Loft Conversion Structure: Floor Loads, New Joists and Steel Beams

Loft conversion floor loads in kN/m², how they reach new joists and steels, plus worked floor beam (33 kN) and ridge beam (20 kN) reactions.

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A loft conversion turns a ceiling that carries almost nothing into a habitable floor. The design load goes from roughly 0.5 kN/m² to about 2.65 kN/m² characteristic (0.65 dead plus 2.0 imposed), so the existing ceiling joists are replaced, new steels pick up the floor and the roof, and every reaction has to be traced down through the walls and foundations.

This page is for technicians and engineers. It covers which loads change, how they reach the new members, and how to get a beam reaction in kN. It does not size members: that needs a competent engineer.

What loads change in a loft conversion?

Sources: BS EN 1991-1-1 with its UK National Annex for imposed loads and the partition allowance (clause 6.3.1.2). Dead loads are typical build-up values. Values are typical and must be checked by a competent engineer for design.

The floor is the headline change, and typical UK floor and roof loads has the existing-versus-new comparison worked through, so it is not repeated here. The rest of this page picks up where that one stops: the steels.

Why are existing ceiling joists rarely reused?

They were sized for a ceiling and a bit of storage. At 0.5 kN/m² characteristic, the ultimate load is about 0.71 kN/m². The habitable floor is about 3.9 kN/m², which is more than five times higher.

Even where the strength happens to be close, the other checks usually fail. Ceiling joists are shallow, sit at wide centres and have no notches or holes left for services. Deflection and vibration for a bedroom floor are far stricter than for a ceiling. They are also often tied into the roof as the eaves restraint, so cutting or overloading them changes how the roof behaves (more on that below).

The usual answer is new, deeper floor joists, sitting beside or between the old ones and bearing on walls and new steels.

How do loads get from the new joists to the walls and steels?

New floor joists are simple one-way spanning members. The load path is the same as any other timber floor:

• Floor area load (kN/m²) goes onto each joist as a line load, which is area load times joist spacing.
• Each joist delivers half its span's load to each support.
• A support is either a wall plate on masonry, or a steel beam.

For a beam, the loaded width is half the span of the joists on each side of it. A beam with 3.6 m joists on one side and 3.6 m on the other picks up 1.8 + 1.8 = 3.6 m. The loaded width article covers cantilevers and continuity, and the free tributary area calculator will give you the area if the plan is irregular.

Two details are easy to miss:

• Joists that run parallel to a wall load it very little. Do not give a wall a strip of floor it does not support.
• Partitions on the loft floor are a line load, not a smear. The 0.5 kN/m² allowance covers movable partitions up to 1.0 kN/m of wall. A fixed, heavier or blockwork partition should be taken as an actual line load on the joists or beam under it.

Which steels does a loft conversion usually need?

Three groups come up on most jobs. Typical arrangements only: every house is different.

• Floor beams. Where the new joists cannot span from wall to wall, steels sit at mid-span or around the stair opening. They are usually fully loaded by the floor, plus any partitions above.
• Ridge beam. When purlins and struts are removed to open up the space, the rafters need a ridge beam to hang from, and the beam needs end supports (gable or party walls, or posts).
• Dormer trimmers and posts. A dormer cuts the rafters, so trimmer steels (or timbers) carry the cut rafters and pass the load to the ridge beam, wall or floor steels. The dormer cheeks and roof add their own weight.

The next two sections work out a reaction for the floor beam and the ridge beam. They stop at the reaction in kN. The section itself comes from the beam design, which is the engineer's job.

Worked example: a steel floor beam reaction

Loft floor joists span 3.6 m on each side of a steel beam. The beam spans 4.5 m between supports. Floor loads: 0.65 kN/m² dead, 2.0 kN/m² imposed (1.5 plus 0.5 partitions). Beam self weight is a placeholder of 0.5 kN/m until the section is chosen.

Loaded width     = 1.8 + 1.8 = 3.6 m
Dead line load   = 0.65 × 3.6 + 0.5 = 2.84 kN/m
Imposed line     = 2.0 × 3.6        = 7.20 kN/m

Characteristic reaction each end (span 4.5 m)
  Gk = 2.84 × 4.5 / 2 = 6.39 kN
  Qk = 7.20 × 4.5 / 2 = 16.20 kN
  Total = 22.6 kN

ULS (expression 6.10)
  1.35 × 6.39 + 1.5 × 16.20 = 8.63 + 24.30 = 32.9 kN

The characteristic figure goes to bearing pressure checks and service checks. The ULS figure goes to the member and padstone design. See characteristic vs design loads for why you keep both.

BS EN 1990 with its UK National Annex also allows expressions 6.10a and 6.10b, and you take the worse of the two. With the domestic imposed load factor ψ0 = 0.7 and the reduction factor ξ = 0.925, 6.10b governs here and gives about 32.3 kN. That is roughly 2 per cent below expression 6.10, which is why many offices stay with 6.10 for hand checks.

Worked example: a ridge beam reaction

The house is 7.2 m deep, so each rafter spans 3.6 m on plan from the wall plate to the ridge. The ridge beam spans 5.0 m between a gable wall and a party wall.

Roof loads on plan: 0.9 kN/m² dead (tiles, battens, rafters, insulation and plasterboard, inside the typical range of 0.6 to 1.0, so check against your build-up) and 0.6 kN/m² variable. The 0.6 is the roof maintenance load from the UK National Annex. Replace it with the site snow load from BS EN 1991-1-3 if that is greater. Beam self weight placeholder: 0.5 kN/m.

Loaded width     = 1.8 + 1.8 = 3.6 m  (half of each rafter span)
Dead line load   = 0.9 × 3.6 + 0.5 = 3.74 kN/m
Variable line    = 0.6 × 3.6        = 2.16 kN/m

Characteristic reaction each end (span 5.0 m)
  Gk = 3.74 × 5.0 / 2 = 9.35 kN
  Qk = 2.16 × 5.0 / 2 = 5.40 kN
  Total = 14.8 kN

ULS (expression 6.10)
  1.35 × 9.35 + 1.5 × 5.40 = 12.62 + 8.10 = 20.7 kN

Expression 6.10b gives about 19.8 kN. Wind is not included, and neither are any dormer or valley loads landing on the beam.

Two cautions. First, the half-and-half split only holds when the ridge beam is a true support: stiff enough that the rafters bear on it rather than leaning against each other. If the ridge sags or is only a ridge board, the rafters thrust the walls outwards, and the old ceiling joists were often the tie resisting that. Second, the beam is only as good as its end supports: a ridge on a post or a nib has to deliver this load on into something real.

Where do the beam reactions go?

Each end reaction is a point load. It needs a bearing, and the wall and foundation underneath need to take it.

• Padstones or spreader plates spread the point load into the masonry. The check is the bearing pressure under the padstone, using the ULS reaction, against the masonry's local bearing capacity.
• Party walls can take a beam end, but the Party Wall etc. Act 1996 notice route applies, and the neighbour's side of the wall is not yours to overload.
• The wall below has to carry the reaction plus everything it already carries. The point load is usually dispersed down at about 45 degrees, so it smears into a line load lower down. Existing masonry that carried a roof now carries a roof and a floor.
• The foundation below that gets the sum, at characteristic values, against the allowable bearing pressure.

Beam to foundation follows a beam reaction through to the footing. If you want to run the whole house at once, you can do the whole building in LoadTakedown.

What extra loads do people forget?

• Dormer walls. A cheek or face wall in timber frame, with cladding and lining, is a line load on a trimmer, a floor steel or the wall below. Take the weight from the build-up and the manufacturer's data.
• New roof covering. Converting often means re-roofing or adding insulation and plasterboard to the rafters. The dead load on plan can move from the bottom of the 0.6 to 1.0 kN/m² range to the top of it.
• Removed struts. Struts and purlin props often land on an internal load-bearing wall. Once they are gone, that wall carries less, but the outer walls and the ridge steel carry more. Do not assume the load vanishes.
• Stairs. A new stair opening cuts joists and needs trimming. The stair itself loads the floor at 1.5 kN/m² with a 2.0 kN concentrated load.
• Eaves restraint. If the old ceiling joists tied the roof, work out what replaces them before cutting anything.

None of these are exotic. They are the usual items that turn a quick check into a wrong number.

FAQ

What is the design floor load for a loft conversion?

A habitable loft floor is domestic category A: 1.5 kN/m² imposed, plus a partition allowance (0.5 kN/m² for light partitions), plus 0.65 kN/m² dead load typically. That makes about 2.65 kN/m² characteristic and about 3.9 kN/m² at ULS. Always check the actual build-up and any fixed partitions.

Can I reuse the existing ceiling joists for a loft floor?

Rarely. They were sized for a ceiling with light storage at about 0.5 kN/m², and the new floor loads are more than five times that at ULS. Deflection, vibration and services holes usually fail first. A competent engineer should check if reuse is proposed, but new joists are the normal answer.

How do I find the load on a loft conversion steel beam?

Work out the loaded width from the joists it supports (half the span on each side), multiply it by the floor area load to get a line load, add the beam self weight, then take half the line load times the beam span for each reaction. Do it at characteristic and at ULS. The beam section needs an engineer.

What loads act on a ridge beam?

The roof dead load and the roof variable load (maintenance or snow) on the plan area it picks up, usually half of each rafter span either side of it, plus the beam's own weight. Add any dormer or valley loads landing on it. Check the end supports can take the reaction and the horizontal thrust.

Do I need to check the foundations for a loft conversion?

Yes, where the new steels land on walls that were not carrying those loads before. The reaction is dispersed into the wall, added to what the wall already carries, then checked against the foundation. Existing foundations often have spare capacity, but you should show it with a takedown, not assume it. Start with the tributary area calculator if the geometry is awkward.