Structure
Table of Contents
Cottage Deep Retrofit - This article is part of a series.
Floor and walls #
This building is weird. I expected the walls to sit on top of the floor structure, as in this diagram from YourHome.gov.au:
…but instead, the walls sit outside the floor structure, supported only by brackets.
The front wall was only held up by these lightweight brackets: (view from below)
To give better support for the walls and roof, I added 190x45 timbers under all four walls.
Another weird thing. The walls are built in sections of about 1200mm width. And the only thing holding these wall sections together is the vertical strips of wood covering the joins. (You can see one in the photo above.) So I added direct fastenings: nails and structural screws. I also added some extra studs where needed (back corners and under windows).
The biggest opening is at the front where there was a sliding glass door. This needs a strong lintel over the opening to support the roof load above it.
You can look up a span table to find what size timber beam is required. In this case it would be 90 x 35 mm. Because the load is vertical, the timber strength depends on it being placed upright. Sadly in this case the 70 x 35 mm lintel was placed flat, so its effective size is 35mm not the required 90mm. Sure enough there is a crack in the middle:
So yeah I put in a new lintel.
Roof #
There wasn’t much to the original roof. The 3m-long metal sheets were held up these few timbers:
And the ceiling under was made up of 50 x 35 mm battens.
Clearly this stuff isn’t strong enough to span the roof by itself. It all depends on:
Exposed internal beams #
What is really holding up the roof are three sizable beams under the ceiling. They are 190 x 35 mm: the same size as the floor joists, and certainly strong enough to span 3 metres.
Next question: what is holding up these beams? Consider the photo above.
What I thought it was: beam directly supported by wall framing
What it actually was: ends of beam just screwed into the wall lining
I’m not a structural engineer, but that seems crazy. To fix it I installed blocks under the ends of the beams, fastened to new framing in the walls behind.
New posts, beams, rafters #
The plan was for the new roof to be much bigger and heavier. To support it, beams would sit on the front and back walls. The beam at the front overhangs 1.3m on each side where the walls angle back, so I decided to put in posts too. They also help to tie down the roof.
I just happened to have these Cypress Pine posts (I had bought them on a whim from Facebook Marketplace) and they came up very nicely with sanding and oiling.
New rafters — 200mm-deep I-joists — span the front and back beams.
Meanwhile, ceiling joists are fastened to the internal beams.
