Genuine money on a full roof.
Soft sheathing means tear off, no exceptions.
Most jurisdictions cap it and inspectors check.
Must reach and bite solid framing.
Ripples telegraph through. Every one.
Costs more, saves the finish.
The deck is sound throughout, the existing roof is reasonably flat, you are within your jurisdiction's layer limit, and the shingles are not so degraded that they are holding moisture. In that situation you are saving real money — tear-off is labour-intensive and disposal is not cheap — with no downside to the finished roof.
First: any soft or spongy decking. Steel fixed to compromised sheathing is a structural problem you have covered up rather than solved. Second: you are already at your jurisdiction's layer limit — most cap it, and inspectors do check. Third: the existing surface is badly uneven. Steel follows the plane it is fixed to, so ripples, humps and curled shingles telegraph straight through to the finished roof and every one will be visible from the street.
You are now fixing through an extra layer, so screws must be long enough to reach and properly bite solid framing or decking beneath. Using the same length you would on a bare deck leaves fasteners with insufficient grip — they hold at install and back out within a couple of seasons. Tell your supplier it is a recover job. If they do not ask what is underneath, ask someone else.
Some installers batten over the shingles with furring strips, creating an airspace and a flat fixing plane. It costs more and adds height at the eaves and around penetrations, but it solves an uneven substrate and improves ventilation. On a badly curled shingle roof it is often the difference between an acceptable finish and a visibly wavy one.
Often yes, and it saves tear-off labour and disposal cost. It requires a sound deck, an existing surface that is reasonably flat, and compliance with your jurisdiction's layer limit. You also need longer fasteners to reach solid framing through the extra layer.
Three situations: any soft or spongy decking, being at your jurisdiction's layer limit already, or an existing surface uneven enough that ripples will telegraph through the steel. In the first case you would be covering a structural problem rather than fixing it.
Longer than you would use on a bare deck — they must pass through the extra layer and still get proper bite into solid framing or decking. Tell your supplier it is a recover job so fastener length is spec'd correctly. Getting this wrong means screws that hold at install and back out within a couple of seasons.
Not always, but they solve an uneven substrate and add ventilation airspace. On a badly curled shingle roof they are frequently the difference between a clean finish and a visibly wavy one. They add cost and raise the roof height at eaves and penetrations.
On a properly ventilated residential assembly, generally not. The risk rises on unconditioned structures — porches, carports, detached garages — where bare steel over open framing will sweat. Call (252) 375-3651 and describe the structure and we will tell you whether to plan for a vapour barrier.
Tell us the size, your county and what you are storing. We quote from current steel pricing — no obligation, and we will tell you if a different size suits you better.
