Modifying roof trusses to increase height for a two-post lift

[Rudi McAnichal]

You might consider installing portal frames, allowing you to remove those two trusses altogether.

Portal frames of my 15ft wide shed are made from 47x150 timber, in pairs to produce 94x150 stanchions and rafters. Stiffness is provided by triangular gussets, cut from 12mm plywood, everything held in place by 12mm bolts (located on the neutral axis of the timber).

There are big washers, and the bolts were tightened so that there is a lot of friction between the plywood gusset and the timber.

 

[addjunkie]

You might consider installing portal frames, allowing you to remove those two trusses altogether.

Portal frames of my 15ft wide shed are made from 47x150 timber, in pairs to produce 94x150 stanchions and rafters. Stiffness is provided by triangular gussets, cut from 12mm plywood, everything held in place by 12mm bolts (located on the neutral axis of the timber).

There are big washers, and the bolts were tightened so that there is a lot of friction between the plywood gusset and the timber.
View attachment 514786

Thats not actually a bad idea. Especially if built and bolted in situ to the original trusses, so as not to disturb the roof.

But I too would get a proper bloke to do some big sums, maybe steel

 

[chrismac]

I used a company called Pasquill down close to you to design my replacement shed roof trusses, basically I told them where I wanted the horizontal brace, how many trusses, along my 7m shed walls, 5m wide, first draft they had them every 400mm, I got them spaced out to 1m approx, with the head room I needed, they then edited the truss size to suit the loadings,
Than for my next shed I made my own, it was 6m wide tin sheets in a sheltered spot, no issues, I would be tempted to get 8x2's(or bigger) and fix them to the existing, or besides and put the horizontal brace where you need it.

Disclaimer I am not a joiner.

This was what I ended up with,

 

[Smallfry]

Truss manufacturers are adamant that they should not be altered in any way.

I would be inclined to replace them, one at a time, with much deeper rafters at minimum 600mm centres, and used raised collars as illustrated by Chrismac.

You could make the collars with steel strip, one each side, and maybe use steel gussets too

Some 6mm or thicker steel laid and fixed along the top of the wallplate under the rafters would stop the wall bowing too.

 

[gaz1]

if your going to change the rafters take the roof off and replace quickly then reroof that lot of it quickly its not worth messing with

but so far everyone is telling you what ive said change your roof trusses

but there is another way but you need to run support beams along the full length of the roof x2 that will allow the roof trusses to be supported and then you can cut the trusses out

 

[gaz1]

if you want to keep the framing as is with no roof being removed you will need to use beams in a certain configuration

the pic gives 2 choices

a frame to support the original beams whilst fastening to them within the frame structure

the lower pic gives full length support before cutting out

 

[Agroshield]

5% of me is thinking: jeez, lads, it’s only a little tin-roof shed.

It is the snow and wind loads on a roof that stress (load) it the most. Not falling down even in heavy rain is just a warm up exercise for it.

 

[minimutly]

I used a company called Pasquill down close to you to design my replacement shed roof trusses, basically I told them where I wanted the horizontal brace, how many trusses, along my 7m shed walls, 5m wide, first draft they had them every 400mm, I got them spaced out to 1m approx, with the head room I needed, they then edited the truss size to suit the loadings,
Than for my next shed I made my own, it was 6m wide tin sheets in a sheltered spot, no issues, I would be tempted to get 8x2's(or bigger) and fix them to the existing, or besides and put the horizontal brace where you need it.

Disclaimer I am not a joiner.

This was what I ended up with,

View attachment 514794

I have 3 of these a long the length of one of my garages, with the horizontals much lower down. I needed to cut one horizontal out to get the lift to work. That lost the anti spreading function. The answer was to fit steel wire ropes from the top of the adjacent peaks down to the where the horizontal was removed.
So my take is you need to fit something like the above, maybe three or more, and tie them diagonally top down, you could use wood I guess. Point is you have to stop wall being spread by the weight of the roof. And just to add, the horizontal shown in @chrismac s drawing doesn't stop wall spread without putting bending force into the rafters, it does however prevent roof spread at the point of fixing. You really need to add the diagonals (in tension).

 

[jo1010]

1. Modifying the middle trusses is not on for a variety of reasons (as above) but mainly because the stability of truss material cannot be trusted in anything but the original configuration.
2. So, your faced with removing those offending trusses and replacing with something substantial...4 or 5 inch rafters in a joiner-cut roof with a high cross tie maybe?....but that would also need at least 4x2 contiuous wall plates and probably structural engineering calcs if you need to satisfy a future house purchase that it was all kosher......and to do that you're probably stripping the roof covering anyway.....

 

[Dapph2]

You might consider installing portal frames, allowing you to remove those two trusses altogether.

Portal frames of my 15ft wide shed are made from 47x150 timber, in pairs to produce 94x150 stanchions and rafters. Stiffness is provided by triangular gussets, cut from 12mm plywood, everything held in place by 12mm bolts (located on the neutral axis of the timber).

There are big washers, and the bolts were tightened so that there is a lot of friction between the plywood gusset and the timber.
View attachment 514786

Making the roof support as internally fitted portals sounds the way to go . Especially as the weather is changing towards a wet windy miserably cold winter .

Also glue the triangular brace plates to the timbers on both side before bolting up & use resin fixed quick fit steel socket feet to hold them secure ( maybe make the feet sockets with anchoring holes yourself ? ) .


To make it a bit easier cut the up rights about 18 inches or so from the top ends and add welded timber dimensioned metal slide up sleeves that are though drilled & bolted up in place once you place the trusses on the verticals . Only cut the unwanted old truss wood off after all replacement trusses are in place and everything else including lights , signal cables & extra power socket , supply cabling etc. has been attached to them .

 

[knighty]

I'd sandwich the original rafters between 8x2, bolt through

either side of the ones being removed, sandwich the bottom beam in 8x2

run some 8x2 above and below those sandwiches bottom beams (flat on them) - so these run parallel to the wall, screw these into the 8x2 sandwiching the bottom beams and all the other bottom beams

then chop out all the stuff to go

 

[Rudi McAnichal]

Also glue the triangular brace plates to the timbers on both side before bolting up & use resin fixed quick fit steel socket feet to hold them secure ( maybe make the feet sockets with anchoring holes yourself ? ) .

Yes, it would seem logical to use adhesive with plywood gussets, although most documents describe use of nails to fix the gussets. My only reason for using use bolts without any glue was that they allowed me to wiggle the columns and rafters into alignment during assembly.

A few years after building the shed, I came across a paper relating to fairly small (6m span) timber portal frame buildings, and it was interesting to see the column and rafter sizes used.

"Timber portal frames vs timber truss-based systems for residential buildings" (Advances in Civil Engineering, 2019)
https://onlinelibrary.wiley.com/doi/10.1155/2019/9047679

 

[Dapph2]

My friend Don was a specialist packing crate maker ( think 60 foot long 14x14 foot packing crates for the shipping of turbines etc .

When he took early retirement he started a rabbit farm , we helped each other with our sheds his four sheds were 20 foot wide by 80 foot long .
His workshop was two old furniture pantechnicon joined back to back with the rear doors removed and a frontal access in one end .

Don drew precise plans , we made 5 mm larch ply master profiles and then drew and cut out perfect panels in OSB to make the portals .

My main job was to cut many different length as per my schedule sheet of 30mm x 40mm into various angled spars to fit glue , glue & screw inside the frames before we glued & clad the outer faces . The internal bracing spars made them tremendously strong

 

[Guineafowl]

Thanks, I’m still here, reading the replies.

The lift is in, and as planned the posts sit just under the truss outer verticals. Strong bearers can now run between the intact trusses, spanning the modified ones, and be supported by the lift posts partway along. I’ve just milled up some Doug fir into 2.5” X 12+” boards so that should be strong stuff.

Apex gussets will be glued and screwed, and probably nailed as well.

 

[knighty]

don't know why I didn't think of this before...

you could put a couple of legs in? put them along side the lift posts so they don't get in your way?

 

[chunkytfg]

don't know why I didn't think of this before...

you could put a couple of legs in? put them along side the lift posts so they don't get in your way?

Thats actually a pretty good idea. Makes total sense and doesnt impinge in the working area at all really.

 

[Guineafowl]

don't know why I didn't think of this before...

you could put a couple of legs in? put them along side the lift posts so they don't get in your way?

Well, yes, the lift posts are the legs.

 

[gaz1]

Thanks, I’m still here, reading the replies.

The lift is in, and as planned the posts sit just under the truss outer verticals. Strong bearers can now run between the intact trusses, spanning the modified ones, and be supported by the lift posts partway along. I’ve just milled up some Doug fir into 2.5” X 12+” boards so that should be strong stuff.

Apex gussets will be glued and screwed, and probably nailed as well.

dont nail it and if you do nail it you want grip nails but screws are better

don't know why I didn't think of this before...

you could put a couple of legs in? put them along side the lift posts so they don't get in your way?

even if i was beaming it the way ive suggested or even fitted posts in at 4 corners

i would still use the beams between them as then i can fasten the roof truss to it making the roof stronger

we do it all the time when building dormers

and because its with the vertical parts of the roof truss its supported at the lower end and a glued part at apex point will strengthen the roof fitting a 4x2 lower at max height will further strengthen that wood roof