Learn to Build – Roof Construction

Learn to Build – Roof Construction


Pitched Roof Construction

The roof can be formed using kiln-dried stress-graded lumber with the sizes, spacings and fixings determined by your draughtsman. Cut single, hipped or gable roofs over 40 degrees must be braced to BS5268.

Alternatively the roof may be formed by pre­fabricated manufactured trusses, erected and spaced in accordance with the manufacturer’s specifications. Roofs may require built-in purlins or supports in either timber or steel.

Steel purlins must be painted to resist rusting and supported on padstones or spreader beams built into the supporting blockwork.

Roofs must be braced diagonally beneath the rafters at 45 degrees from the wallplate end to the ridge using 47 x 22mm timber.

Longitudinal bracing is also required at the ridge and at the ceiling node points where the bracing within the truss comes down to meet the ceiling joists.

It may be omitted where the spacing between the brace nodes does not exceed 3.7 metres.

It is also required at the rafter node points where the distance between them and the wallplate exceeds 4.2 metres.

All roofs must be tied into the walls using lateral restraint straps across at least three timbers as mentioned above.

Wallplates must be tied down to the blockwork/superstructure using proprietary 30 x 2.5mm straps at 2-metre centres.

The roof covering can consist of slate or tile plus associated capping, and verge/eaves details fixed in accordance with the manufacturer’s details for pitch and exposure as detailed on your drawings.

Tiles and slates are to be fixed to 50 x 25mm softwood battens/lathes fixed across the rafters above breathable, untearable roof felt underlay.

In Scotland the roof timbers must be overlaid with rigid sarking boarding beneath the underlay and tile battening.

I personally prefer this method. This sarking is done by nailing sterling boards side by side on top of the rakers,covering the whole roof.

Over these OSB boards is laid 3 layers of breatheable untearable felt laid across the roof, then down the roof, and finally across.

50X25mm soft wood lathes are nailed down the roof at 400mm centres and then the same size lathes are fastened across them at 90 degrees to attach the slates or tiles to.

The spacing of these cross lathes depends upon the size of the slates/tiles.This cross battening, allows an air flow up the roof to diminish the risk of rot.

The Rafters

If your extension is built against a higher building, you will need to fasten a timber wall plate against the higher wall in order to attach your rafters at the desired rake.

This wall plate is fixed by drilling through the timber into the wall and rawl bolting at 450mm centres.

Strong metal hangers can then be secured to this plate at 400mm centres the rafters will strech from these hangers to, and beyond, the wall plate that you cemented and fastened to your new wall.

They should extend at least 150mm beyond the outside front wall. This overhang is to allow for the fixing of the soffit fascia, ready for the gutter.

These rafters are cross nailed into the wall top plate, ensuring first that you are exactly square to the plate, use your folding flooring square, or the 3-4-5 method.

Measure out 150mm from the outer wall at each end of the wall, strech a chalk line between these 2 marks, and saw off the excess.

Important! Ensure that the saw cuts are perpendicular in line with the house.

By this, I mean, by placing a piece of 150mm wide, by perhaps 1200mm 4′ long piece of wood against the house wall in the upright position.

Then mark up the outside of the wood across the pretruding rafter and you have your saw mark. Now you are ready to fit your sarking boards.

Allow a small overhang at the front of the roof to cover the wood/plastic fascia.

From the bottom corner of the sawn-off rafters place a small set square against the sawn face of the rafter and get a level mark against the house wall.

Do this at each end of the front wall, and once again use a chalk line. You now need to fasten a piece of wood against the wall, or individual pieces fastened to the sides of the rafters, and hanging down.

In both cases, the bottom of the wood should rest on your chalk line.

Fix a OSB wood fascia first, then attach a PVC soffit, the type with breather slits in, to the undersides of the rafters and small pieces of wood.

Finally fit the PVC fascia using the special plastic topped pins. All the plastic should be available from the same supplier.The fascia has a lip which wraps under the front edge of the soffit.

Lead Flashing

To seal the top edge of this roof we must instal code 4 225mm wide lead sheet. This is sold in rolls at builders merchants.

Chase out the pointing of the course above the top row of slates/tiles to a depth of roughly 30mm.

Unroll the lead on the tiles and cut to length, slightly longer than the roof length.

Now, with great difficulty, and the help of at least 1 friend, fit the lead into the chased out slit, use wood or folded lead pegs to hold the flashing in place.

Hammer these pegs in place roughly every 300mm.

Once all the flashing is in the slit begin to point it in using quick setting cement. Remove the pegs as you come to them. Whilst tiling the roof.

As you reach each end of a run of tiles cut a piece of this same lead.

Cut each piece 50mm shorter than the length of the slate/tile, bend it lengthways into an ‘L’ shape.

Make one side longer than the other, ie: 50mmX 175mm and turn over the edge, the opposite way to the other bend, of the long side by 10mm.

The longer side slips under the slate/tile, with the small 10mm turn back upwards. Any rain that manages to get under the slate will be channelled down the roof into the gutter.

As you work up the roof, lay each piece of lead down before laying the slate on top of it. The same supplyer of the PVC soffit and fascia should also sell PVC tile edges that slip under the sides, and gives a nice appearance.