A tarp is a reasonable thing to want on a roof that is leaking. On an asphalt roof it is a twenty minute job with a hammer and a box of roofing nails, and the damage the fasteners cause is absorbed by the replacement that is coming anyway. On a slate roof the same twenty minutes can cost more than the storm did.
Slate is stone. It does not flex, it does not seal around a fastener, and it splits along the plane of the stone when something is driven through it. A nail through a slate in the field of the roof does not make a small hole. It makes a crack that runs, and the piece either falls out that week or slides free the next time the roof goes through a freeze and thaw cycle. A crew that nails a twenty by thirty tarp to a slate slope will leave behind forty or more broken slates in an area that originally had three.
There are ways to get a temporary cover on slate that hold in wind and leave the roof exactly as they found it. All of them work by weight or by clamping over the ridge rather than by penetrating the roof surface.
Why nailing through slate is the wrong trade
Every slate on a roof is held by two nails near its head and is covered by the two courses above it. That overlap is what keeps water out. It also means that when you nail through the exposed face of a slate, you have put a hole in the middle of the waterproof layer, in the one place where nothing covers it.
Three things follow from that.
- The hole leaks after the tarp comes off. Every fastener location becomes a new entry point, spread across the slope in whatever pattern the tarp crew used. These leaks are small, slow, and hard to find, and they show up in the ceiling months later as a stain nobody can explain.
- The slate cracks, not just punctures. Stone driven through with a nail splits outward from the hole. Even if the piece looks intact from the ground, it has lost its structure and will fail.
- Every broken slate has to be replaced individually. There is no patching a slate. Each one has to be removed with a ripper, matched for size, thickness, and color, and reset. Forty pieces of avoidable slate tile replacement is a real bill on top of the repair the storm made necessary.
The same logic rules out screws, plastic cap nails, and the strips of lath that some crews screw down over a tarp edge. Anything that goes through the stone is a problem. Adhesives are no better: construction adhesive and roofing cement both bond hard to slate, and removing them takes the face of the stone with them.
Covers that hold without a single fastener
Weighted at the perimeter
The simplest approach is a heavy tarp laid over the damaged area with its edges rolled around weights. Sandbags work. So do lengths of two by four with the tarp edge wrapped twice around them and the roll tied off. The weight has to sit on the tarp edge continuously, not at three points, because wind gets under an unweighted span and lifts the whole cover like a sail.
Roll the tarp edge rather than laying it flat. A rolled edge with a batten inside it presents a rounded profile to the wind and has no loose corner to catch. Corners are where every weighted tarp fails.
Battened over the ridge
For anything larger than a small patch, the tarp should start above the ridge and come down over the damaged slope. The top edge is rolled around a board, carried over the ridge, and weighted or tied on the opposite slope. Water sheds over the top of the cover instead of running under it, which is the failure mode of a tarp that starts partway up the slope.
Where a ridge run is not practical, a batten can be clamped rather than nailed, using the roof edge, a chimney, or existing anchor points. On a steep slope the tie downs should run to the ground or to structural points, never to gutters, which will pull off under load.
What to use and what not to
A heavy woven poly tarp, ten mil or better, holds up in sustained wind. A blue hardware store tarp will shred inside a week and leave you with the same leak and a roof covered in torn plastic. Size it so it runs at least four feet past the damage in every direction, and more above it, because water travels farther uphill under wind than most people expect.
Temporary sealing at flashing, valleys, and chimneys
A lot of storm leaks on slate are not broken slate at all. They are at the metal: a counterflashing that pulled out of the mortar joint, a valley that lifted, a step flashing that separated at a dormer cheek. Those spots are hard to cover with a tarp because the geometry is complicated and a flat sheet will not lie down over a valley or a chimney corner.
For those, a temporary measure is usually a piece of self adhering membrane or a mechanically clamped piece of metal, set so that water sheds over it and nothing is fastened into the slate. It is genuinely temporary. Membrane adhesive bonds to copper and to stone, and leaving it on through a summer makes the eventual slate roof flashing repair harder, because the residue has to come off before new metal is soldered or set.
Never pack a flashing gap with roofing cement as a temporary fix. It works for about one season, then cracks, holds water against the masonry, and has to be chipped out of the mortar joints by hand.
How long a temporary cover should stay on
A temporary cover is measured in weeks, not seasons. Three reasons.
- Tarps degrade. Ultraviolet light breaks down poly. What was watertight in April is brittle and porous by August, and the homeowner usually does not know until the ceiling goes again.
- A covered roof cannot dry. A tarp laid directly on slate holds moisture against the stone and against the deck beneath it. Soft slate and older sheathing both suffer from staying wet, and organic growth under a tarp will discolor the stone.
- Weights move. Every wind event shifts a weighted tarp slightly. Over a few months the edges creep, the coverage stops matching the damage, and the cover starts funneling water into places that were dry before.
Exposure changes the math. Houses on open ground take more wind on a cover than sheltered ones, which is why a tarp on one of the ridgetop slate roofs around Big Island needs checking more often than one on a wooded lot in town. Farmhouses out toward Rustburg with original slate roofs sit in the same kind of open exposure. If a cover has to stay up while slate is sourced, it should be looked at after every storm and re-weighted as needed.
Sourcing is often the reason a cover stays up. Matching reclaimed Buckingham or Vermont stone to an existing roof takes time, and a good match is worth waiting for. What should not wait is the initial stabilization, which is the whole point of emergency roof repair on slate: stop the water now, in a way that does not add to the permanent repair.
Common questions
Can I tarp my slate roof myself?
From the ground or a ladder at the eave, no, you cannot reach far enough to place a cover correctly. From the roof itself, wet or dry, slate is slick and every step risks cracking sound stone. If the leak is contained inside the house with buckets, that is the safer choice until a crew can get there. If it is not contained, call rather than climb.
A contractor wants to nail the tarp down. Is that ever acceptable?
Only outside the slate. Fasteners can sometimes go into a fascia board, a rake board, or a batten clamped at the ridge without touching the roof surface. If the proposal involves nails through the field of the slate, it is not acceptable, and the crew proposing it does not work on slate regularly. Ask specifically where each fastener will land before work starts.
How much of the roof should the tarp cover?
More than the visible damage. Run it at least four feet to each side and carry the top edge over the ridge where you can. Water that gets under the head of a short tarp simply reappears below it, and a homeowner who sees water after a tarp goes on often assumes the cover failed when in fact it was never carried high enough.
Will a tarp cause more damage than leaving the roof open?
A correctly placed weighted cover causes none. A nailed cover causes a great deal. An uncovered opening lets water into the deck, the insulation, and the framing, and wet sheathing that stays wet loses its ability to hold nails, which affects how the replacement slate can be fastened. The order of preference is a proper cover, then an open roof, then a nailed tarp.
Get a proper cover on it
If your slate roof is open and rain is coming, the cover that goes on today determines how much work is waiting next month. We stabilize slate roofs without putting fasteners through the stone, then repair from there. Call +15555555555 to get a crew out and a temporary cover placed correctly.
