Loads the Envelope Adds That Nobody Drew

An attenuation roof holds rainwater deliberately and releases it slowly through a restricted outlet, and the record is explicit that the structure beneath now carries a load that appears during rainfall and disappears afterwards. It asks whether the structure has been assessed for the weight of held water in addition to everything else on the roof, and whether the structural engineer has assessed the roof for the stored water as well as for the build-up and its use.

The record is equally explicit that adding storage to an existing roof changes its loading and the duty on its waterproofing, and is not an addition that can be made without assessment. It also declines to state any storage volume, discharge rate or structural capacity, describing all of them as engineering determinations.

On an inverted roof the ballast or paving is not a finish. The record describes it as the weight holding the stack down against wind and against the tendency of insulation to lift in standing water, and says it is doing a mechanical job, so it cannot be reduced, thinned or partly removed on aesthetic grounds without changing the roof.

That is a load the structure carries permanently and a load that cannot be traded away. The record asks whether the deck has been assessed for the weight the arrangement relies on, including any surfacing intended later, which is the case that most often catches a project: the roof was assessed for the build-up, and the terrace was added to the drawing afterwards.

The trafficked deck record asks whether the structure has been assessed for the weight of the surfacing, its supports and the intended use of the terrace, which separates three things that are easy to treat as one. It also notes that making an existing flat roof accessible changes its structural loading, its guarding requirements and its maintenance regime, and needs professional assessment.

On pedestal paving the load arrives differently again. The record notes that because units are loose, load does not spread the way it does through a bedded surface: a point load on a slab corner arrives at a pedestal head and then at a small patch of whatever lies below, so the protection layer, the insulation where present and the water-resisting layer are all loaded through a concentrated area. What a roof can accept from a terrace and its intended use is a structural question for the engineer responsible for the deck.

The covering record puts one question to professionals about weight, and it runs in both directions. A heavier covering asks more of the structure beneath it. A lighter one changes the balance between the weight holding the roof down and the wind trying to lift it, which is why restraint is reconsidered rather than carried over.

The question is asked at the point the covering is chosen rather than after the old one has come off, because by then the scaffold is up and the decision has effectively been made. It is one of the few items on this list that can be settled entirely on paper.

The warm deck record describes equipment mounted on a roof as applying point loads onto a compressible thermal layer and needing access around it for servicing, and sends whether load is spread, how the support meets the membrane and how maintenance routes are kept clear to the designer. The plinth record adds that the support carries a load concentrated at a position on the deck that may not have been designed to receive one.

A walkway is the quieter version of the same thing. The junction record describes it as bearing on the plane relied on to exclude water, so what the route bears on and whether anything is fixed through the plane to hold it are conditions of the layer underneath. Both are decisions taken long after the structural design and both land on it.

External shading is the clearest case on a facade. The record describes the shading element as cantilevering out and imposing a concentrated, prying load at each attachment point, and states that the wind action it delivers to its supports follows from its area, its spacing and its geometry rather than from how heavy it looks, and is established by structural design. It also notes that wind both pushes and lifts, and that the reversal is often the case the connection has to be designed around.

The important sentence is about where the load goes. Identifying the backing element is described as a design act, because the visible facade in front of it is usually incapable of the duty, and where an arm lands instead on cladding, the cladding's own fixings quietly become structural, which is a change nobody records and no one inspects.

A precast panel facade is hung from or restrained back to the structure at a small number of points, and the record notes that support and restraint have to stay separate, because holding a panel rigidly at more than one bearing point transmits the structure's own shortening and deflection into a stiff cast unit. Fixing positions are decided before the structure is built and cannot be moved afterwards without difficulty.

Guarding delivers a different kind of load. The record describes a force applied at the top of the infill travelling into the frame, down the posts, into the base anchorage and out into whatever the anchorage is fixed to, and says the weakest link is normally the substrate behind the fixing. Substituting one infill for another changes the weight the posts carry and the way force is delivered into them, so it is not a like-for-like swap.

The common shape of all of these is that an envelope decision taken late alters something settled early, and that the person taking it is not the person who settled it. The practical response is a short list of triggers agreed at the start: if the roof will hold water, carry ballast, be walked on, carry plant, change covering weight, or have anything hung off the facade, the engineer is told before the decision is fixed.

None of that requires the client to know what any of it weighs. It requires the decision to be named as a structural question rather than an envelope one, which is what the records do each time they raise it and then hand it to the engineer.