The free diy home improvement guide with answers to your questions on a wide range of do it yourself projects.


First Floor Roof Sections

Previous Stage
Slating the Roof

With the main roof complete, the smaller first floor roof sections are constructed. This is a fairly complicated detail but adds significantly to the appeal of the external elevations of the new extension. If you’ve been following progress, you’ll know from previous articles that the second floor room above sits on some fairly hefty steels and is smaller than the ground floor extension. Along two sides there is therefore a small section of roof spanning between the two.

Hip and main rafters Hip and main rafter junction

The first part of the job is to set out the timber frame work for this. This doesn’t look like a particularly easy thing to do, but the carpenter has plenty of experience with this and soon has the position of the upper line of the rafters marked out as well as the point at which the hip timber will be fitted. Timbers are bolted to the side wall of the second storey wall to support the ends of the rafters. The hip and the first rafter for each elevation are fixed first. These are the most complicated elements as you can see in the pictures. The hip has a compound angle at the top as well as a birds mouth joint where it sits on the wall plate at the other end.

Side rafters fitted

With the three main timbers at the corner fitted, the full length rafters on the side elevation are cut in. These have a birds mouth joint at either end to accommodate the supporting timber at the top and the wall plate below. These are also complicated by the junction with the neighbouring property’s flat roof.

Roof window opening trimmed

As you can see in the picture taken from underneath this elevation, one of the rafters is left out part of the way along this elevation where a roof window is to be fitted. The rafters on either side are then doubled up and trimmers (also doubled up) are fitted above and below the position for the window. Stub rafters are fitted above and below the opening

Timber supporting rafter Birds mouth joint

Timbers are bolted to the rear wall of the second storey to support the ends of the rafters on this elevation. The rafters are again cut with birds mouth joints at either end. These are then fixed in position followed by the infill rafters spanning between the wall plate on both elevations and the hip timber.

Junction with flat roof – felt left loose

Fortunately the flat roof next door was only recently built. This detail for the junction of the two roofs was taken into account and the felt has been left loose here. A 300mm wide sheet of ply is fixed across the ends of the rafters which will allow the felt to be dressed over the junction and the slates laid over the top of that to produce a neat watertight detail.

Timber framing to support soffits Rebated fascia fixed over projecting soffit

On the rear elevation, short lengths of timber are cut and fitted between the ends of the rafters and onto the external skin of the cavity wall. These horizontal timbers will provide the support for the soffit boards along this roof. Where the roof meets the end wall of the house, the timbers are framed out round the end of the wall as you can see in the picture. The low maintenance white plastic soffit board is cut to length and fixed to the underside of these supports. The black plastic fascia board is then cut to length, slotted over the projecting soffit, and fastened to the ends of the rafters with screws covered by black plastic caps.

Roof insulation between joists Soil pipe and roof light

Rigid thermal insulation sheets are cut to size and installed between the rafters. As you can see, there’s not much space, but these rigid foam insulation panels are very efficient in reducing heat loss. The foam is very lightweight and easy to cut making installation a very quick job. With this in place, the roof covering can go on. As with the main roof, a roofing membrane is laid and tacked in place, followed by roofing battens set at suitable intervals. On the side section of roof there are a couple of significant details to work around. There is the soil pipe coming down from the bathroom which feeds through a special flashing detail where it passes through the roof. Adjacent to this there is a newly installed roof light.

Laying the slates and fixing Slates completed

The roofers commence laying slates on the rear elevation first – starting from the bottom, the slates are laid with an overlap of just over 50%. This means that at any given point there are at least two thicknesses of slate on the roof. It’s interesting to watch the process of fixing these slates. Each one is laid in position and a light line scored to mark the position of the batten for fixing. Using the pointed head of the slater’s hammer, the roofer makes a hole for each of the two fixings. The slate is then repositioned and, using the flat head of the hammer, fixed with galvanised nails. At the hip, slates are cut to suit the angle and fixed in place. Work then starts on the side elevation. The slates are laid in a similar fashion. At the roof light and the soil pipe, slates have to be carefully measured and cut to sit neatly round these two details.

Lead flashing over slates completed Sloping flashing being fitted

Finally, the lead flashing is installed. This runs along the top edge of the roof where it meets the wall. It comprises two separate strips of lead. The first is shaped to cover the junction between the roof and the walls and is nailed in position using galvanised nails fixed through the lead towards its top edged. A grinder is used to cut a groove in the wall just above this and the edge of the second lead strip is wedged into it using small off cuts of lead. The lead is then dressed down over the previous lead work before pointing the top edge in the brickwork to form a weatherproof flashing. On the side elevation, where the sloping section meets the back of the house, a stepped flashing detail is needed to accommodate the slope. Sections of lead approximately the same length as the slates are cut and prepared so that they are formed into right angle sections. These are fixed to the wall and lap under each slate on the slope. With these in place a grinder is used as before to cut a slot in the wall and a second strip of lead is wedged into it before being dressed down over the previous sections.

If you live in the Hertfordshire area and are looking for a professional building contractor, you can get in touch with G L Smith and Sons via their website:

Previous Stage
Slating the Roof

G L Smith and Sons