Case Study 1 – Roof Construction

ROOF STAGE

Our case study home is now at roof cover stage, meaning the roof is complete, gutters are on, and the roof is covered.

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This is a traditional progress payment stage, and the elevation of the home is largely in place.

It is an exciting time, as the rooms inside the home are taking on their overall shape, and features such as vaulted ceilings and dropped bulkheads start to appear for the first time.

The overall scale of our home is becoming readily apparent, with the dropped bulkhead in the kitchen along with the 40 cse ceiling in the family areas already starting to give the home a feeling of grandeur.

The external alfresco area is in place, and it adds a level of interest to the rear elevation with the split roof, and the high windows now apparent.

Roof construction is possibly the most complicated supervision stage of the build and it is covered by the light weight timber framing code, an Australian Standard, and your supervisor needs a good understanding of the required performance of certain roof timbers in order to be competent at ensuring your roof is constructed correctly. The wind rating will govern certain tie down requirements however for expediency we will assume the rating as N1 (the most common rating).

Before I give you some things to look out for, I should give you some description of the roof members themselves:

Wall plate – the timber running on top of the walls around the house used to attach the roof members

Ceiling joists – Horizontal timbers sitting on the wall plate under which the ceiling will be lined

Bolt plate – timber piece bolted to the wall used to support other timbers

Hanger – The timber sitting over the ceiling joists spanning a room and used to support an over span ceiling joist

Rafters – the main roof timbers running from an external wall plate on an angle up into the roof.

Ridges – the timber piece where the rafters are supported at the apex of your roof

Hips – running from an external corner of your house into the roof area with creeper rafters coming off them

Valleys – running from an internal corner of your house into the roof area

Valley Boards – running either side of the valley and used to support the valley gutter

Underpurlins – horizontal members supporting the rafters

Struts – also called toms, these are the timbers running from the wall plate to the underpurlin to support the roof

Battens – the horizontal timbers on top of the roof supporting the steel roof sheets

There are also ancillary timbers such as soldiers, outriggers, beams and braces

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Whilst it is unlikely you as the client can easily sight problems, there are some obvious things to look out for:

  1. Materials: most roofs are now constructed out of “Blue Pine” a product with a treatment to resist white ant damage. It is an H2 timber and is for use only inside the roof cavity. Untreated roof timbers are still allowed however they are not often used. Where you have roof timbers externally, they must be H3 where they are not in contact with the ground, and H4 where they are. Where a pine member is exposed it should be green in colour (H3 is treated with Arsenic giving it a green tinge).
  2. Holding Down Straps: in our last instalment we spoke of holding down straps. These are the metal straps coming out the top of your brickwork, and they are used as the first point of tie down for your roof. These straps should be turned down over the closest possible rafter and nailed in place. Some builders will strap these over the roof battens, but as long as you are bugling down the battens (using bugle screws for fixing the batten to the rafter) then this is not a requirement.
  3. Bolting Plates to Steel Beams: ensure there are bolts running through the steel beams and the plate above, not skew nailed or similar.
  4. Struts strapped top and bottom: every strut should be strapped at the bottom to the wall plate, and at the top to the underpurlin. This can be done with triple grips or hoop iron strap.
  5. Rafters triple gripped to wall plates: where the holding down straps haven’t been installed over the rafters or battens, the rafters must be triple gripped to the wall plates. This is especially important when the rafters are attached to a bolt plate, on a garage wall for example.
  6. Battens bugled to the rafters: This is important, but difficult to see from the ground. Each connection on the top and bottom batten and those battens on the hips and valleys need to be bugled. They should also be hit/ miss bugled across the remaining roof as a minimum.
  7. Roof planes not in wind or wavy: each plane of roof should run true from side to side without going into wind, or waving up and down along its length
  8. Dropped bulkheads: these should be strapped from bottom plate to a support beam where over 3 courses in depth. These become heavy roof sections and can sag without this additional support
  9. Beam connections: where two timber roof beams are connected they should be supported by a steel angle plate either bolted or bugled into position
  10. Fascia installation: fascia should be level (eaves gutters are not laid on a fall, a common misconception). Internal and external fascia corners should be square to the line of the fascia.
  11. Downpipes: at a maximum centre of 12m or as often as required to service each plane of roof
  12. Batten positions: the bottom batten should be visible just above the top of the fascia line and should be parallel with the fascia. The battens either side of a ridge should be at about 150 to 180mm from the ridge and run parallel with it.
  13. Gutter installation: box gutters should be cut into rafter sections and supported by gutter boards (along a parapet wall for example). Eaves gutters should be level and corners should be well mitred and sealed from the inside.
  14. Flashing: there may be many different spots for flashings, usually where a vertical surface such as brickwork meets the raking surface of a roof plane. Flashings should be of one piece where possible with a cover flashing in place cut into the brickwork above or underneath cladding. Flashings should also be siliconed down to the roof sheets where necessary.
  15. External framing: Where framing is used as the external leaf of a cavity wall (for installation of lining boards) the external leaf should be insulated where it abuts an internal conditioned space. All framing should have builders paper applied prior to lining externally. It is also a good idea to provide a waterproof barrier (such as Alcor) between the frame and the top brick course.
  16. Tying down beams: all brick piers should have a 10mm rod exposed at the top of the pier, This is attached to the footing and should then be connected to your roof construction. This connection varies with the circumstances, and position of the timbers and rod. It is often necessary to weld plates to the rods and bolt or bugle them into the beams accordingly. This is especially important on verandahs as there is a lot of potential for wind to get underneath the structure and lift the roof.
  17. External beams and posts: should be plumb and level as these are going to be seen easily, and can usually be lined up with other construction such as brick walls.
  18. Colorbond ridges should be straight with consistent line of screws either side.
  19. Roof sheets to be screwed down using rubber stopped hex head screws.
  20. Valleys to be cut straight with a consistent margin either side of the valley gutter.
  21. Whilst it isn’t a requirement, it is a good idea that where you will have a raking plasterboard ceiling (in this home it is under the raking alfresco), that you put sarking under the colorbond sheeting. Sarking is a waterproof builders paper, and will help control condensation on the underside of the sheets moistening the rafters inside the roof space. With a plasterboard ceiling attached directly to the underside of the rafter there is potential for transitional moisture to get to the sheets and cause a shadowing effect along the lines of the rafters.

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Beam to beam plate connection                             Holding down strap over rafter                                         Batten slightly above and level with fascia

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Dropped bulkhead strapped to beam                      Kitchen variation in ceiling heights                                View from doorway leading into Family

The next stage of construction will see the external lining, plastering, and ceilings complete.