The most obvious difference between today’s roofing mortar and that used in the past is the mix. Traditionally lime was added to the mortar. A traditional lime mortar mix comprised of 1 part lime to 2 parts sand/aggregate mix. Horse hair was added to reinforce the ridge bedding.
The introduction of Portland cement has virtually eliminated the use of lime mortar in roofing situations. Its compressive strength and faster curing time make it ideal for roofing applications. Portland cement is a lot easier to mix and with added Plasticizers easier to trowel.
Up to the mid nineties it was common practice to include 4 parts cement to 1 sand/aggregate. Recent recommendations advise increasing the cement to 3:1 in order to strengthen the mortar and promote a secure bond.
The increase in cement proportions could be the reason modern roofing pointing is suffering from cracking and poor bonding. Increased cement content leads to a faster curing time or hydration. The mix simply dries out too quickly.
it is important to ensure that the curing time is controlled especially during periods of warm weather. I use soaked Hessian draped over the roofing to slow the process down. Bungee cords are attached to the edges of the Hessian. The cords hook under the roofing tiles to keep it secure and away from the pointing. The Hessian will also protect against “wash outs” should the weather change. In warmer weather I constantly spray the Hessian to keep it damp.
To promote an adequate bond it is necessary to stop too much suction from the substrate. If the roofing tiles and ridge are too dry they will suck the moisture from the mix before a sufficient bond is made. Where possible the ridge tiles should be primed. Soak them in a large tub for an hour or so and wet down the top course of roofing tiles. If this is not possible a mixture of one part PVA to 3 parts water should be applied to the bonding area. This will prevent the roofing tiles from drawing too much moisture away from the mortar.
Pointing a roofing valley
If you have tried pointing a roofing valley on a hot day you will know the problems associated with cement slumping. You return to a beautifully pointed valley to find the mortar has slumped leaving a gap between tile and mortar. A drier roofing mix is often used in an attempt to alleviate the problem. This is not advisable because the cement mix requires sufficient moisture for successful hydration.
Take a tip from “back in the day” and reinforce the mix. You can use stainless steel products used in the plastering industry. This strengthens the valley and stops the mix slumping. I lay a length of L shaped stainless corner bead each side of the roofing valley. It’s fitted just behind the face of the valley clear of roofing batons and underneath the roofing tiles. Do not hole the valley with your fixing.
If you use the correct mix and control the curing time your mortar will remain secure and durable for many years to come.