Thursday, September 1, 2022

14 Things to know about roof structures

Selecting the right type of wall construction is one of the most important stages in the design of any self-build or home extension project. Yet when it comes to roofs, the decision-making process rarely extends much beyond the choice of tiles or slates.

Roofs are one of the most complex, aesthetically critical and potentially troublesome parts of the build, so it’s a little worrying that more time is often devoted to choosing kitchen units than making sure the roof is fit for purpose. Here are the top 14 things you need to know before putting that roof over your head…

1. Roof design factors

Picking the right ‘hat’ for your building will depend in large measure on what it’s sitting on. The layout of the rooms below and whether the roof needs to span across large open spaces without interruption will be key to the design of the roof structure.

As well as resisting everything the elements can throw at them, roofs also need to look right. Steeper roof slopes are normally required to accommodate habitable attic rooms, although there may be planning restrictions on height to contend with. How steep or shallow the roof is will also have a bearing on your choice of coverings, which need to be technically compatible with the degree of slope.

Once you’ve got an outline roof design, a structural engineer will need to be consulted on how best to achieve this in practice. This task will be a lot more straightforward where roofs are sitting on a simple oblong box without multiple corners or complex level changes. Engineer’s calculations will need to be produced to factor in all predicted loadings including ‘dead’ loads (from the building itself ), ‘imposed’ or ‘live’ loads (from occupants and contents), and those from wind, snow and so on. 

2. Trad vs. modern?

Until the late 1960s, most roofs were constructed in time-honoured fashion by chippies cutting and joining lengths of timber on site. But with the advent of labour-saving prefabricated trussed rafters, traditional ‘cut timber’ roofs were largely superseded in volume housebuilding. In the 1990s, technology once again moved on with the introduction of pre-insulated manufactured roof panels which promised to save even more time on site. Today any of these options, or some combination, could potentially achieve the optimum roofing solution, depending on what best suits the style and function of the building.

3. Cut timber

The blueprint for a standard ‘cut timber’ roof is fairly straightforward. The main roof slopes are formed from a series of rafters cut from standard lengths of timber (traditionally 4x2 inch). In dual pitched roofs the rafters are propped up against each other to form a triangle, with a long length of timber known as a ridge board running along the full length at the apex. Each roof slope is normally given extra support underneath around mid-span typically in the form of thick horizontal timbers known as ‘purlins’, which may need to be supported by struts resting on load-bearing internal walls.

4. Prefabricated trusses

Unlike custom-made traditional roofs, prefabricated triangular trusses are delivered ‘oven ready’ to site and simply need to be positioned in series to form a basic roof structure. Despite the comparative thinness of their timber component parts, a lot of strength is derived from the overall integrity of the structure, typically with distinctive ‘W’ shaped webbing in the centre. The bottom ‘chord’ of the triangle doubles as ready-made ceiling joints with no need for support from purlins, struts and so on.

5. Panelised roofs

Panelised roofs are prefabricated roof slopes delivered to site ready to be craned into position. They are mostly made from thick SIPs (structural insulated panels) comprising two sheets of plywood or OSB with a filling of insulation sandwiched in between. The panels are hoisted onto a prepared supporting framework at ridge, wall level and sometimes also at mid span. 

6. Best roof for quick install and wide span

Take a look around any large residential construction site and the chances are you’ll see lorryloads of trussed rafters awaiting installation. This type of roof structure is still the number-one choice for mass-produced ‘box’ homes. The appeal of manufactured trusses is that they’re much quicker to install on site than the handmade variety; a basic roof structure can be built in a day. Also, because they can span eight metres or more from wall to wall there’s no need for internal load-bearing ‘spine walls’. And because trusses are spaced further apart (600mm centres) than conventional rafters they use less timber than traditional roofs.

Their main drawback is obvious to anyone who’s ever popped their head into a loft — the forest of timber webbing is a major impediment to movement within roof spaces. And because the timber components are so thin it makes them difficult to alter or customise. Today, a number of variations are manufactured for different house styles, but with less advantage in terms of cost and build speed. ‘Attic trusses’ are a popular modern variation with an open centre for attic rooms or subsequent conversion. Trussed rafters are ideal for multiple housing with relatively simple house designs and suitable lifting/handling equipment on site.

7. Best roof for bespoke designs

Traditional hand-cut timber roof structures are ideally suited for bespoke house designs, and for extensions that need to accommodate dormer windows, multiple valleys, hipped roofs, split-level mansard roofs and the like.

If you want your design to feature big, spacious, open ceilings then the custom build approach is normally best, although some types of trusses can accommodate higher-level ceilings. Another advantage of the old-school approach is that the raw materials are readily available, without the lead-in times required for manufactured components. Building the structure on site is also a very forgiving method that can accommodate last-minute design changes. Plus, traditional timber rafters look great if you want an exposed structure.

The main downside is slower speed of construction and the cost of employing skilled trades to get all the detailing right. Also, the amount of timber required is  greater than for the equivalent trussed roof

8. An all-in-one solution

Panel roofs are very quick to install but are comparatively expensive, not least because they need to be craned into place, with important implications for cost and site access. The big savings come later without the need for follow-up insulation work because it’s already embedded within the panel, making panel roofs ideal for habitable loft rooms. They also avoid the need for inserting cumbersome high-level steel beams. But as with prefabricated trussed rafters, factory-made panels are suited best to fairly simple roof shapes.

9. How is a roof structure supported?

Left to their own devices, rafters have a nasty habit of pushing the upper walls causing them to lean outwards, a common defect in period homes known as ‘roof spread’. This is more of a risk where the lower rafters lack sufficient restraint, a role that’s normally performed by bedroom ceiling joists or horizontal timber ‘collars’ that link opposing pairs of rafters together near their base (structural engineers refer to this as ‘triangulation’).

Rafters are also potentially prone to sagging in the middle if unsupported, hence the need on traditional roofs for large purlin timbers along the undersides, which in turn are propped up by struts or hangers resting on a load-bearing internal wall. Alternatively, steel beams are today often specified to transfer loadings while leaving roof spaces clear. Or, as touched upon earlier, modern trussed rafter roofs can span across the entire living space so only require support from the main external walls.

10. What does a roof structure consist of ? 

Like external walls, roof structures comprise a series of layers designed to provide structural strength and weather resistance. Once the roof ’s skeleton is in place, large sheets of breather membrane are draped over the rafters before being secured in place by horizontal rows of battens nailed to the rafters. Roof tiles are later hung from the battens. The membrane underlay has a dual purpose — to provide a secondary barrier against the ingress of rain, and to allow potentially damaging indoor humid air to escape.

In homes where attic spaces are used for habitation the roof slopes play an extra defensive role as part of the building’s ‘thermal envelope’. They need to be fully insulated to minimise heat loss, in line with Building Regs. High levels of thermal efficiency are easier to achieve where insulation is added at rafter level (‘warm roofs’) compared to conventional lofts where insulation is laid at ceiling level (‘cold roofs’).

11. How to join to the wall plate

To prevent new roofs taking flight at the first gust of wind, it’s obviously essential to firmly join them to the sub-structure below. The primary connection is at the point where the roof sits on the main walls. So a long strip of timber ‘wall plate’ is cut by carpenters ready for brickies to bed in mortar on top of the main wall inner leaf. Wall plates must then be secured with long metal anchor straps that extend down the inner face of the walls below, firmly screwed in place. A small V-shaped ‘birdsmouth’ cut is made in the lower inner faces of the rafters so it neatly interlocks into the wall plate before being secured by nailing. With prefabricated trussed rafters special truss clips are used to secure them to the wall plates. In most cases the ceiling joists ends will also rest on the timber wall plates.

12. What is wind loading, and why does it matter?

Unless roof structures are firmly anchored in place they can be vulnerable to ‘wind uplift’, where extreme wind pressure can cause roofs to literally lift off. This is why building control pay a lot of attention to the various ties and anchor straps needed to secure roofs to the main walls.

So one of the most important aspects of any roof design is to ensure that it can cope with predicted wind speeds in the immediate location of the property. For example, roofs on buildings in coastal regions or on the crests of hills will obviously need to be designed to resist higher wind speeds. Generally, the lower the roof pitch, the more wind uplift will be acting on it. In more exposed locations all roof tiles must be nailed in place and ridge tiles must be mechanically fixed. A structural engineer will calculate wind loadings as part of the design process.

13. Interstitial condensation

Because keeping the rain out is the most fundamental task for any roof, the fact that dampness also emanates from inside buildings can sometimes get overlooked. Unless roofs are properly designed, humid indoor air from daily activities like cooking and bathing can result in serious risks to roof timbers if water vapour finds its way inside insulated roof structures. So to protect the roof structure and prevent any risk of ‘interstitial condensation’ from moisture penetration, vapour barrier membranes need to be incorporated within the inner layer of insulation. Care must be taken not to punch holes in these defences when later installing downlighters and cables. A secondary line of defence against damp can be provided by leaving a clear ventilation path under the tiles so that any trapped moisture is free to safely evaporate. 

14. Rising cost of roof structures

The predominant raw material in roof structures is of course timber. And as everyone knows, timber has been one of the worst affected materials from inflation, roughly doubling in price since 2019. Current forecasts suggest prices are likely to keep increasing because most timber is imported and hence vulnerable to currency fluctuations and global trade restrictions, as well being affected by rises in transportation costs. Roofs also consume large quantities of insulation materials which are mostly petrochemical based and hence vulnerable to world energy price fluctuations.\

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