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Timber Supported Roof Guide

What you need to build a Timber Supported Polycarbonate Roof

Whether you are looking to build a rafter supported roof on a Lean To Conservatory, Car port or Canopy the following guide will help you to choose the best products for the job.

Once you are ready for pricing or to buy online please go to our Rafter supported roofing pages.

Timber carports can look stylish and are very effective in keeping the leaves and other garden debris off your car. They can help shelter you from the rain while you get in and out of your car and in the summer can help to reduce heat build-up with the right choice of polycarbonate.

Timber Canopy roofs can be a cost effective and easy to build alternative to a purpose built aluminium canopy.

Once you have your timber framework in place, completing your roof is simple. On the diagram below the following letters represent:

A. Your timber wall plate and
B. Your eaves beam

You now need to choose elements from each of the following that best suit your project:

C. Polycarbonate sheets.

Multi-wall polycarbonate is the most popular choice for carports and canopies. There are many thicknesses and colours available, all of which are suitable but some are more commonly used. As the sheets get thicker they become more rigid and can achieve greater spans between glazing bars.

Polycarbonate thickness - The most popular thickness of sheet for a car port would undoubtedly be 10mm and for a canopy it would be 16mm. 10mm is a very strong, lightweight sheet which is shatterproof and offers an UV protection layer (extruded with the sheet) as does all multi-wall polycarbonate. Many would argue that this 10mm twin-wall polycarbonate is as thick a sheet as you need for a carport or a canopy, and as they are both open sided and exposed it is pointless going for a thicker, more expensive sheet. Certainly insulation doesn’t need to be factored in for these very reasons. The only real advantages of going up to 16mm or 25mm would be to achieve greater spans between rafters and to reduce noise on rainy days. 16mm can generally span up to 900mm centres and 25mm up to 1m. 10mm polycarbonate is noisier when it rains than thicker sheets. As a canopy is generally attached to your home and often outside of a room, noise may be an issue so you may want to go for a thicker sheet.

If you are roofing a conservatory you will probably want more thermal insulation. In unheated smaller conservatories 16mm is sometimes used however, if you want to keep the heat in in the winter and out in the summer, you would want to go for 25mm or even 35mm. The specifications for each can be found in our brochures area of the site.

Polycarbonate colour - In terms of colour, clear lets in the most light, then opal and then bronze. Opal is very popular as it helps to obscure any dirt or debris that can fall onto the roof over time. Bronze is better for prevention of heat build-up and aesthetically ties in well with a timber frame.

There are also options for Heatguard or Bronze Opal which are available from 25mm upwards which have a dual colour for exceptional thermal performance.

D. Rafter Glazing bars.

There are two main types of glazing bars for polycarbonate that are designed for use with wooden rafters. You start and end your roof with a glazing bar and should therefore always have one more glazing bar than polycarbonate sheets. These are available either as aluminium capped screw down bars or PVC capped snap down ones. They are both available in white, brown or grey as standard but the aluminium ones offer the additional benefit of being available in mill finish allowing you have them coated in another colour.

Aluminium capped rafter bars

The strongest and neatest solution is an aluminium capped rafter bar of which The Glazing Shop has bars in widths of either 50mm or 60mm to suit the width of your rafters. Aluminium capped bars screw right through the cap down into the rafter giving a very secure fixing. You then clip in the 'c' section at the top to cover the screws. These are however more expensive than their PVC capped alternatives and for this reason are mostly used in conservatory roofs, high end canopies or roofs in coastal or exposed locations. These bars suit all thicknesses of polycarbonate and are also suitable for glass. To create an end bar simply add an aluminium F section to the ouside of the bar. These are available in white and brown and in anthracite grey to match your bars. They will fill the ouside of the bars, balancing them up and neatly cloaking off the ends of your roof.

PVC capped rafter bars

These are cost effective and versatile and are known as ‘knock down’ or ‘snap down’ bars. The aluminium base bar screws down into your rafter then the PVC top cap is 'snapped' into place over the sheet.

The Glazing Shop offers two types of PVC capped bars:

PVC capped rafter bars - 10mm, 16mm & 25mm

The standard bar PVC capping has dedicated lock in points for 10mm, 16mm & 25mm sheet thicknesses.

PVC capped rafter bars - 25mm, 32mm & 35mm

This glazing bar has locking points for 25mm, 32mm & 35mm sheets. It gives a stronger fix for 25mm so is better for windy locations. 

For standard carports in a non-exposed locations, these are our best sellers.

PVC capped rafter bars are not suitable for glass

For both versions of bar you will need end caps. The PVC capped bars have one end cap included with each bar; the aluminium capped ones need end caps to be purchased separately.

An end bar is easily created from the standard bar by adding a Universal Edge Trim (PVC F section.) Simply leave the gasket out of the outside of your aluminium base bar. Click the prongs of the edge trim in its place. This will balance up the outside of the bar and neatly cloak off the edge of your roof.

E. Sheet Closures.

These are used to close off the ends of the polycarbonate sheets at the gutter end. Designed to stop water and dirt / insects from getting into the flutes of the sheets, they also neatly finish off the front edge of the roof.

The Glazing shop professional sheet closure has drainage holes to increase breathability and prevent condensation forming in the sheet. It also has a co-extruded upper lip which provides an improved waterproof barrier.

F. Roof Edging.

As explained above, to achieve the best and most long lasting finish you would start off and finish your roof with glazing bars. You then have a choice of how to finish off the left and right hand edges of your roof, effectively making those bars into end bars.

The edging sections are available in powder coated aluminium or PVC. 

The Aluminium 'F' sections

These sections are placed in the outside of the bars at the left and right edges of your roof, this balances the bars as the sheet is held on the inside of the bar. The downwards (longest) leg of the 'F' acts as a smooth flat face to neatly cloak of the edge of the roof. It can also be used with the leg facing upwards so you can flash over it when against a wall. 

Sometimes to save money aluminium F sections are used instead of the end glazing bars.

The PVC 'F' sections

Universal Edge Trims are placed in the outside of the PVC capped glazing bars at the left and right edges of your roof to create end bars. These balance the ouside of the bars as the sheet is held on the inside of them. The downwards (longest) leg of the 'F' acts as a smooth flat face to neatly cloak of the edge of the roof. You would leave the gasket out of the outside of the bar so the two pips in the bottom on the edge trim can clip into the bar in its place.

These sections are not designed to be used on their own without a bar as they will not be effective against wind uplift. 


In summary

For a Conservatory you would probably choose the following:
5mm or 35mm polycarbonate sheets
Aluminium Capped rafter bars & bar end caps
Professional Sheet Closures
Aluminium F sections


For a Budget carport you would probably choose the following:
10mm polycarbonate sheets
PVC capped rafter bars & PVC bar end caps
Standard PVC Sheet Closures
PVC universal edge trims

A canopy would fall somewhere between the two depending on your rafter spans and the finish you are trying to achieve.

Ready to order?
You will find all the parts you need in our Rafter supported roofing pages.

Still need advice?
This is by no means and exhaustive guide and here at the Glazing shop we aim to provide as much information as possible for you to make informed decisions about the parts of your rafter supported roof. If however, you are still asking yourself how do I do this? How wide can the centres be between my rafter? What deductions do I need to make for my chosen glazing bar? Or any other timber roof questions please call us at The Glazing Shop on 01536 446395 and we will be very happy to help.