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Exterior veneer trusses

2/19/23       
Orin N Zelenak Member

We have some fascia to fabricate in euopean rift white oak which will be in a covered porch so no rain but full exterior. The veneer would be +/- 0.6mm thick and hot pressed to 3/4" thick Extira mdf. Both sides of the Extira would be veneered and any end cuts sealed.
The fascia will cover an A-frame truss (SEE ATTACHED SKETCH) so there will be a joint between the rafter covers and the chord (cross piece) cover. I'm going to specify that these joints are fitted tightly but not glued.
I've Gene Wengert's comments about LVL spring back and that's a potential concern with the MDF although I think much less so. We could saw all the MDF into blanks first and let them sit outside for a month before veneering them but this might be a waste of time.
Does anyone have any suggestions regarding Extira? Hot press glue type? Veneer thickness? I expect the finish (on site) will either be 2 coats of water based PU or possibly a hardwax oil like OSMO.


Click the link below to download the file included with this post.

EXTERIOR_TRUSS_FASCIA_RFI_2.pdf
2/20/23       #2: Exterior veneer trusses ...
Tom Gardiner

I have glued up layers of Extira successfully with type 2 pva. I have layed up HPL with it as well. I think I would go for PPR glue for the extra water and heat resistance in your application.
Read through the Extira application information sheet for recommended sealers and paints for sealing the "end grain". Extira expands and contracts with humidity so you are wise to not glue the butt joints. I couldn't find a spec for dimensional change but you you should call a tech rep to get that information.
I have no experience with exterior finishes

2/21/23       #3: Exterior veneer trusses ...
BH Davis  Member

Website: http://www.bhdavis.net

The first thing that comes to mind is to ask if these are structural components or strictly decorative? I have to assume the latter.

I wouldn't be too concerned about movement in the MDF. I think regular MDF has a seasonal change rate of about 1/2%. So a 20" wide piece should only move about .1". I don't think that would have a negative impact on your construction plan. Plus Extera is going to have greater density than regular MDF which should minimize the movement even more.

BH Davis

2/22/23       #4: Exterior veneer trusses ...
Keith Newton

Orin, my suggestion if the chord and rafter are on the same plane, where you show tight joint, is to sand a small round over on both edges to accentuate that joint. This will keep it from showing up as a bad fitting joint later if there is some shrinkage.
Otherwise, I would have planned for them to meet at different levels to begin with, rather than flush, which would be typical for timber framing.

2/24/23       #5: Exterior veneer trusses ...
Dave Edgerton  Member

To be honest, i think its a bad idea. Mdf and veneer are meant for interior components. Extira although for exterior is not being used in the manner you describe. Why not use solid 1/4" white oak wood? Why are you using 3/4" substrate and not thinner?
Plastic resin adhesive is the only way to go if you decide this route. Leaving extira to age before use is useless.

Sorry i dont mean to be harsh but i personally beleive this will fail in the long run.

3/1/23       #6: Exterior veneer trusses ...
Bruce H

If you are using MDF, get the stuff made from rice straw. It is waterproof. I have a piece in a jar of water on my desk for six months, there is no change from the original. I used it for window sills and exterior porch columns.


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