This 1949 photograph shows members of Chicago Local No. 17 applying interlocking shingle tile over the felt covered gypsum roof deck of St. Rita's Church in Chicago.
The roof surface should be carefully examined. All projecting nails,
broken pieces of lumber and debris such as chunks of mortar, excess nails,
pieces of brick or stone should be removed so that they will not fracture
the felt when it is applied.
Certain flashing or parts of them should be in place before the felt is
laid. Roofer should check with the sheet metal workers on this point so
that the work between tiler and sheet metal man can be coordinated. In fact
it is very important that these two crafts closely coordinate their work
throughout the entire progress of the job. A roofing job cannot be
watertight unless the tile is properly laid and the sheet metal correctly
applied. If these two applications are accurately carried out, the roof
cannot leak.
Roof surfaces should be smooth, faulty sheathing replaced, openings or
cracks filled in so that the sheathing surface is in proper condition to
receive the felt.
Standard specifications require the use of good quality asphalt felt weighing 30 lbs. per square. On roofs of average pitch one layer is usually specified with two layers for low pitches and on hips, valley and ridges.
During the past few years there has been considerable discussion about
various methods of felting under tile roofs. Some experienced roofing
contractors advocate the use of two layers in all jobs regardless of pitch,
others state one layer is sufficient. In some sections of the country felt
is mopped with hot asphalt after application, sometimes two layers are
applied with this method. We have examined roofs that have been on for over
fifty years and found the felt underlay still to be in excellent condition.
The varying conditions of humidity and temperature, the sudden change in
climatic conditions, the original method of application, the quality of the
felt - all of these affect the life of the underlay.
The asphalt felt should be laid parallel to the eaves, lapped 30" horizontally and 6" vertically. Fasten with large
headed galvanized nails. Extend the felt 6" up vertical walls and 4" over gutter and valley metal.
"WHY USE FELT under slate or tile?"
Felt is used for several reasons. One of the most important is that it
keeps the building dry during construction. As each section of sheathing is
finished the roofer immediately covers it with felt to protect the interior
so that other work can be done without fear of damage by bad weather.
The felt acts as a cushion for the roofing material; this helps to
absorb vibrations and gives resilience at the point of contact between the
roofing tile and the sheathing. The asphalt in the felt forms a seal around
penetrating nails.
Felt has the very important function of preventing drafts through the
roof caused by wind on the outside and also acts as a barrier against escape
of heat from the inside. Obviously the protection which the felt provides
against the two conditions mentioned indicates it is of real value and an
aid to the insulation used.
The underlay is of course of great value when the roofing material
itself is damaged by lightning, struck by objects with sufficient force to
break the slate or tile or in any case where the roofing may be damaged or
in need of repair.
1763 sketches showing the method of applying tile laid on wood strips (battens). This application is still used today with the tiles normally wired to the battens.
The roofing medium most used in Europe is clay tile with slate next in
general usage. Probably 95% of these roofs are laid on open
construction, the tiles or slates being laid on battens or purlins with no
underlay of any kind and no sheathing. This writer has spent considerable
time in various European countries and can definitely state that the roofs laid without felt do not leak unless the roofs have been damaged. This
statement applies to all patterns of tile and slate.
St. Rita's Church in Chicago showing the wooden scaffolding that was used in 1949. Today's scaffolding is metal and the tiles are now carried to the roof by conveyor machinery!
You should check to see what is recommended for your felting needs as many new developments have taken place since this article was published.
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