GB2538849A

GB2538849A - Roofing structures for fully supported metal roofing

Info

Publication number: GB2538849A
Application number: GB1605402.5A
Authority: GB
Inventors: Boyt Paul; Boyt Alan
Original assignee: Individual
Current assignee: Individual
Priority date: 2015-04-02
Filing date: 2016-03-31
Publication date: 2016-11-30
Anticipated expiration: 2036-03-31
Also published as: GB201505806D0; GB2538849B

Abstract

The roof structure includes an array of flat rigid support boards 1 on top of a ventilated support structure, and sheet metal cladding resting on the support boards, wherein each support board has ventilation apertures 2 through it. The support boards may be rectangular and be formed of laminate or particle board. The ventilation apertures may be a regular array of slots running in at least two directions. The area of the apertures may be larger on the lower side 4 of the board than on the upper 3. There may be a barrier coating or liner sheet between the support boards and the metal cladding. Also claimed is a roofing support component in the form of a rectangular board of laminate sheet or wood or chip or fibrous material, and having a set of ventilation slots running in at least two directions. Each slot may be wider on one face of the board than on the opposite.

Description

ROOFING STRUCTURES FOR FULLY SUPPORTED METAL ROOFING

This invention relates to fully supported metal roofing and the support structures for use therein.

The use of sheet metal to provide a weatherproof roof structure is well-known. Although corrugated sheet metal roofing may be supported on a set of rafters or on an inclined set of joists, the use of non-corrugated sheet metal cladding requires that the sheet metal itself is fully supported by an underlying structure. This applies particularly in the case of sheet lead, which sags easily under its own weight, but also to roof claddings of copper, zinc, aluminium or other metal sheet.

A potential disadvantage of the use of sheet metal roofs is that moisture can condense on the underside of the metal sheet, and such condensation can lead to a premature failure of the metal sheeting due to corrosion and of the roof itself arising from damp-induced degradation of the supporting structure. In practice, for roof constructions using wood or timber supporting structures, the moisture can lead to fungal infection or rot; for roofs with metal supporting structures, it can lead to corrosion.

In order to reduce the risk of material amounts of interstitial condensation building up and causing damage, building standards often require, in the case of fully supported metal sheet roofing, the provision of a ventilated zone underneath the roof cladding or deck. In the United Kingdom, the relevant standard is BS5250: 2012 Code of practice for control of condensation in buildings.

While the provision of a ventilated zone below the roof support deck should assist in the dispersion of moisture it is found in practice that problems can still arise because it sometimes occurs that moisture can enter the roof via the laps or joints between adjacent sections of metal roof cladding under certain conditions. This moisture is then trapped between the underside of the metal roof covering and the top of the supporting roof deck.

One standard approach to providing support for a metal sheet roofing cladding material is to provide support in the form of a set of spaced boards, analogously to a set of floorboards, but with gaps between the boards to enable the ventilated zone to extend to the area between the boards and the metal cladding on top of them. In order to satisfy normal structural requirements, the boards need to be made of real sawn wood rather than e.g. particle or composite board. Using timber gives rise to a problem, viz, a tendency for the boards to warp over time producing an uneven support surface for the metal roof covering. In addition, the installation of a set of timber boards is labour-intensive and care needs to be taken to leave "penny gaps" between them in order to allow some ventilation of the space between the boards and the metal sheet.

As an alternative to the use of timber boards it is now more common to use plywood sheets. The grade of plywood should be sufficiently thick to withstand the intended loading, and sufficiently moisture-resistant not to delaminate or rot over the course of time. Both of these requirements render such sheets essentially impervious to water, so while moisture vapour may be removed from a ventilation zone beneath such plywood sheets, any moisture vapour on the underside of the metal and on top of the plywood sheet is effectively isolated from the ventilated zone. Over time, this can lead to a build-up of moisture vapour and subsequent condensation on the underside of the metal roof covering causing damage to the metal roofing and degradation of the roof structure.

It is a common practice to provide a barrier material between the metal sheet 30 and the supporting boards in order to reduce the risk of chemical damage to the metal sheet from chemicals in the boards, and to assist in allowing differential movement between the metal sheet and the supporting boards which occurs due to variations in temperature. Although building paper can be used, e.g. over plywood sheets supporting a lead sheet roof cladding, it is also known that geotextile material can be used. However, this can lead to problems if there is water penetration as the geotextile acts as a wick to attract water by capillary action and then to retain that water between the metal sheet and the support structure.

In accordance with a first feature of the present invention, there is provided a metal-clad fully supported roof structure including a ventilated support structure, an array of flat rigid support boards on top of the support structure and, resting on the array of support boards, sheet metal cladding, wherein the support boards have, formed within each board, ventilation apertures extending from one side of the board to the other.

The support boards are preferably of a standard shape and size and the preferred shape is rectangular. The support boards may be constructed of appropriate thickness laminate or particle board, for example 18mm to 30mm thick The ventilation apertures extending from one side of the support board to the other are preferably in the form of slots, most preferably an array of slots running in at least two different directions which, while conveniently generally running parallel to the edges of the support board, do not necessarily need to do so. In order to enhance the moisture removal ability of the apertures in the support board when in situ in a roof structure, the area of the aperture measured in the plane of the support board is preferably larger at the face of the board which is underneath than at the upper face of the support board.

The array of apertures is conveniently a regular rather than a random array 30 and the density of the apertures in terms of the number and size occupying the area of each support board should be such as to provide that no part of the surface of the support board is more than a given distance, for example 50mm, away from a ventilation aperture.

A highly preferred configuration of apertures is that of a set of slots running in two directions and where no slot in each support board is close enough to any other slot in that board to materially affect in an adverse way the structural properties of the board; in particular its overall bending strength should not be materially affected. To assist in achieving this, no slot is preferably aligned with any other nearby slot. This arrangement preserves the overall structural strength of the board, as well as enhancing the effectiveness of the ventilation zone under the support board connecting it with the space between the support board and the metal cladding and does so irrespective of which way round the support board is laid on the supporting structure, for example an array of joists or rafters.

Although it is conceivable to manufacture particle boards with already-formed slots, using an appropriate moulding process with blades which form the slots needing to be extracted after the board is pressed under heat to cure the resin bonding agent in it, it is generally simpler and easier to take rectangular sheets and cut slots into them using appropriate machinery.

In known fashion, the roof structure may have a lining between the support boards and the sheet metal cladding; alternatively, and preferably because it reduces construction times, the top of each support board may have a liner material on it to reduce chemical interaction between board and sheet metal cladding, or to facilitate thermal movement, or both.

In accordance with a second feature of the present invention, there is provided a roofing support component in the form of a generally rectangular rigid board formed of laminated sheets or bonded particulate wood or like chip or fibrous material and having formed in it a set of slots, the slots running in at least two directions. As noted above, preferably the slots run parallel to the edges of the board but with no slot being aligned close enough to any other slot in that board to materially affect in an adverse way the structural properties of the board. Preferably also no two or more slots are aligned with any other nearby slots so as not to weaken the overall bending strength of the board. Also as noted above, the size of the slot is preferably wider on one face of the board than on the opposite face of the board. The upper face of the board, i.e. the face of the board adjacent the underside of the sheet metal cladding, may be protected with a barrier coating, i.e. for example a polyurethane-based coating, or it may be faced with a lining e.g. of building paper or geotextile material, to assist differential movement between board and sheet metal. In the latter case, the paper or textile may have slots in it registered with the slots in the board.

The advantage of the roofing structure in accordance with the present invention is that it achieves an enhancement of ventilation from the ventilation zone below the roof decking right up to the underside of the metal roofing sheet. The use of an array of support boards means that they can be installed rapidly on a supporting structure such as a set of joists or a set of rafters and, because they are provided with the ventilation slots, they do not need to be carefully set with gaps in between them; rather they can simply be assembled adjacent one another like any other sheet material installation.

An embodiment of the invention is shown in the attached drawing in which: Figure 1 is a diagrammatic view of part of a support board unit; and Figure 2 is a partial section, on an enlarged scale, of part of the board.

The board shown, denoted 1, is made of compressed particle board of generally square shape. The thickness (T) may be e.g. 18mm to 25mm, the length and width being chosen to match standard board sizes, for example 1.22 x 2.44 m. Formed in the board is an array of slots 2. Each slot is around 10cm long. As can be seen in Figure 2, the width of the slot 2 on the upper face of the board denoted 3, is relatively narrow, e.g. about 3mm. Each slot then widens out towards the lower face 4, and the slots may be e.g. 30mm wide at the face 4. The upper face 3 preferably carries a coating of polyurethane-based varnish.

Claims

CLAIMS1. A metal-clad fully supported roof structure including a ventilated support structure, an array of flat rigid support boards on top of the support structure and, resting on the array of support boards, sheet metal cladding, wherein the support boards have, formed within each board, ventilation apertures extending from one side of the board to the other.
2. A roof structure according to Claim 1 wherein the support boards are rectangular.
3. A roof structure according to Claim 1 or 2 wherein the support boards are formed of laminate or particle board 18mm to 30mm thick.
4. A roof structure according to any one of Claims 1 to 3 wherein the ventilation apertures extending from one side of the support board to the other are in the form of slots.
5. A roof structure according to Claim 4 wherein the slots are arranged in an array having slots running in at least two different directions.
6. A roof structure according to any one of Claims 1 to 5 wherein the area of the ventilation apertures measured in the plane of the support board is larger at the face of the board which is underneath than the area of the apertures at the upper face of the support board.
7. A roof structure according to any one of Claims 1 to 6 wherein the ventilation apertures are arranged in a regular array and the density of the apertures, in terms of the number and size occupying the area of each support board, is such as to provide that no part of the surface of the support board is more than 50mm from a ventilation aperture.
8. A roof structure according to any one of Claims 1 to 7 wherein the ventilation apertures are constituted by a set of slots running in two directions, where no slot in each support board is close enough to any other slot in that board so as to adversely affect the structural properties of the board, and no slot is aligned with any other nearby slot in order substantially to preserve the overall structural properties of the board.
9. A roof structure according to any one of Claims 1 to 8 and including, between the support boards and the sheet metal cladding, a barrier coating on the board and/or a liner sheet.
10. A roofing support component in the form of a generally rectangular board formed of laminated sheets or bonded particulate wood or like chip or fibrous material and having formed in it a set of ventilation apertures in the form of slots, the slots running in at least two directions.
11. A roofing support component according to Claim 10 wherein the slots run parallel to the edges of the board and no slot is aligned close enough to any other slot so as materially adversely to affect the structural properties of 20 the board.
12. A roofing support component according to Claim 10 or 11 wherein each slot is wider on one face of the board than on the opposite face of the board.
13. A roofing support component according to any one of Claims 10 to 12 wherein at least one face of the component carries a protective barrier coating.
14. A roofing support component according to any one of Claims 10 to 13 30 wherein one face of the component carries a liner sheet having apertures in it registered with the slots in the component.