The short answer
A new roof covering on its own does little for your heating bills, because tiles and slates are not insulation. The saving comes from insulation laid at the same time, and from a better-sealed roof void. When you strip a roof to the rafters, Part L may require the roof to be upgraded to a U-value of 0.16 W/m²K. The size of the saving depends on what you have now.
Why the covering is not what saves energy
Tiles, slates and stone are there to shed water and survive weather. A layer of clay or limestone does hardly anything to slow heat leaving the house. The thermal barrier is the insulation at ceiling or rafter level, together with how well the ceiling is sealed against warm air rising into the loft.
So a like-for-like re-roof, with the same covering over a loft that is already insulated, should not be expected to change your bills in a way you could measure. What it does is stop water getting into the existing insulation, because damp insulation works poorly. If the old roof was letting water through, there is a benefit, but it comes from repair, not from a new surface. We would rather say that plainly than promise a figure, because a promised saving that never appears on the meter helps nobody.
Where the saving actually comes from
The gain from a re-roof comes from using the opportunity. With the covering off, insulation can be fitted properly, rather than squeezed in around obstacles. In a conventional loft, that means quilt between and over the joists. In a room in the roof, it means insulation between and under the rafters, with attention to the air gap and ventilation behind it. Draughts at the eaves and around pipes can be sealed while the roof is open, and gaps round the loft hatch tidied up at the same visit.
Underlay also plays a part. A well-laid membrane with sealed laps and a ventilated roof void keeps moisture out of the structure, which protects timber and insulation alike. Our answer on new battens and underlay is not about energy, but the layers it describes are the same ones that decide how the roof performs.
We do not quote percentages for heating savings, because they would depend on your house, your existing insulation and how you heat it. A fair estimate comes from the survey, not from a general claim.
What Part L asks for when you re-roof
The Building Regulations put a floor under this. Approved Document L (2021 edition) counts a roof as renovated when its layers are stripped to the rafters and rebuilt. Past a certain scale, the whole roof must then be improved: more than 50% of its surface renovated, or a major renovation touching more than 25% of the external envelope. The target is the U-value in Table 4.3, column (b), which for a roof is 0.16 W/m²K, against a threshold of 0.35.
A U-value measures heat lost through a surface, so a lower figure means a better-insulated roof. If the full thickness would eat into head room, a lower standard can be accepted, provided the insulation and the gap above it fill no less than the rafters' depth. A brand-new roof element is held to 0.15 W/m²K under Table 4.2. Applying these is the job of the roofer and building control, not something you need to calculate. Our guide to Part L insulation upgrades when you re-roof sets out the tests in full.
What to expect and what to ask
Set your expectations by what is being changed. If your loft already has a decent depth of insulation and you are fitting the same covering, expect a dry, sound roof and little change on the meter. If the loft has little or none, or a room in the roof is cold in winter, adding insulation during the re-roof is where a real difference can come from, because the roof is already open and the insulation can go in at the same visit.
Ask the quote to say what insulation is included, with the material and the depth, and to say whether the roof is being brought up to the Part L standard. Reusing sound material can matter here as well. If you are keeping your tiles, see whether old tiles or slates can be reused. And if the rafters turn out to be rotten, the insulation plan may change, which our answer on replacing roof timbers explains.
VAT, bills and the long view
The VAT position differs: fitting insulation in a house roof or its loft is zero-rated for VAT until 31 March 2027, and the roofing around it is standard-rated, so expect the quote to explain how VAT applies. That narrows the cost of adding insulation during a re-roof, and the government notice sets out the rule exactly.
Over a longer span, a roof that stays dry and well insulated also protects the timber, a benefit that never shows on a bill but does show in fewer repairs. The survey records your loft in photographs, and the written quote afterwards covers the whole of the work. We stand behind the finished roof with a 10-year workmanship guarantee, in writing. The main page for the whole job is roof replacement.
If you want a rough check before the survey, go into the loft with a torch. Look at how deep the insulation is against the joists, whether it has gaps around the hatch and pipes, and whether any is damp or compressed. Those observations make the conversation with a surveyor far more useful than a general estimate, and they help us decide whether the insulation needs topping up or replacing.