Why do some modern construction roofs suffer ridge loss in high winds while older adjacent roofs hold up? Similarly, roofs built in the last 10 years are experiencing more cracks in the roofing mortar. What can we learn from the roofing practices of the past?
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 made up of 1 part lime to 2 parts sand/aggregate mix. Horsehair was added to reinforce the ridge bed.
portland cement
The introduction of Portland cement virtually eliminated the use of lime mortar in roofing situations. Its compressive strength and faster cure time make it ideal for roofing applications. Portland cement is much easier to mix and, with the added plasticizers, it is easier to trowel.
Until the mid-1990s, it was common practice to include 4 parts cement to 1 part sand/aggregate. Recent recommendations advise increasing the cement to 3:1 to strengthen the mortar and promote a secure bond.
The increase in cement ratios could be the reason why modern roof shoring suffers from cracks and poor adhesion. Increasing the cement content leads to faster cure or hydration time. The mixture just dries up too quickly.
decelerate
it is important to ensure that curing time is controlled, especially during periods of hot weather. I use soaked burlap on the roof to slow down the process. Elastic cords are attached to the edges of the burlap. The ropes hook under the shingles to keep them secure and away from staking. The Hessian will also protect against “washouts” should the weather change. When the weather is nice, I constantly mist the burlap to keep it moist.
To promote proper bonding it is necessary to stop too much suction from the substrate. If the shingles and ridge are too dry, they will absorb moisture from the mix before a sufficient bond is formed. Whenever possible, ridge shingles should be primed. Soak them in a large tub for an hour or so and moisten the top row of shingles. If this is not possible, a mixture of 1 part PVA to 3 parts water should be applied to the joint area. This will prevent the tiles from drawing too much moisture from the mortar.
Aiming for the roof valley
If you’ve tried to target a valley for roofing on a hot day, you know the problems associated with sinking concrete. You return to a beautiful jagged valley to find that the mortar has collapsed leaving a gap between the tile and the 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 beef up the mix. You can use stainless steel products used in the plastering industry. This strengthens the valley and prevents the mix from collapsing. I placed a piece of L-shaped stainless steel corner bead on each side of the roof valley. It is placed just behind the valley face free of roofing shakes and under shingles. Do not pierce the valley with your fastener.
By using the correct mix and controlling the curing time, your mortar will remain safe and durable for many years.