7 August 2026
Traditional stone roofs vs reinforced concrete: comparing 7 Maltese roof types
A traditional limestone slab roof with torba and screed, and reinforced concrete roofs with torba and screed, with and without bitumen felt and at two insulation thicknesses — compared using Document F Part 1.
Roofs get less attention than walls, but in Malta's climate a flat roof is often the single largest exposed surface on a property — and, as with walls, what it's actually made of matters far more than how it looks from the street. If you haven't already, it's worth reading our guide to what a U-value is and how eight real Maltese walls compare first — this one uses the same method and the same source, just applied to roofs.
We compared seven real Maltese roof constructions — a traditional limestone slab roof with torba and screed, and reinforced concrete roofs with torba and screed, with and without bitumen felt and at two insulation thicknesses — using Technical Document F Part 1 (Building and Construction Authority, effective 1 July 2024) for every thermal conductivity value and every worked example.
A quick note on the numbers
Every roof below uses the same thicknesses for shared layers — 100mm torba, 100mm screed, 150mm limestone slab or 200mm reinforced concrete slab, 3mm dense plaster ceiling finish, 6mm bitumen felt, 80mm insulation — taken directly from Document F's own Appendix B and its own worked roof calculation (Section 9.4), so the comparison isolates construction, not arbitrary thickness choices. U-values are calculated the same way as the wall guide: R = d/λ, U = 1/R total, using Document F's own roof surface resistances (0.04 m²K/W external, 0.14 m²K/W internal). Document F's new-build roof limit is 0.4 W/m²K.
The traditional limestone roof
Limestone slab roof, torba and screed
353mm overall
150mm limestone slabs spanning the roof, a 100mm torba infill layer, and a 100mm cement screed cap on top — the traditional Maltese flat roof. Torba is crushed limestone chippings, not soil, and it's never left as the exposed surface: the screed above it is what actually sheds water.
Reinforced concrete roofs
Waterproof isn't the same as insulated
The standard reinforced concrete roof — torba, screed, no felt, no insulation — comes in at 1.58 W/m²K, nearly four times Document F's 0.4 W/m²K new-build limit. Adding bitumen felt, the single most common roof upgrade in Malta, barely moves the number: felt keeps water out, it does almost nothing for heat.
Reinforced concrete roof, torba and screed
403mm overall
A 200mm reinforced concrete slab with the same 100mm torba and 100mm screed build-up — no waterproofing membrane, no insulation. This is the baseline flat roof found on most Maltese homes.
Reinforced concrete roof, with bitumen felt
409mm overall
The same roof with a 6mm bitumen felt membrane added for waterproofing. It stops water — it barely touches the U-value, because a waterproofing layer and an insulating layer solve two completely different problems.
Reinforced concrete roof, with insulation, no felt
483mm overall
The baseline roof with an 80mm insulation board added, but no membrane — an unusual combination in practice (you'd never skip waterproofing once you're going to this trouble), included here to isolate what insulation alone does to the number.
Reinforced concrete roof, felt and insulation
489mm overall
Bitumen felt for waterproofing and an 80mm insulation board together — the complete, genuinely modern roof build-up, and one of the roofs here that comfortably clears Document F's new-build limit. This is the exact specification behind Document F's own new-build worked example.
Reinforced concrete roof, 50mm insulation, no felt
453mm overall
The lighter, renovation-grade insulation spec — 50mm instead of 80mm, no membrane — from Document F's own major-renovation worked example. It comes out to 0.402 W/m²K: technically just a hair over the strict new-build limit.
Reinforced concrete roof, 50mm insulation and felt
459mm overall
The same 50mm insulation spec with bitumen felt added — Document F's own major-renovation example in full. Felt's small thermal contribution is just enough to tip this one to 0.398 W/m²K, under the new-build limit rather than over it.
All seven, side by side
Sorted from best to worst performing. The insulated roofs sit far out on their own — everything else, regardless of deck material or waterproofing, is bunched together well above the limit.
Dashed lines mark Document F Part 1's roof U-value limits: 0.40 W/m²K for new dwellings, 0.59 W/m²K for major renovations. Values in W/m²K, lower is better.
View as a table
| roof type | U-value (W/m²K) | Status |
|---|---|---|
| RC slab + torba + screed + insulation | 0.28 | Meets new-build limit |
| RC slab + torba + screed + felt + insulation | 0.28 | Meets new-build limit |
| RC slab + torba + screed + felt + 50mm insulation | 0.40 | Meets new-build limit |
| RC slab + torba + screed + 50mm insulation | 0.40 | Above new-build limit |
| Limestone slab + torba + screed | 1.45 | Above new-build limit |
| RC slab + torba + screed + felt | 1.51 | Above new-build limit |
| RC slab + torba + screed | 1.58 | Above new-build limit |
What this actually shows
First, the traditional limestone roof — stone slabs, torba, and a screed cap, no concrete anywhere — actually performs slightly betterthan the standard uninsulated reinforced concrete roof being built today: 1.45 versus 1.58 W/m²K. Neither comes close to Document F's 0.4 W/m²K limit, but it's a reminder that swapping stone for concrete doesn't automatically improve anything, unless insulation comes with it.
Second, both insulated roofs land at practically the same U-value — 0.28 W/m²K — whether or not bitumen felt is present. Felt and insulation are solving two different problems, and a well-built roof needs both, just not for the same reason: felt keeps water out, insulation keeps heat out, and neither substitutes for the other.
Third, and most importantly: insulation is the only thing that moves any of these roofs into compliant territory, and it does so by a wide margin. Adding an 80mm board to the standard concrete roof drops the U-value from 1.58 to 0.28 W/m²K— a roughly 5.6× improvement, comfortably under the 0.4 limit with room to spare. Every other uninsulated roof on this list, including the traditional stone one, is a variation on “not insulated”.
Fourth: thinner insulation cuts it close. Drop to the renovation-grade 50mm board instead of 80mm, and the roof lands right on the edge of the new-build limit — 0.402 W/m²K without felt, 0.398 W/m²K with it. That 0.004 W/m²K gap, entirely down to felt's small thermal contribution, is the difference between failing and passing. It's presumably why Document F pairs the thinner 50mm spec with the more forgiving 0.59 W/m²K renovation limit, and reserves 80mm for genuine new-build compliance.
Wondering what your own roof actually adds up to? Request a free quote and we'll take it from there. We're also working on an interactive calculator so you can build up your own wall or roof construction and see the U-value update live — keep an eye on this space.