A new roof over old slate: yes or no — three checks
The offer to “remove nothing and haul nothing away” sounds reasonable, and sometimes it really can be done. But only if three checks are passed, and the third is the main one.
A new roof can be laid over old slate if two conditions are met: the rafters are sound and there is air under the new sheet. Three things need checking — weight, air and rafters. If even one item does not add up, the old roof is removed. Asbestos is not a ban in itself: intact slate is almost safe, and caution is needed when breaking, cutting and drilling.
Why this question comes up at all
When a house has old slate, demolition looks like a separate cost item: it has to be taken off and hauled away. So an offer to lay the new roof straight over it seems profitable. Sometimes it is honest, sometimes not. The difference is not in who offers what but in the condition of the structure under the slate. You can assess it yourself, before talking to a contractor.
Check 1. Weight
A square metre of slate is roughly three times heavier than metal tiles. Add the old and new roof together and you get a noticeable extra load, and it seems this is the main obstacle.
But in winter snow lies on the same metre, and it weighs many times more than any roof. So with sound rafters weight is not the main question. It becomes a problem where the rafters are already weakened, and that is checked by the third item.
Check 2. Air under the new sheet
The new sheet is laid not on the slate but on battens fixed to the rafters.
The gap is not for looks. Air must move under the metal: otherwise condensation collects on the cold sheet and water starts dripping inside.
What to ask the contractor: what the new sheet is fixed to and where air will move under it. The answer “straight onto the slate” fails this check.
The cost can be calculated in advance, before talking to a contractor. The calculator takes the area of the roof, not of the house.
Calculate a roof →Check 3. Rafters — the most important
You cannot see the rafters from above, so they have to be inspected from the attic. Take a torch and an awl.
| What you see | What it means |
|---|---|
| Dark patches and stains on the boards | The roof has already leaked |
| The awl goes into the wood easily | Rot |
| The ridge is uneven from outside, with a sag | A reason to inspect the rafters especially carefully |
| The ridge is level, the wood is hard, no patches | Check passed |
The ridge itself is checked from outside: it should be level, with no sag.
Why this check matters more than the other two: laying a new roof on rotten rafters means paying twice. It is the most expensive way to save on a check that needs only a torch and an awl.
And what about asbestos
Old slate contains asbestos, and it is worth knowing this without panic. Intact slate, while it simply lies on the roof, is almost safe. The danger is the dust that appears when it is broken, cut or drilled.
Hence the rules if the slate is removed after all:
- remove it in whole sheets;
- wet it with water;
- do not throw it off the roof.
If the new roof is laid over it, the slate stays in place intact. The danger appears where it starts to be broken, cut or drilled, so ask the contractor exactly how the old covering will be handled.
What to ask a contractor who proposes “over the top”
The three checks are easy to turn into three questions. They do not accuse; they simply show whether the proposal has been thought through.
- What about air? What the new sheet is fixed to and where air will move under it. If the sheet is meant to go straight onto the slate, the condensation has nowhere to go.
- Did they look at the rafters? Were the two of you in the attic, did you see the patches and did anyone test the wood with an awl. An offer to “lay over” made without inspecting the rafters should be put off.
- How will the slate be handled? If some sheets still have to be removed, they should come off whole, wetted with water and not thrown from the roof.
A good sign: the contractor raises all three points before you ask about them.
Bottom line: over the top or remove
| Rafters | Air under the sheet | Decision |
|---|---|---|
| Sound | Yes | You can lay over and not pay for demolition |
| Sound | No | The old roof is removed |
| No | Yes | The old roof is removed |
| No | No | The old roof is removed |
Weight does not get a separate row in the table: with sound rafters it is not decisive.
These checks are for a first inspection: so that you come to the contractor with questions, not just a readiness to agree. They do not replace the opinion of a specialist who sees your roof.
Enter the roof area into the calculator as it is. The calculation uses open price lists and needs no call.
Open the calculator →Frequently asked questions
Yes, if the rafters are sound and air remains under the new sheet: the sheet is fixed to battens along the rafters, not directly to the slate. If even one of these conditions is not met, the old roof is removed.
Slate is roughly three times heavier than metal tiles, and together it seems like a lot. But in winter snow lies on the same square metre, and it weighs many times more than any roof. So with sound rafters weight is not the main question; the main one is the condition of the rafters.
Intact slate, while it simply lies there, is almost safe. The danger is the dust that appears when it is broken, cut or drilled. So it is removed in whole sheets, wetted with water, and not thrown from the roof.
Go up to the attic with a torch and an awl. Dark patches and stains on the boards mean the roof has already leaked. If the awl goes into the wood easily, it is rot. From outside, look at the ridge: it should be level, with no sag.
The article describes a general order of checking and does not replace an inspection of your roof by a specialist. The cost of a new roof in the calculator is approximate: it is calculated from the roof area and open price lists, and the exact estimate is confirmed by an engineer.