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neil_9 09-29-2010 07:55 PM

removing rafter ties: garage conversion

I am in the process of converting my garage, and installing a fairly light weight first floor platform, for small amount of storage. Any suggestions will be most welcome. The main issue is how/if its possible to remove some rafter ties in order to make more space above the platform.

Dimensions are as follows:
total garage size: 6 x 4.5 metres (20 x 15')
total platform area: 3 x 4.5 m (10 x 15')
The platform is only in half of the garage space. However, to make the space above the platform more usable, I will need to remove atleast 4 consecutive rafter ties. There are a total of 14 rafter ties across the entire garage, and 1 'massive' rafter tie in the middle of the garage (3x10'') which connects the main structural rafter, which in turn holds the purlin. The other 14 smaller rafter ties are 2 x 3'' timbers, and are spaced around 40cm apart (1'6'') on each rafter, i.e. one rafer tie for each rafter.
So my question is: is it possible to remove four of these smaller rafter ties? and what can I do to compensate?

From what I understand, collar ties will not really help, but perhaps they will be better than nothing, and prevent the roof from sagging a little.

The other idea I had was to put in 'side legs' in place of the rafter ties, and also tie in the rafters to the purlin, thus preventing the rafters from slipping and losing it's 'triangularness'. These 'side legs' would connect from the platform to the purlin, so as to support the purlin, so as to counter-act the weight of the rafter above. The link to photo below sort of gives an indication of the 'side leg' approach:

unfortunately, my floor joists do not connect to the rafters, thus cannot act as rafter ties, so I will need an alternative to supporting the rafters once I remove the rafter ties.

Other dimensions of this build are as follows ( platform timbers):
so far I have used two main beams to support the platform each 4.5m spans, using 2x9' timbers. These main beams are supported by the brick walls using joist hangers - each hanger can support 6.7kN, according to the manufacturer. On top of these spans I have laid (at 20cm intervals) 3m lengths of 2x3' timbers. On top of this is some plywood flooring. All in all, this is not a super strong structure, but I might re-inforce it if I need to take addtional load from the roof, if/when i take out some of my rafter ties. Only 4 rafter ties need to be taken out, so hopefully this won;t weaken the roof too much.

Many thanks for taking the time to read this post, and hope to hear back your suggestions,


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