How to Tarp an Open Roof So It Stays Put Overnight

Why this matters

A planned roof opening, a skylight pulled for replacement, a curb cut for a new rooftop unit, a vent or solar penetration that will not be flashed until tomorrow, is a hole in the customer's house overnight. If the tarp over it lifts at 2 a.m., the call comes from a homeowner holding a flashlight under a wet ceiling, and the repair is now drywall, insulation and a claim on top of the work you were hired for.

Tarps rarely fail because the plastic tears. They fail at the edges: wind gets under an edge that was held by grommets and a few nails, the tarp becomes a sail, and the fasteners rip through. Or water gets in through a fastener hole uphill of the opening and runs under the tarp straight into the hole. Both failures are decided by where the edges and the fasteners are, which is what this card covers.

This is for a planned opening on a roof you chose to open. A storm-damaged roof with water already coming in is a different visit; see related: Emergency Tarp and Leak-Stop Visit. Whether the opening should be made at all given tonight's forecast belongs to The Overnight Open-Work Protection SOP.

Before anyone goes up

Fall protection comes first, and the tarp makes it harder. Roof work on a residential job is construction work, and 29 CFR 1926.501 requires fall protection for residential construction at 6 feet or more above a lower level. The opening itself is a hole in a walking surface, so it is covered before anything else happens on the roof (Step 1). A tarp is slick when dry and worse when wet or frosted: nobody stands or walks on the tarp on a pitched roof, ever. Plan the work so you install it from the roof surface beside it, working from the ridge down.

The ladder extends at least 3 feet above the eave per 29 CFR 1926.1053, is tied off at the top, and is set where the tarp and battens will not be carried across the rungs one-handed. Sheets and battens go up by hand line, not on a shoulder.

Wind decides whether you can install at all. A 10 by 20 foot tarp in a gust can pull a person off balance on a slope. If gusts are already strong when you start, get the cover board down and fastened and stop there. See related: Wind on a Ladder and the Sheet You Carry Up It, and Snow and Frost on a Roof Before Anyone Climbs It for a morning when the roof is white.

Step 1: Close the hole with a fastened cover board

Before the tarp, the opening gets a rigid cover: plywood or OSB cut to overlap the framed opening by several inches on every side and screwed into the framing or the curb. Mark it COVER in large letters.

Why: the tarp is a water barrier, not a floor. A tarp spanning an open hole sags into it, ponds water, and invites someone to step on what looks like roof. 29 CFR 1926.502 sets the cover rules in substance: a hole cover must be able to carry at least twice the load that could be imposed on it, be secured so it cannot shift, and be marked (check the current text). What breaks if you skip it: the tarp becomes the only thing between a crew member and the room below, and the first heavy rain fills the sag until a fastener lets go.

Step 2: Size the tarp to go over the ridge

The tarp runs from the far side of the ridge, over the ridge, down across the opening and well past it. Measure along the slope, not on the plan:

• Far side: enough to fasten a batten a few feet down the back slope.
• Ridge to the top of the opening.
• The opening itself.
• Below the opening: a few feet of tarp past the bottom edge.
• Width: the opening plus a lap of a few feet on each side.

Why the ridge: the uphill edge is where water gets under a tarp. A top edge fastened on the near slope, uphill of the opening, sends every drop that finds that edge running down under the tarp into the hole. Carried over the ridge, there is no uphill edge on the wet side at all. What breaks if you skip it: the tarp sheds the rain on top and channels the rain underneath into the opening.

If the opening is too close to an eave, a valley, or a hip for an over-the-ridge tarp, the geometry has changed and so has the job; call it in rather than improvising a top edge.

Step 3: Roll every edge around a batten

Do not rely on grommets. Each edge of the tarp is rolled two or three turns around a wood batten, a 2x4 or a heavy furring strip, and the fasteners go through the batten and the rolled tarp together into the roof deck.

Why: a grommet concentrates all the wind load on one small ring of plastic. A batten spreads it along the whole edge, and the rolled turns mean a fastener has to tear through several layers to pull out. What breaks if you skip it: the tarp holds until the first strong gust, then unzips grommet by grommet from the windward corner.

Roll the bottom batten so the tarp comes over the top of the roll and down the downhill side, so water running down the tarp flows over the roll and off, not into it.

Step 4: Put fasteners only where water can not reach the opening

Every fastener through the tarp is a hole in the tarp. The rule: no fastener through the tarp uphill of the opening on its own slope, and every side fastener at least the side lap away from the opening.

Why: water that gets through a fastener hole runs straight downslope under the tarp. A hole on the far side of the ridge drains to the far side. A hole in a side batten a few feet sideways from the opening drains down past the opening. A hole directly uphill of the opening drains into it. What breaks if you skip it: a tarp that never moved still leaks, and nobody can find why because the top looks perfect.

Use screws long enough to pass through the batten, the rolled tarp and the roofing and bite the deck, and space them close enough that the edge cannot lift between them. A starting default is one screw every 16 inches; tighten that in an exposed spot.

Do not place battens running across the slope above the opening on the near side. A crosswise batten on top of the tarp dams water and its screws are uphill holes.

Step 5: Tension and check the whole edge

Pull the tarp snug across the slope before the side battens go down, so it lies flat without wrinkles that catch wind. Then walk your eye around all four edges from the roof surface: battens continuous, no gaps at the corners, no tarp flapping loose past a batten end.

Why: a wrinkle is a wind scoop and a corner gap is where lift starts. What breaks if you skip it: a tarp that looked fine from the ladder lifts at one loose corner and peels from there.

Step 6: Protect the inside, and tell the customer

Under the opening, the room below gets a drop cloth or plastic and, where the ceiling is open to the attic, a bucket or pan under the cover board. Tell the homeowner what is over their roof, what they might hear in wind, and the number to call if they see water.

Why: if something does get through, a wet drop cloth is a nuisance and a wet carpet is a claim. A customer who knows to look and call at the first drip turns an event into a quick fix.

Worked example: a skylight out overnight

A roofing crew is replacing a skylight on a 6/12 asphalt-shingle roof. The old unit comes out at 2 p.m.; the new one arrives tomorrow at 9 a.m. The framed opening is 30 inches wide and 48 inches tall measured along the slope. Its top edge is 6 feet below the ridge along the slope. Tonight's forecast: rain after 11 p.m., gusts moderate and inside the shop's overnight-opening limit, which is why the office let the opening be made today.

The cover. A piece of 3/4 inch plywood cut 38 by 56 inches, overlapping the opening 4 inches on every side, screwed into the framing and marked COVER.

The tarp size, along the slope:

• 4 feet down the far side, plus 6 feet ridge to opening, plus 4 feet of opening, plus 4 feet below it, is 18 feet.
• Width: 30 inches of opening plus 36 inches of lap on each side is 102 inches, 8.5 feet.

A 10 by 20 foot tarp covers it: 10 feet of width gives 18 inches more lap than the 8.5 needed, split across the two sides, and the 20 feet gives 2 feet of spare length, which goes below the opening so the bottom batten sits 6 feet down from the opening instead of 4.

  BACK SLOPE
  +======================================+ FAR BATTEN
  |       4 ft down the back slope       |
  +--------------- RIDGE ----------------+
  |S|                                  |S|
  |I|      6 ft ridge to opening       |I|
  |D|                                  |D|
  |E|           +---------+            |E|
  | | 3 ft 9 in | OPENING | 3 ft 9 in  | |
  |B|           | 30 x 48 |            |B|
  |A|           +---------+            |A|
  |T|                                  |T|
  | |        6 ft below opening        | |
  | |   (4 ft planned + 2 ft spare)    | |
  +======================================+ BOTTOM BATTEN
  far and bottom battens 10 ft, side
  battens 16 ft; bottom edge rolled over
  its batten; no fastener through the
  tarp above the cover between the sides

The drawing shows the tarp laid flat: the far batten is on the back slope, the two 16-foot side battens run from the ridge to the bottom edge on the near slope, and the 16 feet is the 20-foot tarp less the 4 feet on the far side.

The battens. Two 10-foot 2x4s for the far edge and the bottom edge, two 16-foot 2x4s for the sides. Each edge is rolled two turns.

The fasteners. At one screw every 16 inches, a 10-foot batten (120 inches) gets 8 spaces and 9 screws, and a 16-foot batten (192 inches) gets 12 spaces and 13 screws. That is 9 plus 9 plus 13 plus 13, 44 screws, all through battens and none through the tarp above the cover between the side battens.

What the layout prevents. The highest fasteners on the near slope are the top of each side batten, at the ridge, 3 feet sideways from the opening's edge plus the 9 inches of extra lap. Water entering there runs down the side, past the opening, and out at the bottom batten.

The next morning. Before anyone touches the tarp, the crew lead checks the roof for frost (it can be slick even when the air is above freezing) and the inside for any drip. Clean ceiling, dry pan. The tarp comes off from the bottom batten up, screws out, and each of the 44 holes gets sealed: the near-slope ones with roofing sealant worked under the shingle tab above the hole so the next course covers it, the far-side ones the same way. The hole count goes on the ticket so the crew can confirm every one was sealed.

What would change the call:

• A forecast with gusts above the shop's overnight limit. Then the skylight stays in until tomorrow, or the new unit goes in the same day. A tarp is not the plan for weather the shop would not leave an opening under.
• A low-slope roof. Water drains slowly and ponds against battens, so the fastener rule tightens and a membrane temporary patch may beat a tarp.
• Tile or slate. Screws through the roofing break pieces; the batten layout gets reworked to fasten into the deck at gaps or ridge, and the crew prices in replacement pieces.

How to know it held

A tarp that held looks identical in the morning to how you left it, and the room below is dry. If the tarp moved, find out where before you take it off: a lifted windward corner means the corner batten or its screws were short, and water stains under the tarp below an uphill line mean a hole you did not intend. Write either finding on the ticket, because the next crew to tarp that roof will repeat it.

References

• 29 CFR 1926.501, Duty to have fall protection (residential construction at 6 feet)
• 29 CFR 1926.502, Fall protection systems criteria and practices (covers for holes)
• 29 CFR 1926.1053, Ladders
• See related: Emergency Tarp and Leak-Stop Visit
• See related: The Overnight Open-Work Protection SOP