Home Contractor's CornerAn Inspector Calls The art of the pitch: keep the tiles on and water out!

The art of the pitch: keep the tiles on and water out!

by Matt Downs

This month the Inspector explains why the pitch of your roof should be perfect.

Pitch; the angle of your roof. It is one of, if not the most, important measurement when designing, specifying and installing your roof. Everything we do on the roof is generally designed to achieve two major outcomes. We want to keep the tiles on and the water out. That is our goal: we want to do it safely and, while we want the end product to look good, tiles on and water out is what we are there to achieve.

Pitch effect

Let’s consider the effect of pitch on these two outcomes. Firstly, and most simply, keeping the tiles on. At a low pitch the weight of the tile will help keep it in place, whereas when the pitch is increased the deadweight resistance is decreased as the pull of gravity encourages it to slide down the roof and the tile is forced to hang from its nibs supported on the batten. Most tiles will hang at a 90º pitch (i.e. vertical) but stronger fixings maybe required to hold those tiles in place.

Now let us consider keeping the water out. Tiles and slates are laid in a way that controls the flow of water down the roof and prevents water from leaking through the junctions between the tiles. Many roof tiles have interlocks formed on their left edge which are essentially water gutters designed to aid this process.

The pitch of the roof will control the speed at which the water travels down its surface. At a high pitch, the water will travel quickly and care must be taken to ensure that the guttering at the eaves can deal with higher volumes of water. At a low pitch, the water will move more slowly, giving it time to move sideways via capillary action across the tile, perhaps aided by the wind. It can flood interlocks and seep between tiles or slates commonly on longer rafter lengths particularly with flat interlocking tiles. In extreme cases wind driven rain can be blown back up under the headlap of the tile or slate causing excessive water ingress beneath the roof covering.

Special care must be taken when dealing with double lap products such as plain tiles and natural slates. These materials have no water channel formed into their surface. They do not interlock. Instead they are laid in a staggered formation we call broken bond, with alternate courses shifted over by half a tile’s width. This means that water falling between the tiles lands in the centre of the tile beneath. To ensure weathertightness the tiles must be laid double lap meaning that one tile covers the next two tiles beneath, rather than just one tile over one, or single lap, which is how an interlocking tile is laid.

The double lapping and staggering of tiles and slates is effective, but only to a point. At lower pitches (below 35º for plain tile and 25º for natural slate) the water will flood the outer roof covering as it is moving too slowly down the roof.

Every roof covering will operate at a minimum and maximum pitch and should never be used outside those recommendations. We want to keep the tiles on and the water out and while we might like the look of our natural slate extension laid at a 15º pitch, we will not like the cost of the repairs when the slates leak and the roof is in need of a complete re-roof.

Tiles on water out!

There are many tiles out there designed to work at a low pitch, some as low as 12.5º so you do have options. Always check your roof pitch and always follow your manufacturer’s recommendations for their tiles or slates. Remember, tiles on and water out!


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