, insulation in the bays, sometimes a vapor retarder (which side it belongs on depends on climate), and drywall.
What a screw does to that wall
A fastener through siding and WRB makes a hole in the one layer whose job is to be continuous. Water running down the face of the WRB meets the fastener, follows its threads inward, and wets the sheathing around it. One hole is a stain. A bracket with four lags, at a spot where water runs every rain, is a soft spot in the sheathing in a few years.
Three things the mounting has to do:
- Give the item a flat surface. Lap siding is a series of sloped steps. Screwing a flat box or bracket across them either crushes the laps, leaves wedge-shaped gaps behind the item that collect water, or both. Vinyl siding also has to stay loose enough to move with temperature; screwing it tight makes it buckle.
- Shed water around the item. A mounting block, a factory-made flanged block or a flashed backer board gives the item a flat face. A head flashing over the top tucks up behind the siding and behind the WRB above, so water coming down the wall is turned out and around the item, not behind it.
- Seal the hole through the WRB in shingle fashion. Where a fastener, conduit or line set passes through, the WRB is taped around it in the lap order: the bottom piece first, then the sides, then the top piece over them, so every edge laps downhill. See related: How to Flash an Exterior Wall Penetration Into the WRB.
Which cladding changes what
- Vinyl siding: use the mounting blocks made for it, which let the siding around them float. Never drive a fastener tight through a vinyl panel.
- Fiber cement and wood lap siding: a mounting block or flashed backer, set into the siding by cutting out its outline.
- Brick veneer: fasten into the brick or the mortar with masonry anchors per the anchor maker's instructions, and do not bridge or fill the air gap behind the brick or drill through the flashing at its base. The wall's weep holes along the bottom course stay open.
- Stucco and EIFS: these are different systems entirely, and EIFS in particular has foam insulation board behind a thin finish coat. A hole made the way you would in lap siding lets water straight into the wall. See related: Stucco and EIFS and the Hole You Cut.
Holding the load
Sheathing alone holds light items: a house number, a small camera, a doorbell. Anything heavier, or anything that gets pulled on (a hose reel, a dish in the wind, a disconnect a tech yanks open, a TV mount on a porch), goes into framing. Find the studs with a scanner and confirm with a small pilot hole before committing. On brick veneer, the brick carries the load, not the wood wall behind it; do not try to reach through the brick into framing.
Worked example: a disconnect on fiber cement lap siding
An electrician is mounting an air conditioner disconnect on the side of a 1995 house clad in fiber cement lap siding with a 7 inch exposure. Say the enclosure is 6 inches wide by 10 inches tall; the real figures come off the enclosure.
Size the backer to catch framing. The scanner finds studs 16 inches on center, at 0 and 16 inches from a reference mark. Each stud's face is 1-1/2 inches wide, so the faces run from -3/4 to 3/4 inch and from 15-1/4 to 16-3/4 inches. A backer 18 inches wide, from -1 to 17 inches, lands on both studs. The electrician makes it 18 inches tall as well, leaving 4 inches above and below the enclosure and room for the conduit fitting at the bottom.
What gets cut. The block's 18 inches of height spans 18 / 7 = 2.6 siding exposures, and where it lands here it takes out part of three courses. The siding is cut to the block's outline with fiber cement shears, which make no dust. The WRB behind it is left intact except where screws and the conduit pass.
Flash the head. A Z-shaped head flashing 20 inches long (the block's 18 inches plus 1 inch past each side) goes over the top of the block. Its upper leg is tucked behind the siding above and behind the WRB: the electrician slits the WRB horizontally just above the block, slides the flashing's leg up behind it, and tapes the slit closed over the leg. Water running down the WRB now lands on the front of the flashing and is thrown out past the block.
Fasten and seal. The block is screwed through the WRB and sheathing into both studs. The conduit to the panel goes through the wall inside the block's footprint, below the enclosure, and the hole in the WRB is taped bottom piece first, then sides, then top. The enclosure mounts to the block with its own fasteners.
The tally. One flat, flashed surface instead of a box screwed across three sloped laps; screws into two studs instead of into sheathing; one wall penetration, taped in lap order and sheltered under the block and the head flashing. The enclosure stays tight when a tech pulls the handle hard.
On a pre-1978 house with painted wood clapboard, the same steps apply, and cutting out parts of three painted courses makes lead dust. The block's 18 by 18 inches is 2.25 square feet, well under the 20 square feet of exterior surface the EPA rule's minor repair and maintenance definition allows (40 CFR 745.83 in substance, check the current text), but the worker protections of 29 CFR 1926.62 apply regardless of area.
What changes the answer
The item is light and the wall is vinyl. A camera or a small light goes on a vinyl siding mounting block, fastened into sheathing, without cutting the siding at all.
A conduit or line set that runs on the surface. Fasten straps into the siding's thick bottom edge where possible, not mid-lap, and keep every strap on the face of the cladding. Even a short strap screw usually reaches the WRB, so use as few as the run allows and seal each one per the siding maker's instructions.
An exterior wall that is a fire-rated assembly, as on some townhouses close to a property line. Penetrations through it have to be sealed with a listed firestop system. Ask the general contractor or the building department before drilling.
References
- OSHA 29 CFR 1926.1153, respirable crystalline silica in construction (Table 1), and 29 CFR 1926.62, lead in construction.
- The siding and mounting block manufacturers' installation instructions.
- See related: How to Flash an Exterior Wall Penetration Into the WRB
- See related: Stucco and EIFS and the Hole You Cut
- See related: How to Scan a Wall Before You Cut or Drill