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How To Fit Polycarbonate Sheeting To A Timber Roof Structure

Diagram A

Our polycarbonate sheeting can be fitted vertically, for the sides of a greenhouse or porch, or even a partially enclosed carport. More often however, it is used on a roof with a gentle slope to permit the easy run-off of rainwater into guttering. The minimum slope should be about 10 degrees.

The sheets have a natural tendancy to expand and contract as temperature changes take place and for this reason they are fitted into special glazing bars down the long edges that are specially designed for this movement. The aluminium glazing bars are screwed to timber supports(joists) at a maximum of 1050mm centres for 16mm sheet thickness and a maximum of 700mm centres for 10mm sheet thickness.

The deduction of 20mm for the centres dimension allows for fitting into the glazing bars and sideways expansion. Once laid in the glazing bars the sheets are held in place with a snap down pvc top capping cover strip - diagram A.

More often than not, sheets will be cut to width as described later but the 20mm reduction in width between the glazing bar centres must always be taken into account - Diagram B.

Diagram B

Effective supports-(purlins)-across the sheet width should be at maximum 1500mm centres so roofs constructed with 3m long sheets would only require one purlin with 4m sheets requiring two.

In general it can be said that mechanical fixings are not required for fixing roofing sheets when glazing bars are used. However, in view of extreme weather conditions which have prevailed in recent years, you may need to consider that some mechanical fixings are used for extra security. These are limited to one button fixing at the bottom of each sheet in the centre and fixed to the front fascia, and one to each purlin, again in the sheet centre.

The starter hole for button fixings should be made in the centre of the sheet cavity rather than through a wall(if possible) and it will need a 16mm diameter hole.(Hole saws available) This is larger than the actual button size but allows for expansion. When making the fixing ensure that the foam washer is in place under the button. Drive in the screw and fit the cover firmly without over tightening as this could lead to sheet distortion. Diagram C.

Diagram C.

Cutting & Preparing Polycarbonate Sheeting
Polycarbonate, despite its strength can be cut using a fine tooth panel saw. When cutting to width, you should endeavour to cut just to the outside of one of the walls as in diagram D.

Diagram D.

When supplied, the sheets have protective polythene film coverings on each side. Do not remove the polythene films until the sheets are cut and ready to fit. The side of the sheet with the printed polythene MUST be fitted to face upwards. If the polythene is mistakenly removed too early and you do not know which should be the top, look carefully at the end of the sheet. The cavities are very slightly shaped as in diagram E and the shaped skin is the one with the special built-on long life protective layer which must be uppermost. Both ends of the sheet should be supplied sealed to prevent the entry of dust and dirt during transit.

Diagram E

When cutting to length the 'top' end should be taped with aluminium foil or breather tape so that sheet has a permanently sealed top end. The bottom cut end of the sheet should be fitted with our anti-dust breather tape - available in 33m rolls-and then a pvc end closure- diagram F.

Diagram F

Cutting will inevitably create swarf in the cavities and this must be removed. This is best done with a suction type vacuum cleaner, but if this proves ineffective, remove the top sealing tape and blow through the flutes with a pressure air line. Reseal the top and bottom end. Every care should be taken to avoid getting moisture in the flutes. Do not, for example, leave sheets with open edges lying on damp grass. Store them in a dry place. Under no circumstances try to remove dust or swarf with water. After fitting the sheets the end closure is fitted between the glazing bars. This is done as in diagram F with a fine line of silicone used to secure the end closure to the sheet. Small holes should be drilled in the bottom of the beads at 500mm centres to allow any trapped moisture to escape. Screw fix the end caps supplied with the glazing bar to conceal the ends of the aluminium glazing bars between the bottom cover strips for a neat finish-diagram G.

Diagram G.

Wall End Fixings
On a sloping roof butting up to a house wall the joint between the wall and the polycarbonate is sealed using flashing which runs along the house wall where it is cut into a mortar joint normally, and at the bottom end, overlaps the polycarbonate. At the end of a lean-to conservatory, the outside edge of the glazing is covered by a special 'F' profile which also protects the top of the gable end cladding. The 'F' section is fitted on the edge of the sheet - Diagram H.

Diagram H.

If an 'F' profile is fitted to a wallplate running down the house wall the same special 'F' profile is used to cover the joint adjacent to the wall. Once the 'F' profile is fitted, flashing is run down the wall and overlaps the 'F' profile.

Sealants & Flashing.
Although our polycarbonate sheet is extremely tough and herd wearing, as with other thermoplastic materials, it can be adversly affected by chemical agents. Care must therefore be taken when choosing items such as sealants and flashing that will come into contact with the sheet.