The floor is a new structural task
Ceiling joists were not necessarily designed to carry an occupied floor. Planning Portal explains that new floor joists may be required, supported by suitable walls or beams, with the stair opening incorporated into the arrangement. The design must consider the whole floor rather than merely add boarding over the existing ceiling. [2]
Give the designer the intended room layout before the structural package is finalised. Ask for the stair opening, bathroom position and proposed partitions to appear on the same coordinated information used for the calculations.
Purlins, struts and ridge beams
Purlins and their supporting struts form part of the roof's support arrangement; Redbridge specifically identifies alterations to these as requiring structural consideration. Removing a strut because it obstructs a bedroom changes the roof's existing load path. [1]
A ridge board in a traditional tied roof should not be confused with a structural ridge beam. Where the design uses a ridge beam, it carries roof loads to designed supports. Changing the way rafters are tied or supported can therefore require more than strengthening the new floor. The engineer must determine the appropriate arrangement for the actual roof. [3]
Ask the engineer to mark which existing members remain, which are altered and what replaces their function. Avoid relying on an unlabelled photograph or a builder's sketch when different parts of the roof are being removed at different times.
Follow the load down through the house
Existing walls and foundations must be considered, including earlier alterations below the loft. Planning Portal notes that an old ground-floor knock-through may have introduced a beam whose capacity needs reconsidering when loads above change. A wall visible in the loft is not proof of uninterrupted support to the ground. [4]
Ask what existing drawings, opening-up or foundation information is needed before the design can be completed. Record unresolved assumptions so they can be checked before the affected work starts.
Fit structure into the usable space
Review the finished floor level and beam positions with the architectural designer before accepting the room layout. Ask to see where the structure affects stairs, ceilings, doors and service routes. This is a coordination exercise: do not instruct the contractor to notch, cut or relocate a member to regain headroom without design review.
Where a beam is inserted into a party wall, the Party Wall Act process may also apply. GOV.UK identifies cutting into a party wall to insert a beam as notifiable work. That procedure is separate from the structural calculations. [5]
Keep the temporary roof safe
HSE requires stability to be planned throughout structural alteration. Confirm who designs and manages support while the old roof members are changed and the new structure installed. Agree inspection stages, retain revisions and do not cover connections before the relevant checks are complete. [6]