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5 Most Common Mistakes When Installing a Slate Roof 

Any construction work can be a hassle.

From not being able to finish it on time to making various mistakes along the way, there are numerous things that can go wrong. The same goes for slate roof installation.

If you have no experience with roofs, perhaps it’s best if you hire a reputable roofing company to do this work for you. However, that doesn’t mean you should necessarily shy away from this kind of work; everything is possible with little time and a goodwill!

Here are some things you need to keep your eyes on during slate roof installation.

  1. Not being able to install the starter course face up

This is probably the first thing to check when installing a slate roof. At the same time, it is something that can easily tell you whether a person is experienced or not. Basically, if you mess things up on this very first step, don’t bother with the rest. Even if you decided to hire a roofer, this is something you need to be able to check. When it comes to starter course slate, it is the only part that should be placed face down while all other slates should be positioned faced up. By doing this, you are able to expose its edges which will, later on, allow you to merge the drip edge of the first course with the flush. However, keep in mind that this is mostly an aesthetic decision. Placing it another way around will not have an impact on the roof or its longevity but is still regarded as a big mistake. Another thing you need to consider is ordering field slates and starter slates separately. This will help you out as well. Most suppliers will have starter slates that have holes on the top side. These holes are used for nails. By doing this, slate can be positioned face down.

  1. Not having enough headlap on the starter course

One big issue occurs when you turn field slates on their side. There is a chance that there will not be enough adjustment allowing a proper headlap. This problem may appear on the first-course exposure and will be really troublesome. Always keep in mind that the regular height of the starter slate is 3 inches or more in comparison to the vertical exposure of the first course of slate. Fortunately, there are certain things you can do to remedy the situation. The best approach is to improve the headlap spacing by dropping the chalk line for the top of the second course. This is precisely the difference between experienced and inexperienced roofers as inexperienced people will not know about this little trick. It is very important for the healap to be right as this will allow for more water to run over it. Given that we are talking about the bottom headlap; this is where most of the rainwater will go over. These bottom slates should always have more headlap compared to the field slates.

  1. You might be missing a shim strip or a proper cant

Needless to say, you cannot do anything without a proper cant or a shim strip. You have to remember that your starter course needs to have the same angle as the other slates. They all have to be properly aligned. Slates will never be placed flat on the roof gave the fact that they are placed over each other giving them a certain angle. However, starter course doesn’t have anything beneath it. This poses a problem because then, the starter course will not have a proper angle. Usually, roofers will fix this issue by using a shim strip or a cant. They would be placed below the bottom edge of the course. Both cant and strip are made from wood which is, for esthetic and functional purposes, the same type of wood used for the rest of the roof. Besides these two parts, you can also rely on a fascia board to change the angle of the starter course. When using a metal drip edge, bear in mind that you can form it with an inverted V groove. The result will be very similar to that provided by a cant or a shim. Basically, although there are some other solutions to this problem, I would still recommend cant. Without it, slates will not be properly placed and there will be a gaping hole between the slates.

  1. Not installing proper side laps

Side laps can also cause you some issues. These side laps should be at least 3 inches wide. You can find them between side butts and the first course of slates. If a person doesn’t know what he’s doing, these overlaps will have less than 3 inches between them. This is an issue because when side butts are too close to each other, there will be a small section where water can penetrate the structure. This will most certainly cause a leak and endanger the integrity of your home. Be careful with this as it can endanger the whole project and render it useless.

  1. Incorrectly installing the starter course prior to the first course

One of the mistakes that people make is installing a starter course prior to the first course. This mistake is usually made due to the name confusion; a person might think that the starter course of slates should be placed first. So, make sure to install them at the same time. The only exception is when both starter slates and field slates have the same width and also, starter slates only have nail holes in the top section. Besides this particular case, they need to be placed at the same time. By doing things this way, you’re making sure that side-laps are proper. Also, this ensures that there aren’t any nail holes in the starter slates in the joint between the first course slates. These holes pose a problem as they lead to leaks. When installing these two sets concurrently, you are able to trim starter slates to a proper size which is otherwise impossible.

When using field slates laid on their side instead of starter slates, you need to add another nail hole. This hole would be positioned on one side of each starter slate. By doing this, you would be able to nail the slate properly given that all slates need two nails. If you choose to go this route, you will be left with a hole that was the original nail hole. It is at the bottom of a slate and it can be a leak hazard.


Although the starter course cannot be seen, it represents the crucial part of a slate roof. This is precisely why there are so many things which can go wrong. Naturally, it is much better if you can do it yourself as this way you can save a lot of money. Nevertheless, you need to be very careful during each step of the way. Sometimes, bad slate roofs can even be a cause of litigation.

With this article, you are on your way of becoming a real slate roof expert. Just make sure to follow these tips closely and you will do well!

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