The short answer
The regulations do not give one thickness. They set a U-value, a measure of how much heat passes through the roof, and the depth of insulation needed to meet it depends on the material and the build-up. Rigid PIR boards reach the target in less depth than mineral wool or timber-based boards. When a flat roof covering is replaced, the roof is a thermal element and generally has to be brought up to the current standard.
The rule is about performance, not depth
Approved Document L, the part of the Building Regulations that deals with energy use, asks for a roof to resist heat loss to a stated level. That level is expressed as a U-value in watts per square metre per kelvin. The lower the number, the better the roof holds heat. Nothing in the rule says a specific number of millimetres. Instead, the thickness follows from the maths: a material with high resistance to heat flow reaches the target in fewer millimetres than one with low resistance.
Because the exact figure for a given job depends on the current document and the type of work, we confirm it at survey, with your building control body if the work is notifiable, rather than quote a number here that may have been superseded. Replacing a flat roof membrane, as opposed to overlaying it, generally brings the insulation upgrade into play.
Why the material changes the depth
PIR (polyisocyanurate) boards are a rigid foam with a foil facing. They are very good insulators for their thickness, which is why they are the usual choice for a warm roof where headroom and threshold heights are tight. Mineral wool is a fibrous quilt or slab. It is cheaper and fire resistant, but it needs more depth to do the same job, and it is normally used between joists in a cold roof where there is room. Expanded polystyrene sits in between.
This is why two roofs that meet the same standard can have quite different build-ups. A warm roof with PIR on top of the deck may add a modest height. A cold roof with mineral wool between deep joists needs a ventilated void as well, and the joists have to be deep enough to hold both. Our guide to warm roofs and cold roofs explains the two approaches.
What else decides the depth
The existing structure sets limits. A low door threshold may leave no room for added thickness above the deck, which can push the design towards insulating below the joists or between them. A roof that has to line up with a neighbouring wall, a window head or an existing upstand has the same restriction.
Tapered insulation is a further factor. Where a fall has to be created, boards are cut to a slope, so the thickness varies across the roof, thinnest at the low edge and deepest at the high point. The U-value is then calculated on an average, and the design checks that the thinnest part is not so thin that cold spots form. The slope itself is dealt with under how much gradient a deck needs.
The vapour layer and ventilation
Thickness alone does not make a roof work. A warm roof needs a vapour control layer below the boards, and a cold roof needs a clear ventilation path above the insulation. Pack the joists full to the deck on a cold roof, and the void disappears, leaving trapped moisture and rot. Our article on flat roof ventilation goes through the failure mode.
Thicker is also not always better beyond the target. Very deep boards can create awkward details at the edges, require longer fixings that must reach the deck securely, and raise the finished level so far that it affects the look of the building or the planning position. A designed build-up beats an arbitrary one.
Fire, fixings and condensation risk
Board choice also affects fire performance and fixing. Foil-faced foam boards are held down with long fasteners and plates, or bonded with adhesive, and the length must reach well into the deck. Mineral wool is fire resistant, which can matter on roofs near boundaries or above living accommodation. Whichever material goes in, the joints between boards should be tight and staggered, because gaps let warm air reach the cold side and condense. A single missed gap can leave a damp patch in a ceiling, however thick the boards are elsewhere.
How we work it out
On a survey we measure the roof, note the depth available at the threshold and the edge, and identify what is already there. We then propose a build-up that meets the current standard in the least depth that will work, and explain it in plain terms. The fixed written quote names the boards and the covering. Where building control approval is needed, we deal with that for you. For more about our systems see flat roofing in Basingstoke.
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