When planning a polycarbonate roof, getting the correct sheet dimensions is essential. One of the most common questions we receive is:
“What size should my polycarbonate sheets be if I am using glazing bars?”
The answer isn’t simply to measure the gap between your rafters and order sheets to that exact width. There are several factors that need to be considered, including:
- The thickness of the polycarbonate
- The maximum recommended sheet span
- The number of sheets required
- The number of glazing bars
- The glazing-bar deduction
- Thermal expansion and contraction
- Cutting tolerances
- The actual dimensions of your timber structure
In this guide, we’ll explain how we calculate sheet widths at The Polycarbonate Store, including a worked example.
Why do you need to allow for glazing bars?
When polycarbonate sheets are installed using rafter supported glazing bars, the glazing bars sit over the supporting rafters and join the sheets together. The glazing bar occupies some of the space between the sheets.
Therefore, if you are calculating the required sheet widths across the total width of a roof, you need to take the glazing bars into account.
For our glazing-bar systems, we allow a 15mm deduction for each glazing bar when calculating the total width available for the polycarbonate sheets. This is important: The 15mm deduction is per glazing bar — not 15mm per side.
Because the glazing bar is positioned between two sheets, the 15mm allowance effectively works out at approximately 7.5mm on either side of the glazing bar.
Our glazing bar profile is approximately 50mm overall, with a 45mm aluminium base.
The important thing: don’t start with the glazing-bar deduction
A common mistake is to start with the roof width and immediately subtract 15mm. That isn’t how we calculate the sheet sizes.
First, we determine how many sheets are required based on the maximum recommended span for the particular polycarbonate thickness. Only once we know the number of sheets do we know how many glazing bars are required. We can then calculate the glazing-bar deductions. This gives a much more accurate result.
Step 1: Determine the maximum sheet span
Polycarbonate sheets need adequate support. If the unsupported span between rafters is too large, the sheets will sag / bow.
Our recommended maximum spans are:
| Polycarbonate | Maximum recommended span |
|---|---|
| 10mm Twinwall | 700mm |
| 16mm Multiwall | 800mm |
| 25mm Multiwall | 900mm |
| 32mm / 35mm Multiwall | 1000mm |
These are the maximum spans we recommend without additional supports or appropriate accessories.
Step 2: Work out how many sheets you need
Let’s use a practical example.
Roof width: 4000mm | Roof projection: 3000mm | Polycarbonate: 25mm Multiwall
For 25mm Multiwall, our maximum recommended sheet span is 900mm.
You cannot use 4.44 sheets, so you round this up to 5 sheets.
Therefore: 5 sheets are required.
Step 3: Calculate the rafter centers
Now that we know we need 5 sheets, we can calculate the approximate spacing between the rafters.
4000mm ÷ 5 = 800mm
Therefore, the rafters would be approximately: 800mm center-to-center
This keeps the sheet span comfortably below the 900mm maximum recommended span for 25mm Multiwall.
Step 4: Calculate the number of glazing bars
If you have 5 sheets, you need a glazing bar between each sheet.
Therefore: 5 sheets = 4 glazing bars
Step 5: Calculate the glazing bar deductions
We have: 4 glazing bars × 15mm = 60mm
Therefore, across a 4000mm wide roof:
4000mm − 60mm = 3940mm
This is the total width available for the 5 polycarbonate sheets after allowing for the glazing bar deductions.
Now divide this by the number of sheets: 3940mm ÷ 5 = 788mm
Therefore, our recommendation would be: 5 sheets @ 788mm × 3000mm
This is the same calculation shown in our Sheet Recommendations guide.
The complete calculation
For our example:
Roof width: 4000mm
Number of sheets: 5
Number of glazing bars: 4
Glazing-bar deduction: 4 × 15mm = 60mm
Width remaining: 4000 − 60 = 3940mm
Sheet width: 3940 ÷ 5 = 788mm
Recommended sheet size: 788mm × 3000mm
Why are the sheets not simply 800mm wide?
This is an important point.
You may have noticed that the rafter centers work out at 800mm, but the recommended sheet width is only 788mm.
That’s because the glazing bars occupy space between the sheets. The 15mm deduction for each glazing bar accounts for this.
So, 800mm rafter centres does not automatically mean 800mm sheet width.
Instead, the total roof width, number of sheets and number of glazing bars are considered together.
Understanding the 15mm deduction
Our glazing bar diagram shows the relevant dimensions.
The glazing bar is approximately:
- 50mm overall width
- 45mm aluminium base
- 15mm deduction used in our sheet width calculations
The 15mm deduction is effectively shared between the two sheets either side of the glazing bar.
Therefore, it works out at approximately: 7.5mm either side
This means the polycarbonate should not be forced tightly against the central metal section of the glazing bar.
There needs to be sufficient room for the sheet to engage correctly with the glazing system and accommodate natural movement.
Polycarbonate expands and contracts
Polycarbonate is a thermoplastic material, which means it naturally expands and contracts as its temperature changes.
A roof exposed to direct sunlight can experience significant temperature changes during its lifetime.
Consequently, the sheets must be installed in a way that allows for this natural thermal movement.
As a general installation consideration, approximately 3mm of movement allowance may be required depending on the particular application and sheet dimensions. However, it is important not to confuse this with the 15mm glazing bar deduction.
These are two different considerations.
The 15mm deduction is part of our calculation for determining the sheet widths between glazing bars.
The expansion allowance ensures that the polycarbonate isn’t installed so tightly that normal thermal movement is restricted.
Always follow the installation recommendations applicable to the specific polycarbonate and glazing bar system being installed.
Don’t force the sheets into the glazing bars
When installing your polycarbonate, the sheets should not be forced tightly into position.
In particular, you should avoid pushing the sheet hard against the central metal prong/upstand of the glazing bar. The sheet needs to be able to move naturally as temperatures change.
Trying to force an oversized sheet into the glazing bar is not a substitute for correctly calculating the sheet dimensions. If your measurements don’t work, stop and check the calculation rather than forcing the material into place.
What about cutting tolerances?
There is another important consideration when ordering cut-to-size polycarbonate: Cutting is not guaranteed to be accurate to the exact millimetre.
There is an industry-standard cutting tolerance of approximately: + / – 5mm
This means that if you order a sheet to a particular dimension, you should allow for the possibility that the finished sheet may be approximately 5mm larger or 5mm smaller than the requested measurement.
For example, a sheet ordered at 612mm could potentially arrive approximately within 607mm–617mm depending on the applicable manufacturing/cutting tolerance.
This is particularly important when sheets are being installed into glazing bars where the available space is limited.
Why we recommend ordering more material if you are unsure
If you are unsure about your measurements, we strongly recommend allowing yourself some additional material wherever possible.
This is particularly important when ordering sheets for a bespoke project. Why?
Because there is a big difference between ordering a sheet that is slightly too long or wide and ordering one that is too short or narrow.
Material that is slightly oversized can be trimmed.
Material that is too small cannot be made bigger.
For example, if you order a sheet 20mm too long, it can be trimmed to the required dimension before installation. If you order it 20mm too short, there is no way to add that missing material back. For this reason:
If you are unsure, it is always better to have additional material that can be trimmed than to order material that is too short and unusable.
This is especially relevant where you are measuring an existing timber structure that may not be perfectly square or where you are working from approximate measurements.
Measure the actual structure
Don’t rely solely on the dimensions from the original construction drawings.
Before ordering, we recommend physically measuring the structure.
Check:
- Overall roof width
- Overall roof projection
- Rafter c/c measurements
- Rafter width
- Roof pitch
- Whether the structure is square
- Whether the rafters are evenly spaced
- Any walls, windows or other obstructions
- Where the glazing bars will finish
A few millimetres can make a significant difference when installing bespoke-cut sheets.
Exitex and Sunwood glazing bars
At The Polycarbonate Store, we supply Exitex and Sunwood glazing bar systems.
Both are designed to provide a practical method of securing polycarbonate sheets over supporting rafters, but the exact components and installation details can vary between systems.
Exitex
Exitex glazing bars are available in systems designed for different polycarbonate thicknesses and are available in various colours.
Sunwood
Sunwood also offers snapdown rafter glazing bar systems suitable for different polycarbonate thicknesses.
Always ensure that the glazing bar you select is compatible with the thickness and type of polycarbonate you are using.
We cannot provide technical dimensions or installation recommendations for glazing bars supplied by other manufacturers that are not part of our range.
What size should my glazing bars be?
Our glazing bar profile is approximately 50mm overall width, as illustrated in our diagram. The aluminium base is approximately 45mm wide.
When designing your timber structure, we recommend allowing for a suitable supporting rafter width so that the glazing bar is fully supported.
The glazing bar should be fixed to the supporting rafter and should not be treated as a replacement for the structural timber beneath it.
Need help calculating your polycarbonate sheets?
If you’re unsure about your measurements, don’t guess. Send us your roof dimensions, photographs or drawings, along with:
- Polycarbonate thickness
- Roof width
- Roof projection
- Rafter measurements
- Rafter width
- Roof pitch
- Preferred glazing bar system
Our team can then help you work through the calculation and determine the appropriate sheet sizes.
You can also use our Sheet Recommendations guide for a worked example of how the calculations are carried out.
Email us: [email protected]
IMPORTANT – CHECK YOUR MEASUREMENTS BEFORE ORDERING
Bespoke-cut polycarbonate is manufactured to the dimensions requested, subject to the applicable +/- 5mm cutting tolerance.
If you are unsure of your measurements, we strongly recommend allowing additional material wherever possible. A sheet that is slightly too large can be trimmed on site; a sheet that is too small cannot be made larger and may be unusable.
Always check your measurements, glazing bar requirements and manufacturer’s installation recommendations before ordering.
Leave a Reply