More than 1 layer of roofing not recommended over 3 8 sheathing.
Roofing nail length second layer.
The length of each nail must be a minimum of 1 inches long and for roof overs atlas recommends a nail length of at least 2 inches.
Flashings for re roofing asphalt shingles.
Nesting each new row below an existing one keeps the new nails 2 inches below the existing which will help minimize any splitting of the sheathing.
Nails of the proper length should penetrate three fourths of an inch into the roof deck.
Roofing second layer of shingles roofing.
If your decking is less than 3 4 inch then a 1 1 2 inch nail will suffice.
Wood shakes minimum 3 4 sheathing wood shakes nail type minimum length 18 straight split 5d box 1 3 4 18 and 24 handsplit and resawn 6d box 2.
Use 1 5 or 1 75 inch roofing nails.
If you are putting a layer of asphalt shingles over an existing layer of asphalt shingles on to a 3 4 inch deck or a plank deck then you should be using 1 1 2 inch nails.
Apply the shingles to all roof planes ending a row of shingles at the top of the plane which is the ridge or peak and cut to follow the profile of any outside corners.
Roofing nails should be long enough to penetrate the roofing material and go 19 mm into osb solid wood plywood or non veneer wood decking or through thickness of decking whichever is less.
Use galvanized roofing nails long enough to fully penetrate the sheathing typically 1 1 2 inches for a second roof and 1 3 4 inches for a third.
Select a length of roofing nails that penetrates completely through the new layer of shingles existing layer and the wood roof deck.
To determine the nail length you should consider the number of layers of shingles shingle thicknesses underlayment and flashings installed on eaves.