Roof

The whole roof, starting from the walls

The Roof has its own button on the LSF card, and it only lights up in plan. Its box has two modes. Single truss places one truss between two walls, on its own (see Roof trusses). By outline is the one on this page: you mark the walls of the outer outline, and it covers the whole outline, with the roof planes, the ridge, the hips, the gables, the trusses and the tile support.

The result is an object, the roof TLn, and not a handful of loose bars. It opens again with one click, changes pitch, tile or eave, and its bars follow the walls when they change.

In the example house: TL1, 2 planes at 30%, ceramic tile, 0.60 m eave, in-line structure. The 7 m sides rise as gables. The result is 14 trusses, 12 common and 2 gable trusses, 22 batten lines and 3 bottom chord bracing lines: 605.2 m of section.

Covering the outline, not a span

The single truss (see Roof trusses) spans a gap: it is born between two walls and stays there. By outline covers an outline. You mark the outer walls, and the tool closes their outline. The roof plane passes on the outer face of the walls, at the level of their top (the end level), and each side of the outline either slopes down or rises as a gable.

A side that slopes down becomes a roof plane and gets an eave. A side that rises stays upright up to the roof, as a gable wall, and gets a gable truss on the wall axis. From the combination come the ridges, the hips and the valleys: you draw none of them.

The same house, two roofs. On the left, all four sides slope down: 4 planes, one ridge and four hips. On the right, the 7 m sides rise as gables: 2 planes and one ridge, which is what the example house uses. Schematic drawing.

One object, TL1, and its bars

Section through a common truss of TL1 over the 7.00 m walls. The bottom chord rests on the top track, at 2.80 m; the top chord rises at 30% up to the ridge; verticals and diagonals between the two; the batten runs on top, at the tile gauge.

The roof classes

Nobody chooses the truss type: it comes from the section of the roof plane at that point. Each type gets a class, with a code and a color.

The label of a bar

The bar clicked in step 9 is TL:TL1:T5:bs2:1: roof bar, of TL1, fifth truss, top chord 2, segment 1 counted from zero (the tooltip says "segment 2 of 5"). In the common truss: bi bottom chord, bs1 and bs2 the top chords, m1 to m7 verticals, d1 to d6 diagonals. The battens are R1 to R22.

Reactive, but not stubborn

The TL bars follow the walls: change a wall and the roof rebuilds itself, with no Ctrl+Z step of its own. A bar you edited by hand is never overwritten: the card lists it with Keep and Redo. Once kept, it no longer rebuilds on its own.

In the calculation, the batten leaves the model, because it crosses the chord with no node; its weight goes by area. The document stays whole.

In-line framing, names and limits

Tile, pitch and structure in the example house

The tile drives the rest. It sets the minimum pitch, what goes under it (batten, purlin or OSB), the spacing of that support and the weight that comes down through the roof. The Brazilian rule, as it is in the catalog:

What the box starts with

It opens with 30%, a 600 mm eave, ceramic tile and In-line structure. Grid only shows up when the market has its spans; Brazil does: truss from 2.5 to 4.0 m, purlin from 1.5 to 2.5 m and rafter from 1.0 to 1.4 m.

Why In-line here

The house is already on the 600 mm module. In-line puts one truss over each stud, and the load goes down in line to the foundation, as item 4.1 asks. Grid spaces the trusses out and adds purlins and rafters: it makes sense on a large span, not on a 7 m house.

Why 2 planes with gables

With the 7 m sides as gables, all the middle trusses are the same (12 TC), and the gable wall closes over the whole wall, with the verticals of the gable truss on top of the wall studs. The 4-plane roof also closes on this outline, with four hips and truncated trusses near the ends: it is the choice when the facade calls for an eave on all four sides.

1. Delete the single trusses

Select the single trusses T1 and T2 (see Roof trusses) and delete them with Delete. The walls stay as they are.

2. Open the plan and the Roof

The Roof button on the LSF card only works in plan; outside it, the tooltip says "Open the plan: the roof is placed over the walls". With walls selected, the box opens with them. With no selection, it takes the walls of the rooms that have no roof. The mode is By outline.

3. Check the outline

The marked walls turn amber, and the one under the cursor lights up. A click adds or removes a wall; the selection window adds several. The box says "6 walls · closed outline" and draws the model with all four sides sloping down: 4 planes, a ridge 1.06 m above the wall top and four hips. The six include the two inner walls, 1 and 4, which the automatic selection picked up.

4. Gables on the 7 m sides

Click the side number on the model, or in the Sides list ("slopes down; click to rise as a gable"). With 3 and 2 as gables: 2 planes, one ridge, no hips.

5. Pitch and tile

At 20% (11.3°) the ridge drops to 0.71 m, and the amber warning shows up: "Walls 5 and 6 are below the 30% of the ceramic tile." Generate stays lit. Go back to 30%.

6. The refusal

With all four sides as gables, nothing slopes down: "No side slopes down: mark at least one side for the roof to come down." Generate turns grey and the model, dashed. Set 5 and 6 back to slope down.

7. Enter, or Generate roof

The message confirms: "Roof TL1: 2 planes, ridge 1.06 m above the wall top. 14 in-line trusses." TL1 joins the Project roofs list, with 30% and ceramic tile, and the plan shows the trusses and the battens over the walls.

One Ctrl+Z

Generating is one Ctrl+Z step, with everything inside. If the grid of a wall does not match the trusses, the tool aligns its studs, and the message adds "studs aligned in N walls". Here it did not add it: the grids already matched.

The 14 trusses

The 12 common ones land every 0.6 m, over the studs. Between T10 and T11 the spacing is 1.8 m: there the walls have no regular stud, and In-line only places a truss where there is a stud. The 2 gable trusses run over walls 3 and 2.

The gable truss

Its verticals land on top of the wall studs, with no diagonal. It has no bottom chord between the ends: the top track of the wall is already there. That is where the gable wall cladding is fixed.

8. Reopen the roof

Click TL1 in the list, or one of its walls in plan. The box reopens as "Roof TL1", with Apply and Delete, and below it the class card: Tesoura comum (common truss), TC, 12 pieces, 324.5 m; Tesoura de oitão (gable truss), TO, 2, 27.3 m; Ripa (batten), RP, 22, 226.4 m; Travamento do banzo (bottom chord bracing), TV, 3 lines, 27.0 m. Total: 605.2 m.

• A click on a class selects all of its bars; the eye hides it.
• Roof sheet creates an A3 with the isometric of the project and each truss type highlighted, and takes you to the Sheets tab, in one Ctrl+Z step.
• Apply rewrites with what changed. Delete (or the Delete key) removes the roof: "Roof TL1 deleted. The walls stay as they are."

9. Tab in 3D

With the Select tool and no LSF tool armed, rest the cursor over a roof bar. The tooltip shows the level, and Tab steps down one at a time: Object, Assembly, Whole piece, Bar, and back to Object. The click selects at the tooltip level.

The fields of the box

• Enter: generates the new roof, or applies to the open roof
• Delete: deletes the open roof; the walls stay
• Esc: leaves the tool

When Generate turns grey

The refusal shows up in red, turns off the button and says what to do. The wall numbers below are examples: the box shows those of your project.

Warnings: Generate stays lit

Loads, wind and diaphragm

The roof with structure carries the load through its own bars. Each share acts on its own area and is split among the bars that hold it:

The steel of the trusses enters through the solver self-weight. The covered walls leave the point load at the top of the stud; a wall with no roof with structure keeps it. In the example house, by the app math with the default wizard values (without the battens weight):

• 40.2 kN: tile: 46 kgf/m² x 89.1 m² of roof plane
• 12.4 kN: ceiling and insulation: 20 kgf/m² x 63 m²
• 20.9 kN: live load: 25 kgf/m² x 85.3 m²

Wind

Each roof plane becomes a roof panel, of an enclosed building, with the roof coefficients of NBR 6123:2023, and the pressure reaches the layer where the tile rests. Each gable becomes a panel, taken by the verticals of the gable truss. With no truss and no rafter: "Light steel framing: roof TL1 has no truss or rafter drawn, and was left with no wind."

Diaphragm

With a gable truss and no bracing drawn on the roof plane, the calculation adds fictitious bars, rigid and pinned, in the plane of each roof plane, and warns: "...The calculation assumed the roof plane as a rigid diaphragm... Check that the board, the battens and the connections ensure this." A strap drawn in the roof plane takes their place.

Assumptions, buckling and deflection

The element of the chord is the stretch between panel nodes and supports, where a web member, vertical or track arrives. Its deflection is measured relative to the straight line between its ends, with its own L.

Purlin, rafter, hip, valley and eave

In the Grid structure and under a tile that calls for purlins, the purlin has its deflection checked at L/180, and at L/120 under wind alone; the rafter, the hip and the valley at L/250, as roof beams. Both are measured on the span between supports (NBR 16970-2, Table 3). At the eave, the cantilever stretch leaves the element: the tip deflection is checked against twice the cantilever length, minus how much the node it springs from moves along with it.

The assumptions in this section are those of a new project: CalcSteel versions the calculation, and the result of an older project does not change on its own.

What stays with the designer

1. The deflection of the whole truss as a beam: L/250 of the span, 28 mm over the 7.00 m of the example house. The engine checks the chord segment by segment.
2. The ceiling installed with the fixing the calculation assumes: with a market ceiling, the bottom chord counts as braced at the board screw spacing.
3. Web crippling (NBR 14762, Annex D) of the chord over the track and at the truss nodes.
4. The edge and corner wind zones, the underside of the eave and the anchorage against suction. The model uses one coefficient per roof plane.
5. The batten and its span between trusses, where the spacing opens up (1.8 m between T10 and T11, in the example house).
6. The connections: the screws at the nodes and the truss heel on the track (see Connections panel).

The path, in one line

Delete the single trusses → plan → LSF card → Roof → By outline → check the outline → sides: slope down or gable → pitch, eave and tile → In-line → Enter → TL1 card → 3D.

The example so far

• TL1: 2 planes at 30%, ceramic
• 14: trusses: 12 TC and 2 TO
• 22: batten lines, 380 mm gauge
• 605.2 m: of section on the roof