How to Weather-Protect an Open Wall or Window Opening Overnight
Why this matters
A window that comes out at 3 p.m. and goes back in at 8 a.m. leaves a rough opening in the wall for seventeen hours. So does a siding section stripped to fix rot, or a hole cut through a wall for a through-wall unit or a new vent. Water that gets into that opening overnight does not stay at the opening: it runs down inside the wall onto the sill plate, into the subfloor and the insulation, and it stays there long after the new window is in and looks perfect.
The protection fails in two predictable ways. A sheet tacked over the opening gets lapped the wrong way, so water running down the wall is directed behind it instead of over it. Or the sheet is fastened loosely, wind works it all night, the staples tear out, and by morning it is hanging from one corner. This card covers the lapping order that sends water out, the fastening that survives wind, and the cover that keeps the opening closed.
Whether the opening should be made today at all, given tonight's forecast, belongs to The Overnight Open-Work Protection SOP. A roof opening is a different geometry; see related: How to Tarp an Open Roof So It Stays Put Overnight.
The rule every step follows
Upper over lower, at every joint. Water runs down a wall. Every layer you add must have its top edge tucked under the layer above it and its bottom edge lying over the layer below it, the way shingles or lap siding are laid. That is what "shingle-lapped" means: at any seam, water running down meets the outside face of the upper piece and flows onto the outside face of the lower piece.
The one place this is routinely reversed is the top. The existing weather-resistive barrier (the housewrap or building paper behind the siding, abbreviated WRB) above the opening has to lap OVER your protection, which means your top piece goes UNDER the WRB above. A head piece stapled on top of the WRB above, with its top edge exposed, catches every drop running down the wall and feeds it behind your sheet.
Step 1: Check the wall before you cover it
Before any sheeting goes on, look at the exposed framing and the WRB around the opening. Soft or dark wood at the sill or the bottom of the jambs means the leak you are about to protect against has already happened, and it is a finding for the customer and the job, not something to cover tonight and forget.
Hazards at this step: cutting or prying around an old window disturbs paint and glazing compound. On a house built before 1978, treat the painted surfaces as lead-containing unless tested; the work falls under EPA's Renovation, Repair and Painting rule at 40 CFR 745 Subpart E when the disturbed area crosses its thresholds (window replacement is covered at any size, because the rule's minor-repair exemption excludes it), and that rule's work practices (containment, no dry sanding, cleaning) and a certified renovator apply. Cutting through a wall for equipment means confirming what is in the cavity first: wiring, plumbing and gas lines run through walls, and the cut stops if any is found until it is rerouted by the trade that owns it.
What breaks if you skip the check: the overnight protection works, the new window goes in, and the rot under it is sealed in place.
Step 2: Flash the rough opening, sill first
Wrap the framing of the opening with strips of housewrap or flexible flashing tape so the bare wood is covered, laid in this order:
- Sill strip first. Across the sill, turned up a few inches onto each jamb at the corners, and down over the wall face below the sill so it laps over the WRB there.
- Jamb strips second. Each one from the bottom of the opening up past the top, its bottom end lying over the turned-up ends of the sill strip.
- Head strip last. Across the top, its ends lying over the tops of the jamb strips, and its top edge tucked up under the WRB above the opening.
Why the order: it is the only order in which every seam is upper over lower. Laid head first, the jamb strips end up on top of the head strip at the corners, and water running down the head strip goes behind them. What breaks if you skip it: the corners, which are exactly where a rough opening leaks, are lapped backward.
To tuck the head strip under the WRB above, make a horizontal slit in the WRB a few inches above the opening, slide the head strip up into it, and tape the slit closed over the head strip. Tape the slit, never the bottom edge of the sill strip: the bottom must stay open so water that gets in can get out.
Step 3: Close the opening with a cover board
Plywood or OSB, cut to fit the opening and screwed into the framing, closes it against wind, rain driven sideways and anyone who might walk through an open wall at night. For a removed window, this is also the security answer: the customer should not go to bed with a hole in the house that a person could climb through.
Why a board and not just plastic: a sheet alone over a 3 by 5 foot opening is a drumhead in wind, and wind-driven rain on a windward wall is pushed through any gap the sheet leaves. What breaks if you skip it: the sheet carries the whole wind load and tears at its fasteners.
Hazard: screwing into the rough opening framing, keep the screws out of the space where a new window's fasteners will go if you can, and check for wiring in the header area before driving long screws.
Step 4: Sheet over the board, lapped into the wall
Over the cover board goes a sheet of heavy polyethylene or housewrap, large enough to lap past the opening onto the surrounding siding or WRB on all sides. Its top edge goes under the siding course or the WRB above, the same way the head strip did. Its sides and bottom lie over the wall face.
Hold every edge with wood lath or furring strips screwed through the sheet into framing, not with staples alone. A starting default is a fastener every 12 inches along each lath.
The bottom edge is battened, not sealed. It is held down so it cannot flap, but it is not taped to the wall. Water that gets behind the sheet drains out at the bottom.
Drip edge: where the opening has no head flashing above it and the siding above is cut back, a short length of metal drip edge or a strip of flashing tucked under the WRB and kicked out over the top of the sheet keeps water running down the wall from sheeting onto the top edge.
Step 5: Look at it the way wind will
Before you leave, stand back and look at the sheet in whatever breeze there is. Any edge that lifts, any bulge between laths, any corner without a fastener, is where the sheet will start working loose tonight.
What wind does to a loose sheet: a flapping sheet does not just let water in at the moment it lifts. It pumps air in and out behind itself, pulling water through the gap, and each flap works the fastener holes larger until the plastic tears through them. Poly that is tight and battened lasts a stormy night; poly that is loose often does not.
Worked example: a window out overnight on a west wall
A window crew removes a failed 36 by 60 inch window from a vinyl-sided wall at 3 p.m. The replacement is on site but the crew finds the sill plate soft at the left corner, and the repair and the install move to tomorrow at 8 a.m. Tonight's forecast: rain after 1 a.m., with wind from the west, onto this wall.
Step 1, the check. The soft wood is photographed and probed: it is soft about 2 inches in from the left corner along the sill and does not extend up the jamb. The house is from 1985, so the paint is outside the RRP rule's pre-1978 scope. The finding goes on the job and to the homeowner tonight, with the plan to cut it out and replace it before the window goes in.
Step 2, the strips. The crew cuts strips 12 inches wide from housewrap.
- Sill strip: 36 inches across plus 6 inches up each jamb, 48 inches long, laid across the sill and folded down the wall face below.
- Jamb strips: each 60 inches of opening plus 6 inches past the top, 66 inches long, laid over the turned-up ends of the sill strip.
- Head strip: 36 inches plus 6 inches lapping over each jamb strip, 48 inches long, its top edge slid up into a slit cut in the WRB 4 inches above the opening.
The slit is taped. The bottom edge of the sill strip is not.
WRB above (laps OVER head strip)
---- slit, taped -----------------------
+----------------------------------+
| HEAD STRIP 48 in, laid LAST |
| ends over jamb tops |
+---+--------------------------+---+
| J | | J |
| A | ROUGH OPENING | A |
| M | 36 x 60 in | M |
| B | (cover board goes | B |
| | in after strips) | |
| 66| |66 |
| in| |in |
+---+--------------------------+---+
| SILL STRIP 48 in, laid FIRST |
| ends turned up under jambs |
+----------------------------------+
bottom edge open: water drains out
Step 3, the board. A piece of 1/2 inch plywood cut to fit inside the opening, screwed through cleats into the jambs.
Step 4, the sheet. The siding has been cut back 8 inches around the opening, so the sheet needs to reach past that onto intact siding. The crew sizes it at 36 inches of opening plus 8 inches of cut-back plus 12 inches of lap onto siding on each side: 36 plus 16 plus 24 is 76 inches wide. Height: 60 inches plus 8 inches of cut-back above and below plus 12 inches of lap above and below: 60 plus 16 plus 24 is 100 inches. The top 12 inches go up under the siding course above, which the crew unlocks with a siding zip tool and lets snap back over the sheet. Lath goes on both sides and the bottom, screwed every 12 inches. On a 100-inch side that is 8 spaces of 12 inches with 4 inches left over, so 9 screws plus one more at the top end, 10 in all; on the 76-inch bottom, 6 spaces with 4 inches over, 7 screws plus one at the end, 8 in all.
Step 5, the wind check. At 4:30 p.m. there is a light breeze from the west and the sheet does not move. The crew walks inside and checks the cover board from the interior side: no daylight around it.
The morning. At 8 a.m. the interior wall below the opening is dry and the sheet is tight. Water has run down the outside of the sheet and off the bottom lath. The sheet comes off top-down only after the siding above is unlocked again.
What would change it:
- An east wall with tonight's west wind. The lapping and fastening are the same; the opening is on the lee side and the sheet has an easier night, but the laps still matter because rain runs down every wall.
- Stucco or brick instead of lap siding. There is no course to tuck the top under. The head strip still goes under the WRB if it is exposed; where it is not, a strip of flashing tape bonded to the cladding above and lapping over the sheet's top edge does the job, removed carefully in the morning.
- A larger opening, a missing wall section several feet wide. The cover goes up in pieces, and the sheet goes up in shingle-lapped horizontal courses starting at the bottom, each course's top tucked under the one above, 6 inches or more of lap at every seam.
How to verify it held
In the morning, before anything comes off, check the inside first: the floor and wall below the opening, the cover board edges. Then the outside: sheet tight, every lath in place, water marks running down the face of the sheet and off the bottom, not behind it. Water inside with a tight sheet means a lap is backward; check the head and the corners first, because that is where the order of the strips decides everything.
References
- 40 CFR 745 Subpart E, Residential Property Renovation (EPA RRP rule)
- See related: How to Tarp an Open Roof So It Stays Put Overnight
- See related: The Overnight Open-Work Protection SOP