Mastering Roof Inspections: Slate Roofs, Part 11

Look for evidence of repairs.

Slate roofs may be repaired using several different methods, and there is some disagreement about which method is best. Each method requires removing the damaged slate, and the first step is to remove the fasteners using a slate ripper.

The user slides the ripper up beneath the damaged slate and yanks back on the tool, hooking the nail in a notch in the ripper head and then pulling the nail loose. Breaking the nail is not desirable because the stub left protruding from the roof sheathing will interfere with installation of the new slate and may prevent it from lying flat.

Nails may also bend toward the workman. The ripper can be used both by pulling and pushing, so a bent nail can still be removed by pushing the ripper toward the roof peak.

Once the fasteners are removed, the new slate can be installed. There are several methods for holding a new slate in place. Three methods are relatively common and acceptable:  hooks, copper strips and copper tabs.

Hooks

Using the hook method, a hook is nailed into the roof sheathing between existing slates. The hook should be placed so that the protruding part will support the new slate with the proper amount of exposure. Typically, the butts in the same course align.  However, in a roof with staggered butts, this may not be the case.

The two photos above show a slate hook installed as it would be installed for a repair. You don’t see one single, missing slate because these photos were taken of a display model.

Copper Strips

You may see copper strips installed between slates. Using this repair method, a replacement slate has a new hole punched in a location that will align with the gap between existing slates, once the new slate is installed.  The new slate is slid into place and nailed, and the nail head is covered with a strip of copper that has been bent slightly convex. Because the strip is not flat, once the copper strip has been slid into place, the weight of the overlying slates will hold it in place.

Copper Tabs

You may also see copper tabs used for repairs. After the damaged slate has been removed, a copper tab is nailed into place so that the nail penetrates the roof sheathing between the existing slates. The new slate is slid into place, and the bottom of the tab is then bent up around the butt to support the replacement slate.

Crucial to the success of this method is that the copper tab must be made of a gauge heavy enough to support the slate against sliding snow. If a light-gauge metal is used -- and light-gauge, galvanized sheet metal is sometimes substituted for heavy-gauge copper -- the tab may straighten, allowing the slate to be carried away with the snow.

Improper Repairs

You may see a variety of improper repairs. These include:

• exposed sealant. Slates should not be repaired using a sealant.  If you see sealant protecting the roof, call it out as an improper or temporary repair;

Repair using sealant

• construction adhesive. The person attempting the repair may successfully remove the broken slate but may not be familiar with the methods for fastening a new slate into place. Gluing slates into place with construction adhesive is not an acceptable practice. Different rates of expansion and contraction may lead to failure of the adhesive, causing the slate to slide off the roof, which is very dangerous;
  
  
• exposed fasteners. Fasteners left exposed may eventually loosen or back out, leaving a hole through which moisture can enter the roof assembly;
  
  
• using cracked slate. A workman may sometimes realize that he doesn’t have enough replacement slates to complete all repairs. You may find damaged slates that have been re-used. If the lower corners are cracked, the workman may spin the slate 180º so that the broken butt is hidden by the overlying slates. This will leave the nail holes exposed and these are sometimes filled with a sealant in an attempt to prevent leakage. This is not an acceptable repair. The sealant will eventually dry and shrink, leaving a potential  avenue for leakage; and
  
  
• non-matching slates. Matching slates may be difficult to find, and workmen may substitute slate of a different color, or of a type that will eventually change color. Similar-looking slate may color-fade or may fail before the rest of the roof if it’s of a type much less durable than the existing slate.