A slate roof has been a sign of quality and craftsmanship for centuries. A slate roof can last 60-200 years with with the right materials and a little maintenance. That’s a lifespan you simply won’t find with other roofing materials like asphalt or wood. Not only is it long lasting, but it is very tough and very beautiful. Sometimes installed in unique patterns utilizing the multitude of different colored slates available, patterned slate roofs stand out unlike anything else.
In this post I’ll give you a little history of slate roofs in America and how to repair and maintain them. A slate roof may not be in your budget but if you already have one on your house then knowing its benefits and how to care for it so it can last centuries is a big money saver. A lot of the information in this post was gleaned from the National Parks Service Preservation Brief 29 The Repair, Replacement and Maintenance of Historic Slate Roofs which is a very useful publication on the topic. Check it out if you have the time.
History of Slate Roofs
Prior to the opening of the first commercial slate quarry in America in 1785 in Pennsylvania all slate was imported from North Wales in the UK. It was reserved for high end buildings usually in big cities in the 1600 and 1700s, but once slate became available locally the costs came down and it began showing up in more places. As the railroads spread so did the proliferation of slate roofs and by the 1870s the US had become a net exporter of slate with over 200 quarries in operation largely in New England and mid-Atlantic states like Vermont, Pennsylvania, New York, and Virginia.
The slate roofing industry was at its peak between 1897 and 1914 across the country before loosing ground to other less expensive materials like wood and asphalt shingles which could be more easily mass produced and shipped. Combined with a lack of the skilled labor required to install a slate roof compared to other roofing materials slate quickly fell from its peak of popularity to become the specialty item it is today.
There has been a recent resurgence in slate’s popularity thanks in part to historic preservation efforts and education about the the benefits of the material.
Picking the Right Materials
Slate is available in a variety of colors. The most common are grey, blue-grey, black, various shades of green, deep purple, brick red, and mottled varieties. Slate varieties are classified as either “fading” or “unfading” and they mean exactly what you think. The fading varieties will change color much like wood weathers to a soft grey. These slates should be expected to change color with exposure though it is not a sign of deteriorating. If you are looking to match an existing roof it’s important to get a variety of samples from manufacturers and understand if they are fading or unfading so the match will last more than a few months.
Historically, slate was 3/16″ thick and modern slate is sometimes 1/4″ to 3/8″ thick so double check the thickness of slate on your house before buying replacement slate. Widths and lengths ran the gamut and you will find dozens of different sizes of slate as well as unique shapes other than rectangles. If you’re not tied to matching an original type of slate then you have a lot of options to choose from. Not all slate is created equal and some varieties will last longer than others so keep that in mind as you shop.
Time has shown that the Vermont and New York slates will last about 125 years; Buckingham Virginia slates 175 years or more; and Pennsylvania SoftVein slates just north of 60 years. Those are sizable differences and definitely something to consider with new or replacement slate roofs.
How to Repair Slate Roofs
A slate roof is not terribly difficult to repair though it may seem daunting at first to the novice. Slate should never be repaired using mastics, adhesives or other glues. Slate roofing requires a couple very specialized tools that you really can’t do the job without. Below is a list of slate roofing repair tools you may need:
The first step in replacing a broken or otherwise damaged slate tile is removal and for that you’ll need your slate ripper. Slide your slate ripper up underneath the tile needing removal and catch the nail on the hook end. Pull aggressively to pull the nail free of the sheathing and repeat this step for the other nail.
Once the slate is out you have two options for installing a new slate tile. They both work and the method is entirely personal preference in my opinion.
Repair Method 1 Slate Hook
Nail a stainless steel slate hook into the sheathing between the two slate tiles directly below the slate you are replacing. Position it so that the bottom of the hook lines up with the bottom of the tiles in the course you are working on. Make sure it is hammered flush.
Use your slate ripper to pry the upper course of slate up just enough to slide your replacement tile into place and slide it down snuggly into the cradle of the slate hook. The new slate will be held in place by the hook at the bottom, the slates on either side and the slate overlapping it above and will not need nails directly.
Repair Method 2 Nail & Bib
In this method you’ll insert you replacement slate and then nail it in place in the space between the two slates above it so you are only nailing through the replacement slate using the smaller head spiral nail mentioned above. Spiral nails have greater holding power than smooth nails and since it will only have one nail that holding power is imperative especially for large tiles. You’ll want to pre-drill a hole in your slate before nailing to prevent breakage.
To prevent water penetration through the nail you’ll need to flash it with a bib. The bib is a small piece of galvanized steel or copper flashing that you cut little slits into the sides of to help it hold in place beneath the slate. Then simply slide the bib into place below the slate and above the nail so it keeps things dry.
Selecting the length of the nail for slate work will be based on ¾” minimum wood deck, the underlayment and two times the thickness of the slate. The most common nail lengths are 1 ½” and 1 ¾” with a 3/8” diameter head, but check the measurements you’ll need.
Below is a simple video from The Durable Slate Company showing both of these methods in action.
Slate Roof Care
Slate roofs don’t need a lot of maintenance in general. The less foot traffic they get the longer they will last so keep off the roof as much as possibly and when you do go up to inspect things wear soft soled shoes like tennis shoes rather than hard boots.
Other than a visual inspection every couple years to check for loose, cracked, or scaling slate tiles and proper tree trimming to keep things from contact with your roof they don’t need any regular care. The inspection of slate tiles for damage and prompt replacement of any deteriorated tiles will protect the roof as a whole and help you avoid larger repairs down the line.
If you live in an area where the roof gets heavy growth of moss or other organic substances then a cleaning with a non-acidic cleaner like D2 Biological Solution will clean off the roof without damaging the slate. You’ll want to spray the cleaner on using a pump sprayer and simply let it sit to work its magic. There is no scrubbing or washing necessary. Though it can be applied and scrubbed if you want more immediate results, I prefer this less invasive method. You’ll notice results in 1-2 weeks.
What did I miss? What challenges have you had with your slate roof or what are the other benefits you have noticed if you have a slate roof that you think other people need to know? Let me hear your thoughts in the comments below.
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I love old houses, working with my hands, and teaching others the excitment of doing it yourself! Everything is teachable if you only give it the chance.