After years of supplying roofing sheets and helping customers with their projects, we know that the small installation details deserve just as much attention as the sheets themselves.

A roof may look straightforward, but incorrect support spacing, overtightened fixings or a poorly finished wall junction can cause problems once the weather changes. Taking time to prepare properly is usually easier than putting things right afterwards.

This guide brings together practical advice from The Polycarbonate Store for installing multiwall polycarbonate, corrugated PVC and corrugated polycarbonate roofing.

Our starting point is simple: choose the correct system, store it properly, work in suitable conditions and follow the instructions for the products you have purchased.

1) Plan the complete roof before ordering

The sheets, supports, fixings and finishing components need to work together. Avoid buying the sheets first and working out how to secure them afterwards.

Before ordering, establish:

• The roof width and horizontal projection.
• The proposed pitch and available height.
• The supporting framework and spacing between supports.
• The roofing material, thickness and profile.
• The glazing bars or fixings required.
• How the roof will meet any walls.
• Where rainwater will drain.

For replacement roofs, check the existing structure rather than assuming it will suit the new material. Similar-looking sheets may have different support requirements.

Understand which system you are using

Multiwall polycarbonate sheets are commonly fitted using glazing bars.

Rafter-supported glazing bars require supporting rafters beneath them. Self-supporting glazing bars are a different system, with their own span limits and requirements for support at each end.

Corrugated PVC roofing sheets and corrugated polycarbonate roofing sheets generally use overlapping profiles secured to suitable supports.

Do not transfer the installation instructions from one system to another simply because both products are plastic roofing sheets.

Check your measurements carefully

For multiwall roofing, the width of the sheet must allow for the chosen glazing bars, their required sheet engagement and the expansion allowance.

Use our glazing bar deductions guide before finalising your sheet widths.

With corrugated sheets, calculate the roof coverage using the effective cover width after overlaps, rather than the full sheet width.

The sloping roof length will also be longer than the horizontal projection. Check which measurement you are using before ordering.

2) Give the roof sufficient pitch and support

Our rule of thumb: recommend 10° unless the system says otherwise

At The Polycarbonate Store, we generally recommend a 10° roof pitch unless the system being used specifies otherwise.

Different products have different minimum slopes. Follow the requirements for the complete system rather than treating one angle as suitable for every roof.

A suitable slope helps rainwater drain towards the gutter and reduces the risk of standing water. On multiwall roofing, the flute direction also matters for drainage within the sheet.

At 10°, the height difference is approximately 176mm for every 1m of horizontal projection.

Remember that degrees and percentages are different measurements. A 10° pitch is not the same as a 10% slope.

Do not guess the support spacing

Sheet thickness, profile, roof dimensions and local wind and snow loads all affect the support requirements.

Our sheet size and rafter spacing guidance is a useful starting point for multiwall projects. Corrugated products must be checked against their own support tables.

A thicker sheet does not remove the need for a suitable framework. Neither does adding extra screws make an inadequately supported roof suitable.

Choose the correct rafter width

For our standard rafter-supported multiwall installation, we recommend rafters 50–60mm wide when using a 50mm-wide glazing bar. Centre the glazing bar on the rafter and ensure it is supported along its full length.

Rafter width alone does not determine structural suitability. Timber depth, span, connections and expected loads must also be considered.

Use light-coloured timber supports

Timber rafters, purlins and support surfaces beneath the roofing sheets must be light-coloured, preferably white. This is particularly important with corrugated PVC roofing.

Dark-painted or dark-stained timber absorbs more heat from sunlight. Beneath transparent roofing, this can create concentrated heat where the sheets meet the framework, increasing the risk of distortion and damage.

Before fitting:

• Paint the upper surfaces of timber supports white or another suitable light colour.
• Include dark support surfaces directly beneath the sheets, even if the rest of the visible structure remains darker.
• Use a coating compatible with the roofing material.
• Allow paint and timber treatments to dry fully before installation.
• Fit any specified support tape or profile accessories according to the system instructions.

Do not assume that UV protection prevents heat damage, or that a strip of support tape automatically makes dark timber suitable.

3) Store the goods correctly before installation

Installation problems can begin before the first sheet reaches the roof.

Store sheets and long accessories horizontally, with even support, in a cool, dry, shaded and well-ventilated location. Remove outer transit wrapping as advised, while retaining protective surface film where supplied.

Do not leave sheets or PVC profiles tightly bundled in direct sunlight. Heat can build up within the stack, potentially causing distortion or making the protective film difficult to remove.

Protect multiwall flute openings from water and debris, and keep tapes, gaskets and fixings clean and dry.

Read our storage advice for sheets and accessories if installation will not take place immediately.

4) Choose suitable weather and working conditions

A free weekend is not always a suitable installation window. Check the forecast, the condition of the materials and the application requirements for tapes and sealants.

Aim for dry, calm conditions, with clean, dry materials and temperatures within the product instructions.

Never walk directly on plastic roofing sheets. Arrange safe access and handling before starting, and use a competent installer where the work is beyond your experience.

Summer installation tips

Warm weather can make installation more comfortable, but strong sunshine brings its own problems.

• Keep sheets and accessories shaded until needed.
• Avoid laying materials on hot paving or leaving them in a vehicle.
• Do not leave protective film exposed to prolonged sun.
• Check the permitted application temperatures for tapes and sealants.
• Maintain the specified expansion allowances.

A sheet surface can become considerably hotter than the surrounding air. Do not assume the day’s air temperature tells you whether an adhesive can be applied successfully.

Winter installation tips

Cold weather requires extra care, especially when working with PVC.

PVC can be less resistant to impact at low temperatures. Avoid dropping sheets, striking edges or forcing profiles into position.

Check for frost, ice and surface moisture before handling or fitting materials. Tapes and sealants may need warmer conditions than the sheets themselves.

Do not fit products outside their stated working conditions simply because they are intended to withstand winter weather once installed.

Wet or windy conditions

Postpone work if rain, slippery access or wind makes handling unsafe.

Large, lightweight sheets can be difficult to control in gusts. Do not leave loose sheets unsecured on a roof.

Dry-looking surfaces may still carry dew or condensation. Check them before applying adhesive products, and follow any stated curing or weather-exposure times.

5) Multiwall polycarbonate installation tips

Fit the UV-protected surface upwards

The standard multiwall polycarbonate supplied by The Polycarbonate Store has UV protection on one surface. This surface must face upwards towards the sun.

Identify it from the printed film or markings before removing them. Protective film on both surfaces does not mean that both surfaces have UV protection.

Our UV protection guide explains this in more detail.

Run the flutes down the slope

The hollow channels should run from the high end towards the low end of a typical pitched roof. Do not turn sheets sideways to make a width fit. The flute direction is important for drainage and the correct installation of the sheet.

Position the sheets correctly and allow for thermal movement

When placing multiwall sheets into our standard rafter-supported glazing bars, do not push the sheet edges tightly against the central metal upright.

Rest the sheet on the rubber gasket, positioning its edge at the back of the gasket while leaving approximately 2–3mm between the sheet edge and the metal upright. Check that the sheet remains properly supported and has the engagement required by the glazing system.

This clearance allows movement across the sheet’s width. The sheet must also be able to move along its length, so leave the end clearances specified for the chosen system.

Do not wedge sheets tightly against walls or other fixed components, or restrict their movement by overtightening fixings. The required allowances depend on the sheet dimensions, temperature changes and glazing system.

Remove cutting debris before taping

Support sheets properly when cutting and use a suitable fine-toothed tool.

Clear swarf from the flutes with a vacuum or suitable clean, dry compressed air before applying end tapes. Avoid introducing water into the channels during preparation.

Use the correct tapes and sheet closures

For a typical straight, pitched multiwall roof:

• Apply aluminium sealing tape at the upper end.
• Apply anti-dust breather tape at the lower end.
• Protect the lower taped edge with a suitable sheet closure.

We also recommend an upper closure where the system allows, particularly on freestanding structures.

Keep the lower ventilation and drainage route clear. Do not fill the closure or open flutes with silicone.

Read our sheet closure installation guide for fitting advice.

Fit glazing components without forcing them

Use the correct glazing bars for polycarbonate and match their capacity to the sheet thickness.

Check alignment and sheet engagement before securing the caps. A component that will not fit may be incorrectly sized, misaligned or obstructed.

For the standard snap-down system described in the FAQs below, press the PVC top cap into place by hand. Do not use a rubber mallet.

6) Corrugated PVC and polycarbonate installation tips

Check the profile and effective coverage

Corrugated sheets must overlap in the intended way. Profiles that look similar may differ in wave height, spacing or shape.

Check compatibility when replacing individual sheets or buying matching accessories. Do not force a sheet into a neighbouring profile.

Follow the specified overlaps

Side overlaps and end overlaps depend on the product, pitch and installation conditions.

Where sheets join along the roof slope, the upper sheet must overlap the lower sheet so water runs over the joint.

Follow the manufacturer’s instructions for laying direction relative to prevailing wind and rain. Do not assume that a larger overlap can compensate for an insufficient roof pitch.

Use the correct fixings and washers

Fixings must suit the sheet profile and supporting material. Their position, length, washer type and spacing all matter.

For many corrugated roofing products, primary fixings are positioned through the crests. However, check the instructions for your particular sheet and distinguish roof fixings from any separate lap-fixing details.

Browse the appropriate corrugated PVC accessories or corrugated polycarbonate accessories rather than choosing fixings by appearance alone.

Allow movement around fixings

Plastic sheets expand and contract as temperatures change.

Where pre-drilling is required, use the specified clearance-hole size. Some proprietary fixings create their own expansion clearance; use these only as instructed for the relevant product.

Tighten fixings sufficiently to seat the sealing washer correctly without crushing the corrugation or distorting the sheet. More pressure does not automatically produce a better seal.

Use matching infills where specified

Profile-matched infills may be required at particular edges or junctions. Use the correct shape and fit them in the positions stated by the system instructions.

Do not pack gaps indiscriminately with expanding foam or sealant.

7) Finish wall junctions, edges and drainage properly

Flash the wall-to-roof connection

A roof fixed against a wall needs a suitable weathering detail at that junction.

Do not assume that the sheet ends, a wall plate or a narrow bead of silicone will provide the complete flashing arrangement.

Use products compatible with the surfaces involved. Our flashing tape advice explains why compatibility matters.

Finish exposed side edges

Where a multiwall roof has exposed outer sheet edges, use the appropriate edge profile for the glazing system.

An aluminium F-section can finish the outer side of the sheet and connect it to the supporting rafter or wall, as appropriate. Other systems use a matching PVC finishing profile alongside a snap-down bar.

These side-edge components are different from sheet closures and glazing bar end caps.

Position the overhang correctly

Most installations use a sheet overhang of 50–100mm beyond the final support. Do not exceed 150mm, and follow any smaller maximum specified for your particular product.

Position the lower edge so water discharges into the gutter. Excessive unsupported projection can leave sheets vulnerable to movement, while too little may let water track behind the gutter.

Plan guttering before finishing

We strongly recommend guttering to collect run-off and reduce splashing below the roof.

For roofs built from separate sheets and glazing components, guttering needs to be sourced separately. Complete canopy systems may have integrated drainage, so check the specification.

8) Carry out a final check

Before putting the tools away:

• Check sheet orientation, alignment and support.
• Confirm that overlaps and glazing components are correctly seated.
• Look for overtightened fixings or distorted profiles.
• Check that sheet closures, side profiles and glazing bar end caps are fitted where required.
• Keep drainage routes clear.
• Complete the specified flashing details.
• Remove remaining protective film promptly, as instructed.
• Clear cutting debris from the roof and gutters safely.

If dripping occurs later, identify its source before attempting a repair. Water at a joint may indicate a leak, while droplets across the underside may be condensation.

Read our condensation advice before covering gaps with additional sealant.

For help choosing sheets and compatible accessories before installation, contact The Polycarbonate Store with your project details.

9) Roofing installation FAQs

Can I install polycarbonate or PVC roofing myself?

Some systems are suitable for competent DIY installation. Your ability to construct the framework, interpret the instructions and work safely at height is just as important as fitting the sheets. Use a competent installer if you are unsure.

What roof pitch should I use?

Our general recommendation is 10° unless the system specifies otherwise. Always follow the minimum pitch for the complete roofing system.

At 10°, allow approximately 176mm of height difference per 1m of horizontal projection. Degrees and percentage slopes are different, so check which measurement the instructions use.

What size rafters do I need for a 50mm glazing bar?

For our standard rafter-supported multiwall installation, we recommend rafters 50–60mm wide when using a 50mm-wide glazing bar.

Centre the bar on the rafter and ensure it is supported along its full length. The required timber depth and spacing depend on the roof design, span and loads.

Can I install roofing sheets over dark-painted timber?

The timber support surfaces beneath the sheets must be light-coloured, preferably white. Dark timber absorbs more heat and can create concentrated heat beneath transparent roofing, particularly where the sheet meets the support.

This is especially important with corrugated PVC. Paint the relevant support surfaces before installation and allow the coating to dry fully.

How far apart should supports be?

There is no single spacing suitable for every sheet. It depends on the material, thickness, profile, span and loading requirements. Use the relevant support tables before building the framework.

How do I fit rafter-supported glazing bar bases?

For the standard aluminium-base, PVC snap-down system:

1. Centre each glazing bar on its supporting rafter.
2. Ensure the base is supported along its full length.
3. Drill through the centre channel of the aluminium base, between the toothed upright legs.
4. Our standard guidance positions the end fixings approximately 75mm from each end, with intermediate fixings no more than 400mm apart.
5. Secure the base to the timber using the specified wood screws.
6. Fit the required gaskets and check that they are seated correctly.
7. Place the polycarbonate sheets carefully onto the prepared bases, maintaining the required engagement and expansion clearances.

Use screw dimensions suitable for the bar and timber. Follow the particular bar manufacturer’s instructions wherever they differ; these details do not apply automatically to self-supporting bars or other glazing systems.

How do I secure the PVC snap-down top caps?

Align the top cap with the aluminium base so its legs enter the matching slots.

Press it into place by hand, working progressively from the high end towards the low end and checking that it engages evenly. Ensure the seals sit correctly against the sheets.

Do not use a rubber mallet. If the cap will not engage, check the sheet thickness, alignment and gasket positions rather than forcing it.

How do polycarbonate sheets expand and contract?

Polycarbonate expands as it warms and contracts as it cools. This is a normal property of the material and needs to be allowed for during installation.

Movement occurs in both length and width. On a typical multiwall roof, the length runs down the slope, in the same direction as the flutes. Because sheets are usually longer than they are wide, the total movement is generally greater along their length.

For our standard rafter-supported glazing arrangement, keep the sheet edges approximately 2–3mm away from the central metal uprights, supported on the rubber gaskets. Separately, provide the required clearance at the sheet ends for movement along the roof slope.

Always follow the specific sheet and glazing-system instructions. A small side gap alone does not provide all the allowance needed for a long sheet.

How do I finish the exposed left and right edges of a multiwall roof?

Use an edge profile compatible with the sheet thickness and glazing system. With a suitable aluminium F-section, the sheet fits between the profile’s prongs. Fix the profile to the rafter or wall as appropriate, retaining the required allowance for movement.

Some snap-down systems use a matching PVC F-section or gable finishing profile with the outer glazing bar and top cap. These profiles finish the roof’s side edge; they are not substitutes for glazing bar end caps.

Browse our F-sections for polycarbonate to check the available options.

Where do plastic glazing bar end caps go?

Fit the matching plastic end cap to the exposed front end of the glazing bar, with a rear cap where required by the roof detail.

Seat it fully and follow the system instructions for any compatible sealant or retaining method.

A glazing bar end cap finishes the bar. A sheet closure fits across the taped open end of a multiwall sheet. Both have different jobs.

Do all multiwall sheets need fixing buttons?

Their use depends on the glazing system, sheet layout and fixing requirements.

Where compatible with the chosen system, we recommend a fixing button near the top and bottom of each sheet, positioned over the upper and lower supporting beams. These provide additional restraint to help prevent the sheet slipping down the roof slope.

Fix into the supporting timber, keeping buttons at least 50mm from cut edges. Use the specified oversized holes and avoid overtightening so the sheets can still expand and contract.

Follow any specific retaining or fixing arrangement required by the system manufacturer. Fixing buttons do not replace proper edge support or a suitable glazing bar system.

How do I install fixing buttons?

Choose a button that matches the thickness of the multiwall sheet.

Our standard fixing-button advice is to:

1. Mark fixing positions over suitable supports.
2. Drill holes approximately 2mm wider than the button shank to allow movement.
3. Position buttons at approximately 300mm intervals and no closer than 50mm from cut edges.
4. Drill between the ribs where possible and remove swarf from the holes and flutes.
5. Fit the foam gasket beneath the button head.
6. Insert the shank through the prepared hole and secure it to the support with the appropriate screw.
7. Tighten without crushing the sheet or excessively compressing the gasket.
8. Push the screw-hole cover into place.

Where specified or appropriate for the fixing, a small amount of polycarbonate-compatible silicone may be applied around the screw top before fitting the cover. It does not replace the gasket or correct tightening.

Follow the sheet manufacturer’s requirements where hole sizes or fixing patterns differ. See our fixing button installation advice for further details.

How far should sheets overhang the final support?

Most installations use 50–100mm. Do not exceed 150mm, and follow any smaller maximum specified for the product.

The edge must discharge into the gutter correctly without leaving an excessive unsupported section.

How many corrugations should overlap?

Follow the instructions for the exact profile and roof conditions. Side overlaps and end overlaps are different details, and requirements can vary.

Use the effective cover width when calculating how many sheets to order.

Can I screw directly through corrugated sheets?

Only use the specified fixing method. Many systems require pre-drilled clearance holes, while some approved fixings create the required clearance during installation.

An ordinary screw driven through without provision for movement may damage the sheet.

Can I install roofing sheets in winter?

Possibly, provided the conditions are safe and within the requirements for the sheets, fixings, tapes and sealants.

Avoid frost, ice, wet bonding surfaces and temperatures outside the stated limits. Handle cold PVC particularly carefully.

Is a hot summer day the best time to install?

Not necessarily. Strong sunlight can overheat materials and affect adhesive application. A dry, calm day with moderate temperatures is generally easier to work in, subject to the product instructions.

Can I fit sheets while they are wet?

Do not apply tapes or sealants to wet surfaces unless the product specifically permits it. Water inside multiwall flutes should be addressed before installation. Rain also increases the risks involved in access and handling.

Can I use any silicone or flashing tape?

No. Use products confirmed as compatible with the roofing material and adjacent surfaces. General-purpose products may be unsuitable.

Check compatibility, application temperatures and curing requirements before use.

Can I walk on polycarbonate or PVC roofing?

No. Never stand or walk directly on the sheets. Impact resistance does not make them a safe working platform.

Why is my roof dripping underneath?

Possible causes include a leaking joint, failed flashing, overflowing gutter or condensation.

Check where and when water appears. Do not assume every drip means the sheet is defective or that adding silicone will solve it. Read our condensation guidance for further information.

Are sheets damaged during storage or installation replaced free of charge?

The customer is responsible for correct storage, handling and installation. The Polycarbonate Store does not provide free replacements or cover costs arising from damage caused by you or your installer, such as unsuitable storage, incorrect cutting, incompatible products or overtightened fixings.

This is separate from a genuine supply discrepancy, defect or misdescription, which should be reported promptly for assessment. Where replacement is the appropriate remedy for a supply issue for which we are responsible, it will be provided without charge.

Your statutory rights are unaffected. Please refer to our Terms and Conditions for further information.