Flat Roof Joist Calculator

Enter roof dimensions, joist grade and spacing to get a complete timber schedule

Construction type

Cold roof Warm roof

Cold roof: insulation between and/or below joists, ventilated void above. Most common for extensions.

Joist span (m) - wall plate to wall plate

Roof length (m) - perpendicular to joists

Joist width (mm)

Joist depth (mm)

Timber grade

Joist centres (mm) 400 mm is standard for most flat roofs. 600 mm where deck loading allows.

Eaves overhang (mm) How far joists project beyond the wall plate. Typically 150-300 mm.

Noggin rows One row at mid-span is standard. Add rows for spans over 3 m.

Wall plates

Wall plate size

Include firring pieces (for drainage fall)

Cutting waste allowance: 10%

10%

Job Costing (optional)

Joist timber per linear metre

Noggin timber per linear metre

Wall plate per linear metre

Firring pieces per linear metre

Labour per m²

Delivery (flat fee)

UK Market Prices Fills in material cost fields with current UK market prices. Review and edit before applying.

Span check advisory

Results

Total Joists
-
-

Joist Length

incl. eaves

Total Timber

linear metres

Deck Area

Joists

Joist span (clear)-

Joist length (incl. eaves both sides)-

Number of joists-

Total joist timber (with waste)-

Noggins

Noggin length (per piece)-

Noggins per row-

Total noggins-

Total noggin timber (with waste)-

Wall Plates

Wall plate size-

Total wall plate (with waste)-

Firring Pieces

Fall ratio-

Max firring depth (high end)-

Total firring timber (with waste)-

Timber Summary

Total all timber-

UK Building Regulations

BS 6229, Approved Documents A, C & L

Span guide (C16/C24, 400 mm centres)

Indicative maximum spans for domestic flat roofs (0.5 kN/m² dead + 0.75 kN/m² imposed). Verify with full TRADA tables or a structural engineer for spans over 4 m.

Size	C16	C24
47 x 97	1.60 m	1.85 m
47 x 120	2.05 m	2.38 m
47 x 145	2.55 m	2.97 m
47 x 170	3.06 m	3.57 m
47 x 195	3.58 m	4.18 m
47 x 220	4.12 m	4.80 m

Cold vs Warm Roof

Cold roof - insulation sits between and below the joists with a ventilated void (min 50 mm) above. Most common for extensions.

Warm roof - insulation sits above the structural deck, outside the vapour control layer. No ventilation void needed. Better thermal performance but adds height.

Both types require a minimum fall of 1:80 to drain rainwater (BS 6229:2011).

Joist sizing tips

Span - clear distance between wall plates, not including bearings
Bearing - joists must bear at least 75 mm onto each wall plate
Deflection - flat roofs are deflection-governed (L/250). Deeper joists reduce bounce
600 mm centres - only use where deck is 18 mm+ OSB or plywood and loads are light
Noggins - prevent joist rotation and provide lateral stability. One row at mid-span minimum

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Flat Roof Joist Calculator