Ordering Roof Trusses for a Vault Under a Taller Roof

The loading block, and the roof standing above yours

The loading block asks for a snow or imposed roof load, a ceiling load, and often a wind speed or an exposure category. It expects one number per line, and for this roof one number for snow is wrong before the pen touches the paper, because the new surface sits in the lee of a taller one and everything the wind lifts off the old roof has to come down somewhere.

Every snow code treats that step as a load case of its own rather than as a slightly larger uniform figure. ASCE/SEI 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, builds a leeward drift from the run of upper roof the wind crosses and a windward drift fed by the lower roof itself, and takes whichever is worse. EN 1991-1-3, Eurocode 1 Part 1-3, reaches a comparable surcharge through its own drift arrangements, and the National Building Code of Canada through its accumulation factors. The arithmetic is not interchangeable between them; the physical claim is identical in all three, and none of them permits the pile to be averaged into the field load.

Two dimensions size it and both are measured on the existing house rather than on your extension. The step is taken surface to surface at the abutment — old roof surface down to new — which is precisely why the heel figure from the opening section feeds into this one: raise the heel and you shrink the step. The fetch is the run of existing roof the wind travels across before it reaches that wall. Both get scaled off the survey rather than paced, because the drift grows with fetch and a value taken from the wrong elevation of the house produces a confident answer to a different question.

Then write the result on the form as a load case, not as a remark. Truss design software will accept a triangular or trapezoidal surcharge applied over a stated width from the wall, so give it the peak value at the wall, the horizontal extent, and the frames it applies to. A note in the margin saying there is a big roof next door gets read by whoever takes the enquiry, not by the engineer who sizes the webs.

The usual outcome is two truss types on one small roof: the frames nearest the existing wall are engineered for the surcharge, the rest for the balanced case, and the two look identical from the ground. They come off the same wagon with different marks stencilled on them. Set one in the other's position and the roof is wrong in a way no inspection will ever find.

The vault you sold, and the vault the bottom chord gives back

A scissor truss is described by two pitches, and the number that matters to the client is neither of them. The top chord pitch is fixed from above by the covering's minimum slope and from outside by the ridge height the consent allows. The bottom chord pitch is chosen from underneath, by what the ceiling has to clear. What the room gets is the difference between the two rises at the centreline, and it is smaller than most people expect the first time they work it out — halve the difference in pitch and you halve the vault, but halving the pitch difference costs almost nothing on the elevation, so the temptation to trim it is constant.

Treat the result as a framing dimension and not as a finished one. The plasterboard, the service zone if there is one, and any downstand at the abutment all come off the underside of that bottom chord. It is also worth checking the height at the wall rather than only at the ridge: at the bearing the vault gives you nothing at all, the ceiling plane starts at the heel line and climbs from there, and a room reads as generous or mean at its edges more than at its centre.

How far the two pitches are allowed to diverge is the fabricator's call rather than yours. The shallower the bottom chord runs, the less depth is left for webs near the heel and the more the frame deflects, so a deep vault under a shallow roof is the request that comes back refused or re-priced with a heavier chord. Ask the question at enquiry stage, when the answer is a phone call, rather than at approval stage when it is a redesign.

One consequence of the profile deserves a line on the drawing rather than a discovery on site. Scissor trusses move outward at their bearings as they load, and the truss design drawing commonly nominates one bearing as fixed and the other as sliding or slotted for that reason. That is not a detail to improvise: strap both ends hard into masonry and the frame's own movement is delivered straight into the wall head, where it shows up as a crack along the top of the plaster that nobody can explain.

The heel is a wall-height decision wearing an insulation label

A standard heel brings the top chord down onto the bottom chord at the wall plate, which leaves almost no depth for insulation at exactly the point where the ceiling is coldest and the wall below stops helping. The raised heel — energy heel on most order sheets — is a vertical block that holds the two chords apart there. Its stack-up is the depth of insulation you intend to maintain over the plate plus the depth of the bottom chord, and the fabricator adds clearance above that for the ventilation baffle so the soffit-to-ridge path stays open. Quote the first two, let them add the third, and confirm the total on the returned drawing.

How much depth to ask for is a code question rather than a preference. The adopted energy provisions — the International Energy Conservation Code, ANSI/ASHRAE/IES Standard 90.1, or in England the current Approved Document L — set the ceiling requirement for the assembly, and some editions offer a reduced ceiling value specifically where full depth is achieved over the wall plate. That allowance exists because the eave is where ceilings fail, which is a reasonable argument for sizing the heel from the insulation rather than sizing the insulation from whatever heel arrived.

Then follow the consequences outward, because a heel is not a free upgrade. It raises the fascia, moves the gutter, changes the soffit width and the net free area of vent you can fit into it, lifts the ridge against a height that may already be at the limit of the approval, and raises the point at which the new roof meets the old wall — which means more flashing, a longer chase or tray, and a taller course of brickwork to make good. On this particular job it also does one useful thing: every increment of heel takes the same increment off the step between the two roofs, and the step is what governs the drift, so the two figures need settling together rather than in sequence.

Deflection is the part the client can see

Structural adequacy and a ceiling that looks right are two different standards, and the truss will be engineered to the first. Bottom chords acting as ceilings are commonly checked against a live-load deflection limit expressed as a fraction of the span, with the tighter fraction applied where a brittle finish is involved. The limit that applies comes from the adopted code and the design standard behind it; what matters at ordering stage is that a span which passes comfortably can still move enough for a plastered vault to open a hairline along the apex, and that this is a specification decision rather than a workmanship one. If the ceiling is going to be plastered rather than taped, say so on the enquiry.

Camber belongs on the enquiry rather than on the surprise list, because it is a fabrication instruction: the plant builds an upward bow into the jig, set against dead load, so that the finished ceiling arrives level rather than below it. What the fabricator needs from you is not the camber figure but the finish — a vault that will be skimmed and a vault that will be boarded and taped are two different specifications even when they are the same truss, and only one of the two forgives movement.

Partitions under a bottom chord need their own decision. A truss rises and falls seasonally with moisture content across its depth, and a stud wall nailed hard to the underside of a chord that moves will crack its ceiling joint every winter and close it again every summer. The remedy is a slotted or clipped connection that restrains the partition laterally without hanging it from the truss, detailed at design stage rather than negotiated with the plasterer.

Anything hung from the roof also belongs on the sheet: a loft ladder, a flue for a stove, a ceiling fan on a vault, a rooflight breaking the chord line, a rail of kitchen units at the abutment, an air handling unit in the void. Trusses are efficient because nothing in them is oversized, which is another way of saying there is no spare capacity waiting for a load nobody mentioned.

Counting them, and the things on the schedule that are not trusses

Quantity looks like the easy box. Take the length of the roof, divide by the spacing, add one for the frame at the far end, and the number is close — but only close, because the last bay is a remainder rather than a full space, and the frames at each end of a small extension are often not trusses at all. A gable frame is a different component with vertical studs at board centres, a girder counts as two or three plies on one line, and a valley set over the existing slope is a sequence of diminishing frames with its own schedule. Count those separately or the delivery note will agree with the order and disagree with the roof.

Spacing itself is not free choice. It is bounded above by what the sheathing or the batten can span, by the ceiling board's own span rating, and by the fabricator's economics, and bounded below by the point at which you are paying for frames you do not need. Widening the spacing to save four trusses and then upgrading every sheet of decking to suit is a trade that has to be priced as a whole rather than settled at the truss stage.

Then look at the wagon before you confirm the date. Trussed frames for a rear extension are long, floppy in the flat, and heavy enough in a bundle that the crane or telehandler has to be booked alongside them; the access down the side of a terraced house is frequently the constraint that decides whether the roof is craned over in one lift or carried through in pieces. Confirm the offload arrangement at ordering, because a delivery that cannot be received is charged for twice.

Lead time is the last box and the one that quietly governs the rest. Once a batch is released to fabrication the timber is cut and the plates are pressed, so a change after release is a new order rather than an amendment and the frames already built belong to you. Worth knowing on a job where the abutment detail is still being argued over with a neighbour: holding the enquiry for a week costs a week, and holding it until after release costs a roof.