Contrary to common belief, roofing tiles are not “blown off” a roof. The wind creates an upward force as it travels over the roof, causing the tiles to be sucked upwards. It is important to understand this subtle distinction in order to build a more secure roofing system. Securing tiles against upward lift is the key to keeping the tiles on the roof.
As the wind travels over the roof area, it causes the air pressure on top of the roofing to decrease. At the same time the pressure underneath the roof increases. The increase in pressure inside the loft space results in a positive pressure on the roof tiles. The negative pressure above causes the tiles to be sucked upwards, lifting the tail of the tile. The lower the pitch of the roofing, the higher the forces will be.
If the tiles are being sucked upwards rather than blown, the old method of roof tile fixing may not be the best. Traditionally roof tiles are nailed or pegged at the head or top of the tile. This secures the tile against the force of gravity but has little effect on upward tile lift. In fact the only thing securing the bottom of the tile is the tiles own weight.
Tests in the United Kingdom have proved that a roofing tile clipped or secured at the bottom can withstand 5 times the force of a head nailed roof tile. Tiles can be clipped or wired to ensure the tail is secured against wind uplift. This also stops the annoying rattle or chatter sometimes associated with wind swept roofs. The chatter occurs as tiles are lifted and dropped by the wind vortex. I prefer to use a bead of silicone between the tile courses to act as a second line of defence against tile chatter.
It is important to note here that modern roofing tile clips must be installed correctly if they are to have any effect. Tile clips are usually nailed in close proximity to the interlocking channel. Because of the risk of damaging the tile, there is a tendency to move the nail too far away from the interlock. The clip must be positioned at right angles close to the tile. The roofing clip should be securely bedded to prevent the tile working loose. In my opinion, the old system of wiring the bottom of the tiles is superior to modern clipping. The wire fixes to an eyelet on the bottom of the tile and in between tile courses onto the supporting baton. This has the effect of clamping down the tile. I have re-roofed wired roofs that are over 50 years old and the wire is still clamped tight.
The most important roofing component when it comes to preventing wind damage is the roofing felt. Roofing felt or sarking is placed under the batons before the tiles are fixed. The primary function of all roofing felt is to prevent wind damage. The waterproofing of the paper is far less important than its ability as a wind barrier. The sarking prevents the increase in pressure inside the loft, thus preventing wind uplift. It must be fixed securely with 150mm overlaps. An extra baton should be placed at the point where the sheets overlap to prevent wind travelling between sheets. If the felt is not correctly secured the wind force will be focused on the weak spot causing more problems.
All ridge and hip tiles need to be mechanically fixed in areas prone to high winds. Roofing mortar alone should not be relied upon to hold these tiles secure. Once the bond between tile and mortar is broken the tile can work loose. They should be nailed, clipped or wired.
The areas most lightly to be submitted to wind uplift are the eaves, verges and ridges. In these areas it is prudent to take extra precautions with your tile fixing. Nail and clip all tiles around the periphery.
A little extra time taken at the installation stage will ensure the roofing tiles stay secure in heavy weather. The extra cost is outweighed by the savings in maintenance and piece of mind.