60 years ago you could buy a 15 year shingle that would last 20 years…
Q: What is shiplap?
A: They are boards of varying widths ( come in 6″-8″ & 10″) that have an opposing lap cut out of the edges. Basically just lap about 3/4″ over each other.
Q: Is it a good idea to put shingles over shiplap?
A: At one time shiplap was all that was used. All roof decks had it. Years ago the shingle manufacturers did not require underlayment on the roofdeck below the shingles. This allowed them to breathe. The shiplap stayed dry & there was very little movement from the boards. Then came the laminated shingle & underlay was required on all the decks. This sealed the attic spaces. If the attic is properly vented then the shiplap is fine, if not then the moisture in the attic will expand the underside of the boards & cause buckles in the roof. Not so critical with an interlocking shingle( which is no longer made) but with a laminated shingle it can create humps in the shingle & any humps at the nail line can retain water & rust out the nails( especially “Electro Galvanized ” nails)
Q: How do I determine if I have proper venting?
A: Not so easy sometimes. Most of these are old houses & have been renovated several times over. I have found houses that have been re-insulated & had insulation blown between all the rafters very much limiting the air flow. A lot of old houses also had no soffits venting as we know them today (perforated aluminum, vinyl or vented strips under the eave) Then to dress up these old houses they put in aluminum soffits & did not cut the holes through to the attic. Looks vented but isn’t. I have even seen good soffit venting but no roof vents or holes cut too small. Remember air does not move without intake & exhaust. Another problem I run into on a regular basis is bath & kitchen fans blowing up into the attic or out to the soffit (but not through the soffit) This moist air coming out of the house will saturate the shiplap boards. This is not a good scenario with plywood either, usually causing mold because the plywood seals tighter
Q: Should I re-sheet with plywood?
A: Plywood will eliminate the buckling problem completely, but still needs to be properly vented. Shiplap does work but requires good intake & exhaust. I am sure I am not the only one that has been deceived by what looked like venting but wasn’t. Most of the shiplap on buildings has been on there for 2 or 3 possibly 4 roofs so it may not be in the best condition for another roof.
Note: If you think there is something not quite right, you need to bring it to the attention of the roofing contractor so a course of action can be taken to prevent future problems.