About this calculator
This calculator works out the insulation for a pitched roof insulated between the rafters: slabs, an optional second layer under the rafters with its service battens, and the vapour control layer (VCL) with its laps and tape. It also gives the thermal resistance of the insulation layers.
Enter the slope (length from eaves to ridge and length along the eaves) and the number of slopes, or just the area; the rafter spacing, width and depth; the slab thickness and number of layers. Each bay between two rafters gets slab strips cut slightly wider than the gap, so they hold by clamping.
Formula
rafters per slope = ceil((B − rafter width) / spacing) + 1 (one flush with each end)
bay clear width = gap between rafter faces
strip width = bay clear width + allowance
strip length = Σ over bays of that width: A × slopes × layers
slabs per width = the fewer of
ceil(strip length / (floor(slab W / strip) × slab L)) (slab width across)
ceil(strip length / (floor(slab L / strip) × slab W)) (slab length across)
packs = ceil(ceil(Σ slabs × (1 + waste %)) / slabs per pack)
R (insulation) = Σ thickness / λ (LST EN ISO 6946, per layer)
VCL sheets/slope = ceil((A − lap) / (roll width − lap))
VCL rolls = sheets laid one after another along B off the rolls,
a new piece lapping the last where a roll runs out
lap tape = (sheets − 1) × B per slope + end laps × roll width
In area mode every bay is taken at the full spacing (strip width = spacing − rafter width + allowance, strip length = area / spacing × layers), and VCL rolls are estimated as area / ((width − lap) × (length − lap)).
Worked example
A gable roof, two slopes 5 m × 10 m, rafters 50 × 200 mm at 600 mm, two layers of 95 mm slabs 610 × 1170 mm (λ 0.035), 8 slabs per pack, 5 % waste:
rafters = ceil(9950 / 600) + 1 = 18 per slope
bays = 16 × 550 + 1 × 300 mm clear → strips 560 and 310 mm
560 mm = 16 × 5 m × 2 slopes × 2 layers = 320 m of strip
610 across: 1 strip of 1.17 m per slab → 274; 1170 across: 2 of 0.61 m → 263
310 mm = 20 m of strip: 1 × 1.17 m → 18; 3 × 0.61 m → 11
slabs = 263 + 11 = 274; +5 % → 288 → 36 packs of 8
R = 0.190 / 0.035 = 5.43 m²·K/W (insulation only)
VCL = 4 sheets per slope, 8 × 10 m off 50 m rolls → 2 rolls
tape = 2 × 3 × 10 m = 60 m → 3 rolls of 25 m
FAQ
Why is the slab cut wider than the gap? So it clamps between the rafters and does not sag or leave gaps. The 10 mm default is a placeholder: use the insulation manufacturer's instruction.
Is the R value the roof's thermal resistance? No. It is R = d / λ of the insulation layers only (LST EN ISO 6946 for a homogeneous layer). The rafters cross the between-rafter layer and conduct more heat; the roof's total R and U-value need the standard's method for inhomogeneous layers and every layer of the roof — use the U-value calculator for that.
When is a ventilation gap needed? When the underlay over the rafters is not breathable, the roof is ventilated and a free air gap must stay above the insulation. Baunetzwissen gives a minimum free height of 2 cm from the German DIN 4108-3; the German roofers' rules ask for 3 cm on rafters up to 7.5 m, 4 cm up to 10 m and 6 cm above. With a breathable (diffusion-open) membrane the insulation may fill the full rafter depth. Follow the roof's design either way.
What if the insulation is thicker than the rafters? The calculator warns. Put the extra thickness in a layer under the rafters (between service battens), or deepen the rafters with battens.
Which standard applies? LST EN ISO 6946 for R = d / λ. Nothing governs the take-off itself; DIN 4108-3 is a German standard, not adopted in Lithuania, and was only read in a summary, so it is named but not cited.
Assumptions and limits
Standard: LST EN ISO 6946 (thermal resistance of a homogeneous layer) only.
Defaults to replace: Paroc eXtra 95 mm slabs 610 × 1170 mm, 8 (5.71 m²) per pack (a retailer listing); λ 0.035, the 10 mm clamping allowance, the 50 mm under-rafter layer, 1.5 × 50 m VCL rolls, 100 mm laps, 25 m tape rolls, battens at 600 mm in 3 m lengths and 5 % waste are placeholders.
Geometry: each slope is a rectangle; hips, valleys, dormers and roof windows are not modelled. The first and last rafter sit flush with the slope's ends. Strips of one width are cut end to end, so a short length left in one bay continues in the next; the narrow offcut beside a cut strip is not reused, and is counted as waste of the slab — the 5 % allowance is for damage and cutting errors only. The VCL runs parallel to the eaves; tape is counted on laps only — edges to walls and between slopes are reported as a length for the adhesive or tape of your system. The under-rafter layer fills the space between service battens; without battens it is counted over the full area.