The rubberwood tree is the tree which produces the latex from which rubber is made. The trees are grown in plantations in countries such as Malaysia. When the trees stop producing latex they are removed and milled into wood products for furniture and joinery. It has also been used in Malaysia for making MDF. If your windows will be exposed to the weather, rubberwood would not be suitable unless it has had a permanent treatment against fungal attack (wood rot). The reference book 100 Malaysian Timbers rates it "non-durable". 

The truss will have to be isolated from fire from below. It would seem to need fire resistant plasterboard on both sides of the wall, not just one, with the plasterboard continuing along the bottom chord of the truss. It is somewhat similar to Figure 72 in our Technical Design Guide #02 where a fire-rated floor system continues over the top of a fire rated wall. However, we don't have a standard detail for this situation and suggest you seek advice from a fire engineer.

Although our fire reports do not include an assessment of timber flooring on a rubber acoustic underlay, our technical department points out that the test for critical radiant flux (CRF) is a surface spread test, so substrates should not be an issue unless there is burn through. Therefore the figures quoted in Table 6 of our Regulatory Information Report 41117.11 would hold good in your case.

Our Technical Design Guide number 9 provides detailed guidance on the installation of timber floors. A copy can be downloaded free of charge from our website via this link https://www.woodsolutions.com.au/publications. Whether you need an expansion gap at all depends on a number of factors, including initial moisture content of the flooring, local climate, etc. Note the advice in our Guide to the effect that floors are generally more prone to shrinkage in cities with drier internal environments (e.g.

The Baigents were a prominent timber industry family in New Zealand for over a century, operating as H.E. Baigent & Sons until they were bought out by Carter Holt Harvey in the late 1980's. The stamp "BAIGENT 'S' MSG AS1748 90 x 35" identifies the sawmiller (Baigents), the 'S' indicates that the timber was seasoned, MSG confirms it was machine stress graded, and AS 1748 is the Australian Standard to which it was graded, but oddly enough the actual stress grade is not specified.