Mending a corrugated plastic roof

Mending a corrugated plastic roof

Before you buy corrugated plastic to make the repair, measure the profile of the existing plastic on the roof. The sheets may have a round or a box profile and the difference between the lowest and highest points of the profile can vary from 1&1/2in (38mm) to 6in (150mm). If the new plastic does not exactly match the old in profile, it will not make snug overlaps. The length of the screws you use must be the difference between the low and high points of the profile plus at least 1in (25mm) to penetrate the wood.

Overlaps at the sides should finsh in a ‘valley’ of the corrugation, not on a ‘peak’. This prevents water from entering at an overlap. If you are renewing a complete panel on the roof, allow the same overlaps as on the rest of the roof.

To reduce the cost of the repair, you can fit a patch, for which you may be able to buy a cheap offcut.

However, the patch will have to be the width of a full sheet and extend over a roof timber at top and bottom to be screwed in place.

Cut sheets to size indoors in cold weather. Plastic becomes brittle in low temperatures and could crack.

Do not walk on the roof. Kneel on scaffold boards secured with sandbags so that they will not slide.

You will need Tools Sharp knife, tack lifter, screwdriver, fine-toothed saw, hand drill with blunt twist bit, or electric soldering iron with 3/16in (5mm) bit, steel measuring tape.

Materials Enough corrugated plastic sheeting to make the repair with adequate overlaps, No. 8 galvanized screws of appropriate length, protective screw caps, transparent waterproof glazing tape.


1. Cut away the flashing at the top of the damaged panel.

2. Prise off the screw caps with the tack lifter and take out all the screws securing the panel. Carefully remove the panel.

3. Cut the new panel if necessary to match the length of the old one. Hold the saw at a shallow angle and support the sheet on both sides of the cut If you have to cut to width, cut along valleys.

4. Place the new panel in position on the roof, overlapping onto the old ones at the sides. If there is another panel above or below it, make sure that the bottom edge of a panel higher up the slope laps onto the panel below, so that water running down the slope cannot penetrate a gap.

5. Make screw holes through the peaks that are over timbers. Make the holes at intervals of about 18in (460mm) across the panel immediately above the cross timbers. You can either melt the holes with a tine soldering iron, or start them with a bradawl, or use a blunt bit in a drill.

6. Drive in the screws, do not over tighten then or the plastic may split.

7. When the screws are fixed, push on screw caps. They will click into place.

8. Fit new flashing strip (see Replacing a flashing ) where the plastic sheet meets the wall. Press it down well into the valleys.

9. Seal the edges where the layers of plastic sheeting overlap at the sides with strips of the glazing tape.


1. Use a felt pen to mark cutting lines on the damaged panel showing the area for removal. Make the top line just below a timber and the bottom line just above a timber.

2. Remove the whole panel as described on the left.

3. Cut the patch to overlap the guidelines on the old panel by 3in (15mm) at top and bottom. Then cut along the guidelines on the old panel to remove the damaged part. Follow the advice for sawing given in instruction 3AA


4. Lay the plastic for the bottom of the slope in place first. Make screw holes across the bottom if necessary as in instruction 5 AA

OOVVEE. Drive in the screws across the bottom edge. Do not over tighten.

5. Lay the next piece of plastic up the slope, let its bottom edge overlap 3in (75mm) onto the piece below.

6. Drill screw holes through peaks on the overlap at 18in (460mm) intervals. Drive screws through into the timbers but do not over tighten them.

7. Lay the top piece of plastic sheet in place overlapping the previous piece by 3in (75mm).

8. Drill and screw the bottom edge as for the previous piece.

9. Drill holes if necessary at the top edge and screw it in place.

10. Push caps on all the screws.

11. Fit new flashing and seal the overlaps with glazing tape as for a whole panel.

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