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Mix Roofing Cement


How to Mix Roofing Cement.

How to mix roof mortar or roofing cement. A guide to mixing and applying roofing cement for a durable crack free finish.  




In recent years, the roofing on a lot of new build housing has suffered from tile loss due to wind damage. Ridge and hip tiles have been removed from the roof during storms. This may seem unsurprising. What is unusual is that older adjacent houses suffered less wind damage to the roofing. Correspondingly, roofing built within the last 10 years is more prone to cracking and loosing bond with the tiles. The roofing cement simply fails. So what is going wrong and how can we avoid it.

Traditional Roofing Mortar

The biggest difference between today’s roofing cement and traditional mortar is the mixture. Today we use Portland cement with sand aggregate. The roofing mortar on older houses consists of two parts sand or aggregate to 1 part hydrated lime. To reinforce the roofing mortar horse hair was added.

Modern Roofing Cement

The compressive strength of Portland cement is far greater than the mortar of the past. Moreover, it has greater bonding qualities and is easier to trowel. Mixed and applied properly it provides a far superior roof mortar. The majority of mortar failure is due to incorrect application.

Problems with Modern Roofing Cement

Before the mid nineties it was good practice to use four parts sand to one part cement. The British building code of practice for slating and tiling now recommends the use of 3 parts sand to 1 part cement. This provides more durable roofing cement with superior bonding qualities. It also shortens the curing time which is where the problems arise. The increase in cement speeds up hydration, the process which allows cement to harden. If the curing time is not controlled the cement simply dries out too quickly. This results in cracks and poor adhesion to the roof tiles. Below you will find some proven methods for achieving the perfect mix.

A good day to lay Roofing Cement

You will need a dry day. Overcast and not too hot is perfect. Do not lay roofing cement if there is a likelihood of frost. The moisture inside the cement expands on freezing and blisters the roof mortar. If you get caught out by rain or frost you will need to cover your work.

Mixing good quality Roof Cement

Use quality materials. That means quality Portland cement and graded filtered sand. Use clean water and as little additives as possible. Modern Portland cement already has plasticizers incorporated so there is no need to add any. Under no circumstances use diesel, petrol, washing up liquid or any other improvised plasticizer. They don’t work and will only ruin the mix.

Mix 1 part cement to three parts sand.

Don’t guess the quantities, measure them out accurately before you start. Make sure that the sand and cement are thoroughly mixed dry before you gradually add the water to achieve the correct consistency. It needs to be wet enough to achieve hydration without causing too much “slump” when laid. If you were to lay a ridge tile on top of the mortar, it should sink into the bed about 5mm without causing the roofing cement to spread excessively. Too wet and it will be difficult to point, too dry and it will not bond to the roof. Additives such as Iron oxide or cement dye should be added in accordance with the manufacturer’s instructions. Use the exact amount of dye for each new mix or the ridge mortar will look patchy.

Soak the roofing tiles

In order to provide a good bond between cement and tile, it is important to stop excessive moisture suction from the substrate. If the tiles are laid dry, they will suck moisture away from the cement impeding hydration. So it is a good idea to soak the ridge/hip tiles in water before they are laid. This will help to stop the mortar drying out too quickly and will prevent cracking.

Slow down the curing time

The number one reason modern roofing cement fails is because it is allowed to dry/cure too quickly. The mix dries out before a sufficient bond is achieved. It is essential to ensure that the curing time is controlled. This is even more important during periods of warm weather. Once you have finished pointing the roofing cement, cover it with damp Hessian or old wet carpet cut into metre strips. This will help retain moisture and prolong the curing time. This covering will also prevent “wash outs” in the event of rain. Leave the Hessian laid on top of the ridge until the end of the day, or overnight if possible.

All this may seem like a lot of extra effort. However, if you use a good mix and curb the curing time, the roof mortar will be well bonded, crack free and last in excess of thirty years.